load cell technology vpg-01

7/29/2019 Load Cell Technology Vpg-01

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Appl icat ion Note VPG-01

VPG TRANSDUCERS

Load Cell Technology

Load Cells and Weigh Modules

For technical support, contact in Americas [email protected],in Europe [email protected], in China [email protected],

in Taiwan [email protected]

www.vpgtransducers.com1

Document Number: 11866Revision 21-Feb-2012

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The heart o any weighing system is the load cell. Whilstthey are not exciting to watch, load cells are highly accuratetransducers which provides the user with inormationnot generally obtainable by other technology due tocommercial actors.

Load cells are designed to sense orce or weight under a

wide range o adverse conditions; they are not only the mostessential part o an electronic weighing system, but also themost vulnerable. In order to get the most benet rom theload cell, the user must have a thorough understandingo the technology, construction and operation o thisunique device. In addition, it is imperative that the userselects the correct load cell or the application and providethe necessary care or the load cell during its lietime.Understanding these important issues and properlymaintaining the load cells will ensure trouble ree weighingor a long period o time.

Load cells may be damaged because o (shock) overloading,lightning strikes or heavy surges in current, chemical ormoisture ingress, mishandling (dropping, liting on cable,etc.), vibration, seismic events or internal componentmalunctioning. This article will ocus on the Do's andDon'ts or load cells as well as on basic system design.

1. Load Cell Selection

Load cell selection in the context o trouble ree operationconcerns itsel primarily with the right capacity, accuracyclass and environmental protection, rather then witha particular measuring principle like bending, shear,compression or ring torsion. While saying this, it should

also be recognized that a particular measuring principlemight oer distinct advantages in terms o overloadcapabilities or the ease o mounting. The dierentprinciples o operation will thereor be discussed shortly.

1.1 Strain gage load cells

The sensing or spring element is the main structuralcomponent o the load cell. The element is designed in sucha way that it develops a strain, directly proportional tothe load applied. Sensing elements are normally made ohigh strength alloy steels (nickel plated or environmentalprotection), precipitation - hardened stainless steels, heattreated aluminium alloys, or beryllium copper alloys.

By bonding strain gages to a precisely machined element,the orce applied can be identied in terms o resistancechange. The strain gages, usually our or a multiple o our,are connected into a Wheatstone bridge conigurationin order to convert the very small change in resistanceinto a usable electrical signal. Passive components suchas resistors and temperature depending wires are used tocompensate and calibrate the bridge output signal.

1.1.1 Bending load cells

Sensing elements which are subjected to bending momentsare widely used, in many congurations, or commercialtransducers. Bending beams oer high strain levels atrelatively low orces, which makes them ideal or lowcapacity load cells.

Furthermore, in case o a beam with a symmetrical cross-section about the bending axis, there are always twosuraces subjected to equal strains o opposite sign. Thisoers convenient means or implementing a ull bridgecircuit, while temperature compensation is relatively easy.

Most products using the bending principle are o theparallelogram or double bending type.

Figure 1. The most important actors or a weighing

system are linked like a chain; each link requires

attention or a long term successul operation.


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Bending as a measur ing principle oers excellent linearity.Bending beams have relatively high strain levels withgreater defection compared to other measuring principles.This in turn means that although the cell is subjected togreater static overload, mechanical stops are more easible.The dynamic overload capabilities are excellent because othe typical high defection.

1.1.2 Shear load cells

Shear (beam) load cells have become increasingly popularor all types o medium and high capacity applications.Shear as a measuring principle oers a standard prole ora given capacity, good resistance against side loads and arelatively small sensitivity to the point o loading.

At section A-A o the beam, a recess has been machined ineach side, leaving a relatively thin web in the centre. Just asin a structural I-beam, most o the shear orce imposed bythe load is carried by the web, while the bending momentis resisted primarily by the fanges. At the neutral axis,where the bending stress is negligible, the state o stresson the web is one o pure shear, acting in the verticaland horizontal directions. As a result, the principle axesare at 45 to the longitudinal axis o the beam, and thecorresponding principal strains are o equal magnitudeand opposite sign. Pairs o strain gages are installed onboth sides o the web and connected in a ull-bridge circuitor load measurement. Although it is more dicult toinstall the strain gages in some orm o recess, they canreadily be sealed and protected against environmentaleects.

Low capacity shear load cells are diicult to produce,because they require a very thin web to obtain the necessarystrain levels. High capacity shear load cells are usual based

on dual shear webs in a beam conguration, as single endedbeams become expensive and cumbersome to mount.Shear beam load cells are relatively insensitive to thepoint o loading and oer a good resistance to side loads.This simplies its use in many weighing applications. Theoverload capabilities are usual slightly better compared tobending beams, although mechanical stops are less easiblebecause o minimal defection.

1.1.3 Compression load cells

Compression load cells can be based on shear, bending,ring torsion or column measurement. The columnload cell has a history which dates back to the earliest

strain gage transducer. As indicated below, the columnelement consists o one (single column) or more ( multiplecolumn) members.

Although conceptually simple, the column element has anumber o specic characteristics which makes these loadcell types dicult to design and produce. The columnitsel should be long enough, with respect to its crosssection, to provide a uniorm strain eld, unaected byend conditions. Since the column conguration is subjectto second-order eects rom o-axis or o-centre loadcomponents, provisions must be made to minimize these,or example by using two diaphragms at the upper end othe column.

Column load cells are inherently non-linear due to thechange in cross section, while deorming under load(Poisson's ratio). This non-linearity can be compensatedor with semi-conductor gages, connected in the plus andminus excitation lines. The output o the semiconductorgage thus serves as a eedback or adjusting thebridge voltage in the opposite direction to that o the non-linearity error.

Figure 2. Load cell types 1042 and 363

Figure 3. Principle o shear-web sensing element

Figure 4. Multi-column load cell type CSPM


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Single column load cells become tall and dicult to handle(heavy), when designed or very high loads. Low prolecanisters can be obtained i the load is carried by three ormore columns, each column with its own set o gages. Thecorresponding gages rom all o the columns are connectedin series in the appropriate Wheatstone bridge arms. Theresult is not only an overall low prole, but also an improved

perormance when the cell is o-centre or o-axis loaded.

Compression type load cells don't suer rom themomentum typically associated with beams. The ultimateoverload capabilities are thereor excellent. However, therelatively small defection makes these load cell types moresensitive to shock loading.

1.1.4 Ring torsion load cells

The ring torsion measuring principle is relatively new, andideally suited or the capacity ranges which are typicallyserved by shear and bending beams. VPG Transducersload cell model RLC is a low prole, stain-less steel ringtorsion load cell, based on a ull bridge circuit o ourcircular strain gauges. The strain gauges are bonded to aring-shaped part o the element which will bend when loadis introduced. This process will cause a decrease o thering diameter at the top, while the bottom experiences adiameter increase. Hence, two gauges are compressed andtwo gauges are in tension, when the unit is loaded.

The geometrical design o the sensing element providesenhanced specications in terms o creep and hysteresis

compared to shear and bending as a measur ing principle.

Due to its compression loading mode, the unit does notsuer rom the momentum typically associated withbeams, and is thereore an inherently saer device, whilemaintaining an extremely low prole. Mechanical overloadprotection is established by the pre-determined distancebetween the load introduction ring and the base plate.Ring torsion load cells have a very low defection, whichmakes them ideal or high speed weighing, but they arealso more sensitive to shock overloading.

1.2 Capacity SelectionOverload is still the primary reason or load cell ailure,although the process o selecting the right load cell capacitylooks easy and straight orward on rst sight. Capacityselection requires a undamental understanding o theload related terms or load cells as well as the load relatedactors associated with systems. The load related terms orload cells are:

Load cell measuring range:

The range o values o mass or which the result omeasurement should not be aected by an error exceedingthe maximum permissible error.

Sae load limit:

The maximum load that can be applied without producinga permanent shit in the perormance characteristicsbeyond those specied as a percentage o the measuringrange (i.e. 150%).

Ultimate overload:

The maximum load that can be applied without physicaldestruction o the load cell; specied as a percentage o themeasuring range (i.e. 300%).

Sae side load:

The maximum load that can act 90 to the axis along whichthe load cell is designed to be loaded at the point o axialload application without producing a permanent shit inthe perormance beyond those specied as a percentage othe measuring range (i.e. 100%).

Summary:

A load cell will perorm within specications until the saeload limit or sae side load limit is passed. Beyond this point,even or a very short period o time, the load cell will be

permanently damaged . The load cell may physically break atthe ultimate load limit.

The actors that contribute to the weight load on the load

cells are: zero tracking, initial zero setting, dead load,maximum scale capacity, location, and specic actors likewind orces or seismic events.

In addition to these, it is oten necessary to derate the loadcells (use a higher capacity) because o:

Shockloading

Dynamicinuences(agitators)

Offcentreloadingtothescale

Offcentredistributionofdeadload

Thepossibilityofanoverloadweight

Figure 5. Cross section o load cell type RLC


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The dierence between normal or static overload andshock overload is oten misunderstood and needs urtherexplanation. Static overload is dened as a gradual increasein weight over and above the rated capacity o the load cell.

Systems can be protected by incorporating mechanicalstops, or by selecting load cells with a higher rated capacity.

Shock overload can be dened as a sudden change in weight,within a very short period o time, over and above the ratedcapacity o the load cell. This situation specically occurswhen a relatively small non-elastic item is dropped rom aconsiderable height on the scale. Systems can be protectedby incorporating shock insulation pads, or by selectingload cells with a higher rated capacity. Mechanical stopsact to assist the protection against shocks.

Particular care must be given to load cells with a lowdefection, as they are more sensitive to dynamic overload.

Both types o overload result in a sudden change o zerobalance, the temperature compensation on zero is also

aected.The ollowing calculations and table should be used tocalculate the correct load cell capacity:

LCcap = [Dead load + (Live load*Fa)]/N + Ft + Fw

where:

N Number o load cellsFa Dynamic load actorFw Eect o wind orce (or hoppers)Ft Combined eect o zero setting devices:

Ft = [Live load * Zero setting devices(%)]/ N*100

* Determine the value based on the load cell's measuring principle (low

defection 6 value high).

For example:

A platorm scale with a capacity o 1500 kg is built withour load cells. The scale has an initial zero setting o 16%and a zero tracking o 4%. The dead load equals 100 kg.The load cell capacity should be:

Ft = [1500*(16*4)] / 4*100 = 75

LCcap = [100 + (1500*4) / 4] + 75 + 0 = 625 kg

Depending on the load cell's measuring principle, the

required load cell capacity varies between 895 and 1250 kg(derate 30 to 50%).

It is important to veriy the output per scale division with therequired minimum signal level or the measuring device toensure compatibility. The output per division (in V) can becalculated by:

(UE*S*Live load*1000) / N*LCcap*n

where:

UEExcitation voltageSRated output load celln Number o scale divisions

For example:The above scale conguration is built with 4 load cells,output 2mV/V, 3000 divisions, rated capacity 1000 kgand an excitation voltage o 10V. The output per divisionwill be:

(10*2*1500*1000) / 4*1000*3000 = 2.5V

1.3 Accuracy

Load cells are ranked, according to their overallperormance capabilities into diering accuracy classes.Some o these accuracy classes are related to standards

Figure 6. Static overload

Figure 7. Shock overload

Safe overload

Ultimate overload

LC range

Safe overload

Ultimate overload

LC range

Fa De-Rate*

Platform scale 1.4 30 50%

Weighbridge 1.4 30 50%

Single cell application 1.3 20 40%

Single cell hybrid bridge 1.2 10 30%

Hopper, equal load distribution 1.1 10 30%

Hopper, unequal load distribution 1.2 20 40%

Hopper, with agitator 1.3 20 40%


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which are used in legal or trade weighing instruments,while other accuracy classes are dened by the individualload cell manuacturer.

Depending on the standard and the perormance o aparticular load cell type, an alphanumeric "accuracygrade" is given to the product. The alpha designate reersto the specic accuracy class, while the numeric part reersto the number o divisions.

VPG Transducers manuactures products meeting NTEP,OIML and in-house speciications. These product aredesignated:

Az Products meet the NTEP requirements or

class III applications.

Bz Products meet the NTEP requirements orclass IIIL applications.

Cz Products meet the OIML requirements orclass III and IIII applications.

Note "z" represents the number o divisions (x1000), i.e. A3, B10,

C6, etc.

Most weighing systems use load cells where their workingor measuring range is well below their rated capacity. Inthese situations, the values or the load cell utilization andminimum verication interval (vmin ) are important.

The minimum verication interval is dened as the smallestvalue o a quantity (mass) which may be applied to a loadcell without exceeding the maximum permissible error. Itis specied as Emax /, where Emax represents the load cell'srated capacity and represents a value which is speciedby the load cell supplier.

The minimum measuring range can apply over any parto the measuring range between the minimum dead load(Emin) and the rated capacity (Emax).

A load cell may be used over a working range larger that itsminimum utilization.

The terms above the central horizontal line are xed by thedesign o the load cell, while the terms below are xed onthe conditions o use and cell perormance.

1.3.1 Approved systemsLegal or trade weighing systems require load cells whichare certied according to the National Type EvaluationProgram (NTEP) or OIML recommendation R60 (Europe).The requirements in terms o load cell accuracy or theabove mentioned systems are:

1) Select a cell which is certi ied according to theappropriate standard, i.e. products designated "Az" orclass III applications.

2) For each load cell, the maximum number o loadcell intervals shall not be less than the number overiication scale intervals. For example; a 3000

division class III scale requires A3 load cells.3) The minimum load cell veriic ation interval shall

satisy the condition:

vmin e*R / N

where e represents the scale verication interval and Rrepresents the reduction ratio o the load transmittingdevice (hybrid scales).

R = Load acting on the load cell(s)Load acting on the receptor (scale)

For example:

A ully electronic scale (R=1), with our load cells and ameasuring range o 6t divided into 3000 divisions requiresload cells with the ollowing vmin:

vmin 6000 * 1 vmin 1 kg3000 4

1.3.2 Non approved systems

The load cell selection or non-approved weighing systemscan be based on the specied error percentages which areindicated on our datasheets.

1.4 Environmental ProtectionNo area o load cell operation causes more conusionand contention than that o environmental protectionand sealing standards. Although our industries havein-depth standards and test procedures to dene load celland weighing system perormance, no standards havebeen developed to cover product suitability or specicenvironmental conditions.

In the absence o such standards, most manuacturershave adopted the International Protection system (IP/IEC529 or EN 40.050) or National Electrical ManuacturersAssociation Standards (NEMA publication 250). Dene

No Load Emin Emax Safe Load Ultimate LoadMaximum Measuring Range

Measuring Range

D Dmin max

Figure 8. Graphic relationship o load related terms


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the level o sealing or their products. Both standardsare good test procedures or environmental sealing whenapplied to the products or which they were intended -those being electrical enclosures, but they are not very wellsuited to load cells.

1.4.1 IP Classifcation

The IP standard describes a system or classiying thedegree o protection provided by the enclosures o electricalequipment:

Protectionofpersonsagainstaccesstohazardousparts inside the enclosure.

Protectionoftheequipmentinsidetheenclosureagainst the ingress o solid oreign objects.

Protectionofequipmentinsidetheenclosureagainstharmul eects due to the ingress o water.

Unortunately, no denition is given or the term "harmuleects". Presumably, or enclosures, the main problemwith water could be one o electrical shock to persons incontact with the enclosure, rather than malunctioning othe unit. Furthermore, the standard only relates to wateringress and ignores moisture, chemicals, corrosion, etc.

The commonly used categories to describe load cellsealing are:

IP65 Protected against low pressure jets o waterrom all directions, limited entrance allowed

IP66 Protected against strong jets o water e.g. oruse on ship decks, limited entrance allowed

IP67 Protected against the eects o immersionbetween 15cm and 1m

IP68 Protected against long periods o immersionunder pressure

1.4.2 NEMA Classifcation

Classications in the NEMA system run rom NEMA 1 toNEMA 12, but load cell manuacturers concern themselveswith NEMA 4 and NEMA 6. Unlike the IP system, NEMA

does concern itsel with environmental conditions such ascorrosion, rust, reezing, oil and coolants.

NEMA 4 enclosures are intended or indoor and outdooruse, providing a degree o protection against windblowndust, rain, splashing water, and hose directed water.However, no consideration is given or the eects ointernal condensation. Nema 4X enclosures meet the samestandards as NEMA 4 and are constructed o 304 stainlesssteel or other material oering equal corrosion resistance.

NEMA 6 enclosures are used where there is a chance otemporary immersion. This standard calls or the highestpart o the enclosure to remain submerged in water, with its

highest point 1.83 metres below the surace or 30 minutes.NEMA 6P enclosures are used where prolonged immersionmay occur and resistance to corrosion is needed.

While it may seem that NEMA standards oer someadvantages over the IP system or corrosion resistance,they only relate to external corrosion o enclosures. Thisis very limited when applied to the more complex load cellconstruction and the dierent eects o corrosion or wateringress.

1.4.3 Damp Heat Cycling (IEC 68-2-30)

The IP and NEMA standards don't deal with internalcondensation or moisture within the enclosure. However,

moisture or condensation is o vital importance in correctload cell operation.

Moisture may enter the inside o the load cell over a longperiod and have a catastrophic eect, especially whenacids or alkalies are present. One test used to determinea load cells ability to withstand moisture or condensationis the Damp Heat Cycling Test. The object o the IECstandard is "To determine the suitability o components,equipment, or other articles or use and storage underconditions o high humidity when combined with cyclicaltemperature changes".

It is obvious that this standard is a much more useul

classication than the IP or NEMA rating when it comesto dening load cell environmental suitability.

1.4.4 Load cell construction

Besides a given IP-rating or NEMA-classication loadcells should also be classied according to their design interms o cable entry, material o construction and gagessealing method.

Figure 9. Load cells certifed to OIML R-60 are

tested to withstand 12 damp heat cycles.

100

90

80

70

+50

+25

Zeit (h)

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Whilst it is relatively common to weld-seal critical areason a load cell body, one potential problem area is the cableentry. A variety o methods are used to make sure cells areproperly sealed at this area.

In most load cells the main cable enters through aconventional cable gland directly into the gage area.Regardless o how well the gage area is sealed, moisture andsolvents can penetrate either around the gland or throughthe centre o the cable itsel. Oten, temperature changescause a pumping action to occur, pushing moisture downthe inside o the cable. Entry also can be via a leakingjunction box or through a damaged part o the cable. Thiscan take some time to reach critical areas, but once there it

will become sealed in place and do critical damage.

An improvement on the basic cable gland is a water blockat the point o cable entry. Here, the main cable terminatesat or example a small circuit board with on-going wiresleading to the gage area. The block is ully potted toprevent moisture or other contaminants rom reaching thecritical areas.

The best solution is the use o a glass-to-metal cableentrance. This prevents any contamination romreaching the gage or other critical areas. In addition, themanuacturing process used must keep the load cell ree

rom residue contaminations.

1.4.5 Corrosion

The corrosion resistance o load cells is a very complexsubject, one that is urther complicated by the variety oavailable congurations. As a result it is only possible touse standard corrosion charts as a guidance or load cells.In addition, the ollowing actors must be considered:

Surfacenish

Weldareasaroundseals,bellowsandcups

Thicknessofseals Varyingconstructionmaterials

Highstresslevelsatloadingpoints

Cablematerial(PVC,PURorteon)

The environment itsel plays an important role in how aparticular load cell type behaves in practice. Salt water, orexample, has dierent corrosion eects depending on thelocal circumstances. Stainless steel in stagnant salt wateris subject to crevice corrosion and a regular wash down isnecessary to avoid degradation.

Unortunately the term stainless steel has becomesynonymous with "no corrosion, no problem and no

maintenance". While stainless steel load cells usually oeroptimum protection in most environments, other actorsshould be taken into account. In certain applications,painted or plated load cells may oer better long-termprotection.

An alternative is wrap-around protective covers. These canprovide good environmental protection, but can be sel-destructive i corrosive material is trapped inside the cover.

1.5 Summary

Selecting the wrong load cell or an application in termso environmental compatibility can have ar reaching

consequences in terms o costs, saety and productreputation. Current classiications all well short odening adequate environmental standards or load cells.

The users should compare like-or-like eatures whenselecting products rom dierent manuacturers. I indoubt, they should ask pertinent questions relating to:

Constructionoftheloadcell

Cableentrymethod

Pastexperiences

For applications in harsh environments, additionalprotection or the load cells may be needed to assure

their reasonable working lie. This can be achieved withenhanced scale designs and the use o additional coatingson the load cell, such as paints, greases and plating. Thescale or system design should minimize the possibilityo material build-up around the cells. I appropriate, thedesign should also provide mechanical protection rom theeects o direct water and solvents whilst cleaning. Sealingcompounds and rubbers used on some load cells candeteriorate when exposed to chemicals or direct sunlight.Because they embrittle rubber, chlorine-based compoundsare a particular problem.

Figure 10. Water block cable entry


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2. Installation

The installation o load cells into a practical ieldapplication requires careul attention i the system is to besae and accurate.

It is a common misconception that a load cell can beconsidered as a solid piece o metal on which hoppers orplatorms can be supported. The perormance o a load celldepends primarily on its ability to defect repeatably underconditions when load is applied or removed. Furthermore,i more than one load cell is used then the defection andoutput o each individual cell should be similar on eachload point.

To satisy the above requirements, load cells are mainlyused in conjunction with special mounting systems ratherthen being mounted rigidly between platorm/hopper andoundation. Load cell supports should be designed to avoidthe ollowing eects to the load cell:

Lateralforces

Bendingmoments

Torsionmoments

Offcentreloadingtothecell

Vibrationtotheloadcell

These eects not only compromise the perormance o the

load cell, but they can also lead to permanent damage.

I major load movement is anticipated, stay rods shouldbe used to restrain a platorm (weighbridge) or vessel. Stayrods are installed horizontally and should not transer anyorces to the vessel or scale in the vertical direction, whilehaving sucient strength in the horizontal direction to be

able to absorb side orces. The length o the rods should bechosen as long as possible, as this has a avour-able eecton reducing vertical orces.

The arrangement o the stay rods depends on the plan viewgeometry o the structure. In most cases our rods give thebest results.

Stay rods provide stability and accuracy, specially orsystems with agitators. They should be installed careully(exactly horizontal) and without any stress. Stay rodsshould not be conused with saety rods, which are installedsimilar, but provide a dierent unction. Saety rods arelet loose during normal operation. The are an extra saetyeature in the event o wind orces, seismic activity ormechanical ailure o mounts or load cells.

Saety rods are strongly recommended or those systemswhere one o the above events could seriously aectpersonnel saety or where one o the above events couldlead to extensive damage.

In order to assure perormance, load cells should be placedon exactly the same horizontal level. Never use mountingbolts to pull uneven suraces together; shim plates shouldbe used as appropriate.

The preerred orientation o the load cell depends primarilyon its design. The load should always be transmittedvertically through the load cell in the way which it wasdesigned to measure orce.

S-type load cells should be mounted in such a way thatside orces are reduced to a minimum; they should neverbe mounted rigidly (even only at one side) between the

Figure 11. Rubber anti-vibration pad

Figure 12. Sel aligning mount, based on a rocker pin load

introduction. The top plate is held captive to provide lit-o

protection and to restrict the horizontal movement.

Figure 13. Placing rods as indicated in the let drawings will

cause high stresses in the stay rods or rotation o the vessel

Figure 14. Load transmission

Wrong Right

Wrong Right


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structure and hopper. The load cell must be orientated insuch a way that the cable entry does not aect weighingaccuracy.

In terms o saety, attention should be paid to use the ulllength o thread, while considerations should be made toprovide an external back up system.

To prevent load cells rom being damaged duringinstallation, it is strongly recommended to use dummiesor mounting assemblies that can be "locked". Load cellsshould be handled with care, especially those with a lowrated capacity or with metal bellows construction.

Single ended beam load cells are subjected to a momentumand require high quality bolts or sae operation. Theamount o torque on these bolts is specied and should bemet to achieve the maximum perormance.

2.1 Load cell cables

Special attention should be paid in preventing the load cellcable rom being damaged during and ater installation.Never carry load cells at their cables and provide drippingloops to prevent water rom running directly into the cableentry.

Load cells are produced with a our- or six-wire cable. A

our-wire cable is calibrated and temperature compensatedwith a certain length o cable. The perormance o the loadcell, in terms o temperature stability, will be compromised ithe cable is cut; never cut a four-wire load cell cable!

A six-wire load cell cable has two additional wires which canbe used to actually measure the excitation voltage at the loadcell in order to eed this inormation back to the indicator.A six-wire load cell is not part o the load cell's temperaturecompensating system and can be cut to any desired length.However, it should be recognized that the parallel connectiono multiple six-wire load cells results in an equal potentialdierence over all cells. All load cell cables should thereorbe shortened to the same length.

2.2 Junction boxesThe junction box is an essential part o the system andshould be protected to at least IP65 or NEMA 4. Select thelocation o the junction box based on the environmentalconditions; NOT on the ease o installation.

During the installation ensure that no moisture entersthe load cell cable beore and during installation. A bago drying agent (silica gel) may be enclosed to absorbmoisture. However, the drying agent should never makecontact with any non- insulated wiring in the box.

Use junction boxes with high quality terminals or usesolder connections. The components used or corner

correction should be absolutely temperature stable.

2.3 Welding

Avoid electric welding ater installation o the load cells. Iwelding is necessary and the load cells can not be removedthen disconnect each individual load cell cable rom thejunction box or measuring device.

Place the clamp earthing electrode o the welding apparatusin the close proximity o the weld to avoid a current paththrough the load cells. Furthermore, connect a fexiblecopper lead over each load cell.

Figure 15. Correct installation o S-type load cell

Figure 16. Correct installation o junction box;

cable entries downwards and dripping loops

Figure 17. Electrical welding ater installation o the load cells

Earth clamp

Welding

Earth clampWelding

WRONG

RIGHT


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2.4 Lightning protectionModern weighing systems rely heavily on high perormanceelectronic components, but the eatures that make thispossible also makes these components more vulnerable tothe disruption and damage that can be caused by lightningor over-voltage in general.

Investigations indicate that a lightning strike within a 900tradius o the geometrical centre o the site will denitely havea detrimental eect on the weighbridge. Nor is such damageconned only to earth strikes, since cloud-to-cloud strikesare equally capable o producing an electromagnetic pulse(EMP) o sucient strength to cause damage.

In most cases, the actual load cell damage is a directresult o a potential dierence, well in excess o 1000 volts,between circuit and housing.

It should be recognized that a high potential dierencebetween housing and load cell circuit can be caused by:

Ariseoflocalearthpotentialwhenalightningstrikeisdissipated through the ground.

Severeover-voltagesorelectromagneticpulsesviathemain power supply.

The damage in both cases might result in a completeburn out o the component, but this is not always true. Itis possible that only a part o the load cells circuit or one

o the strain gage's glue layer is damaged. As a result thescale starts to drit and/or will not hold its return to zero.Sometimes these problems appear weeks ater the actuallightning strike!

It is obvious that a high level o protection againstlightning strikes can only be established i the completesystem is protected. The decision to protect a system orpart o a system should be based on: the location (urbanarea or open ield), the average ground lash density(ask or statistics at the appropriate institute), the costsincurred during a breakdown period, the expenses or thereplacement o parts and workmanship, and the insurancecompany requirements (or premiums!).

Protection can be considered in two stages; external andinternal.

2.4.1 External protection

External protection entails bonding the steel roo othe cabin where the indicator is located or any nearbystructure or a high protection mast in such a matter asto provide a preerential point o discharge and saelyconduct the surge to earth via conductors.

In act considerable controversy surrounds such externalprotection . A weighbridge or in general a scale is not anattractive point or lightning to strike, but a 60t mast has an

attractive radius o 240t. Lightning which might otherwisehave struck a building or tree 180t or more away will now becaptured to produce a current surge to ground within the veryneighborhood o the weighbridge!

Unless very stringent precautions are taken such a surgewill produce an electromagnetic induced pulse which willundoubtedly cause severe damage to the weighbridge.

Figure 18. Load cell and indicator are both connected to a

separate earth, at a considerable distance rom each other.

Figure 19. The potential at the earthing points will rise as

a direct result o a lightning strike (to earth). However, both

points will not rise to the same potential because o the

ground resistance. Although the excitation voltage remains

10V, the potential dierence between circuit and housing

increases ar above an acceptable value.

Figure 20. External lightning protection

10V

0V

0V

0V

10V

Circuit to housing = 10V

1010V

200V

1000V

1000V

1010V

Circuit to housing = 810V

Current paths Equipotential plane

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2.4.2 Internal protectionInternal lightning protection sets out to provide potentialequalisation throughout the whole system by deining acentral point about which all the electrical systems can foator by using Surge Protection Devices (SPDs).

Surge protection devices are designed to control line-line and line-earth voltages to levels acceptable to theequipment. An SPD incorporates combinations o gas-lled discharge tubes or high current surge diversion andzener diodes or secure voltage clamping with minimalleakage. For AC power applications, varistors are otenused because o their higher power absorption capability.Most SPDs are connected in series, similar to shunt diodebarriers or intrinsically sae systems.

Any device which works by diverting large currents to alocal ground must have a low impedance connection tothat ground. This means that the bonding connection mustbe o low resistance (well below 0,5), short in length andas direct as possible without sharp bends. Veriy the earthconnections at least twice a year and coat all connectionswith a good antioxidant grease.

Any external connection such as AC power lines,communication ports and the signal/excitation cableis a potential source o surges or transients. Central tothe provision o lightning protection is thereore theinstallation o an SPD at all external connections:

3. Maintenance

Maintenance is oten overlooked or ignored by both loadcell users and service companies. However, the regularservice and maintenance o load cells in a weighingsystem will greatly improve their long-term reliability andperormance as well as greatly reduce their sensitivity tocorrosion. Maintenance inspections can be divided intotwo categories:

Routine:Perormed at periodic intervals, it includes the removal oany material or debris buildup rom around the load cellsand mounting xtures. Serious damage can occur to theload cells i mounting systems do not unction correctly.Any damage or degradation o surace coatings shouldbe remedied and all cables and junction-boxes should bechecked. To minimize the eects o fooding, any drainagesystems in the pit should be ree rom debris. Whererequired, regular wash down o the load cell should becarried out to prevent chemical attack.

Adhoc:

Made immediately ater any adverse or unexpected events

such as fash foods, gales, seismic activity or electricalstorms.

In General; careul consideration must be given to anyreason or ailure. I this has occurred as a result o ingresso water or chemicals, then continued deterioration o anyother load cell(s) in the system can be expected, resulting inmechanical ailure. This ailure can have serious saety andcost consequences. Always remove the load cell with careand attach a label with comments to the problem or modeo ailure. Never cut the cable at the gland to acilitateremoval; load cells cannot be tested by us without cables!

Figure 21. Systems with unavoidable multiple

points o earthing should use an SPD to divertsurges to a local ground. By doing so, the whole

system will rise and all at the same potential.

Figure 22. All external connections should also be protected.

safe systems1010V

1000V

1000V

1000V

1010V

SPD

weighbridge

Cabin

J-box

RS 232 COM Port

AC Power supply

Ind.

~

~

~

~


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Do select the right load cell or the application in terms otype and environmental compatibility.

Do choose the right capacity.

Do consider the required accuracy class.

Do consider all environmental aspects beore makingthe nal choice (whether they are always or occasionallypresent at the place(s) the load cells will operate).

Do provide or any additional environmental protection atthe design stage.

Do design-in adequate over/under load protection as wellas protection rom other mechanical damage (e.g. physicalabuse, rodent problems).

Don't make the choice based only on price - cost oownership is more important.

Don't allow load cells to operate above their rated capacity.

Don't over speciy - look at overall system limitations onaccuracy (e.g. mechanical pipe work, vibration etc.).

Don't ignore that hurricane or food that comes once every2 or 3 years.

Don't build in water / debris traps.

Don't assume "it" will never happen, and never use the loadcell as a mechanical use.

Don't orget to provide adequate protection or the loadcell cable, near the load cell i possible.

Do use dummy load cells prior to installation.

Do store and handle load cells careully prior to and duringinstallation, and try to keep copies o the Certicate oCalibration in a sae place. Check load cells beore tting

or correct model, capacity, thread combination, etc.

Do check that any threaded ttings screw smoothly intothe load cell beore nal assembly.

Do use high quality bolts with the recommended torque.

Do check that adequate and accurately tted mountingsuraces are provided.

Do use care when tightening mounting bolts and restraintssuch as tie-bars.

Do use lock nuts appropriate on threaded ittings,especially i vibration is present.

Do check cable color code or load cell prior to connection VPG Transducers has two basic color codes.

Do use good quality connecting terminals/junction boxes.Solder joints i possible.

Don't carry out electric welding near load cells i possible.

Don't orget to check speciic storage and operatingtemperature ranges or the load cells.

Don't ever carry load cells by their cables!

Don't orce bolts or other threaded assemblies.

Don't use mounting bolts to pull uneven suraces together- use shims as appropriate.

Don't use excessive orce when tting / tightening mountingbolts or hardware, especially on low capacity cells.

Don't twist "S" cells when tightening threaded ttings.

Don't cut load cell cables unless necessary, perormancemay be eected.

Don't allow moisture to get at any interconnections.

Don't allow load cell to be the electrical link betweenground and metal weigh structure.

DOs and DON'Ts o Load Cells Summary

Installation And Fitting

Load Cell Selection and Design


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Do regularly inspect load cells and weigh system especiallyater extreme weather conditions ( electrical storms,fooding, seismic activity, etc.) and also beore and aterthe seasons.

Do check or corrosion damage to the load cell andmounting hardware. I practical, carry out cleaning andany remedial work (paint or other protective coating)beore it is too late.

Do give special care and attention to critical areas o theload cell such as metal bellows, seals etc. Those eaturesare important in the operation and perormance o theproduct.

Don't allow build up o debris around load cell or mounts.

Don't allow any drains to become blocked with leaves orother debris.

Don't disconnect and just re-calibrate one or more loadcells in a system i they cease to unction. Mechanicalailure may have catastrophic eects.

Do remove load cell with care and attach a label withcomments relating to the problem or mode o ailure.

Do return a copy o the Certicate o Calibration with theload cell i available.

Don't cut cable at the gland to acilitate removal - please -we cannot test load cells without cables!

DOs and DON'Ts o Load Cells Summary

Maintenance

Finally

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