auto1 hand out part 2

7/23/2019 Auto1 Hand Out Part 2

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Remote Control – Pressure switch and pressure transmitters are used to

measure pressure in the system and gives a corresponding signal to thecontroller. Sometimes, switches are used to give alarm signals or as anemergency switch to cut-of electrical signals. Pressure transmitter givesa standard output o 4 to ! m" or !. to #.! $ar.

A. Level Measurements

#. %nits o level – &evel may 'e measured in terms o innage orullage. (or S) units, we use millimeter, centimeter, meter orpercentage. )n *nglish system, inches, eet and percentage arethe usual units used.

. &ocal Reerence – " sight glass serves as a local indication orlevel measurement. Sounding tape is also used to veriy theactual level o the process. ) ever a level alarm is indicated in thecontrol system, we chec+ed i it is true 'y using theseinstruments as reerence or 'y visually chec+ing the actual level.

. Remote Control – &evel switch and level transmitters are used tomeasure level in the system and gives a corresponding signal tothe controller. Sometimes, switches are used to give alarmsignals or as an emergency switch to cut-of electrical signals.&evel transmitter gives a standard output o 4 to ! m" or !. to

#.! $ar.



B. Temperature Measurements

#. %nits o temperature – emperature is measured using the S)units o Celsius or Centigrade, oC. )n "S) system, (ahrenheit, o(,is used. here are also other units o temperaturemeasurement, /elvin and Ran+ine.

/elvin 0 Celsius 1 2Ran+ine 0 (ahrenheit 1 43!Celsius 0 56 7(ahrenheit – 8

. &ocal Reerence – " 'imetallic device or a 9uid e:pansion deviceserves as a local indication or temperature measurement. Somealso used an inrared radiation type o device or special purpose.;thers use change-o-state devices to indicate temperatureranges.



. Remote Control – emperature switch and temperaturetransmitters are used to measure temperature in the system andgive a corresponding signal to the controller. emperatureswitches are used to cut-of electrical signals that e:ceed aprede<ned limit. he most common sensors connected to

transmitters are the thermocouples and the R=s 7metallic8. hermocouples give an output signal o millivolts and the R=such as Pt#!! gives an output measured in ohms. hese signalsare then converted into standard instrumentation signals.

hermocouple – consist o two diferent metals that are >oined atone end. his >unction is use to measure a process. he >unctionis said to create current 9ow due to the See'ec+ *fect. heoutput is in millivolts and the device has a polarity.

R= – consist o pure metal that changes resistance astemperature changes.

*:ample? Pt#!! 0 Platinum, with #!! ohms at !oC Pt#!!! 0 Platinum, with #!!! ohms at !oC

Temperature is defined as the energy level of matter which can be evidenced by some change

in that matter. Temperature sensors come in a wide variety and have one thing in common:

they all measure temperature by sensing some change in a physical characteristic.

The seven basic types of temperature sensors to be discussed here are thermocouples,

resistive temperature devices (RTDs, thermistors), infrared radiators, bimetallic devices,

liquid epansion devices, molecular change!of!state and silicon diodes.

Thermocouples



Thermocouples are voltage devices that indicate temperature by measuring a change in

voltage. "s temperature goes up, the output voltage of the thermocouple rises ! not

necessarily linearly.

#ften the thermocouple is located inside a metal or ceramic shield that protects it from

eposure to a variety of environments. $etal!sheathed thermocouples also are available withmany types of outer coatings, such as Teflon, for trouble!free use in acids and strong caustic

solutions.

Resistive Temperature Devices

Resistive temperature devices also are electrical. Rather than using a voltage as the

thermocouple does, they ta%e advantage of another characteristic of matter which changes

with temperature ! its resistance. The two types of resistive devices we deal with at #$&'"

&ngineering, nc., in tamford, *onn., are metallic, resistive temperature devices (RTDs) and

thermistors.

n general, RTDs are more linear than are thermocouples. They increase in a positive

direction, with resistance going up as temperature rises. #n the other hand, the thermistor has

an entirely different type of construction. t is an etremely nonlinear semiconductive device

that will decrease in resistance as temperature rises.

nfrared ensors

nfrared sensors are noncontacting sensors. "s an eample, if you hold up a typical infrared

sensor to the front of your des% without contact, the sensor will tell you the temperature of the

des% by virtue of its radiation ! probably +- at normal room temperature.

n a noncontacting measurement of ice water, it will measure slightly under /-* because of

evaporation, which slightly lowers the epected temperature reading.

0imetallic Devices

0imetallic devices ta%e advantage of the epansion of metals when they are heated. n these

devices, two metals are bonded together and mechanically lin%ed to a pointer. 1hen heated,

one side of the bimetallic strip will epand more than the other. "nd when geared properly to

a pointer, the temperature is indicated.

"dvantages of bimetallic devices are portability and independence from a power supply.

2owever, they are not usually quite as accurate as are electrical devices, and you cannot

easily record the temperature value as with electrical devices li%e thermocouples or RTDs3

but portability is a definite advantage for the right application.

Thermometers

Thermometers are well!%nown liquid epansion devices. 'enerally spea%ing, they come in

two main classifications: the mercury type and the organic, usually red, liquid type. The

distinction between the two is notable, because mercury devices have certain limitations

when it comes to how they can be safely transported or shipped.



or eample, mercury is considered an environmental contaminant, so brea%age can be

ha4ardous. 0e sure to chec% the current restrictions for air transportation of mercury products

before shipping.

*hange!of!state ensors

*hange!of!state temperature sensors measure 5ust that ! a change in the state of a material

brought about by a change in temperature, as in a change from ice to water and then to steam.

*ommercially available devices of this type are in the form of labels, pellets, crayons, or

lacquers.

or eample, labels may be used on steam traps. 1hen the trap needs ad5ustment, it becomes

hot3 then, the white dot on the label will indicate the temperature rise by turning blac%. The

dot remains blac%, even if the temperature returns to normal.

*hange!of!state labels indicate temperature in - and -*. 1ith these types of devices, the

white dot turns blac% when eceeding the temperature shown3 and it is a nonreversible sensor which remains blac% once it changes color. Temperature labels are useful when you need

confirmation that temperature did not eceed a certain level, perhaps for engineering or legal

reasons during shipment. 0ecause change!of!state devices are nonelectrical li%e the

bimetallic strip, they have an advantage in certain applications. ome forms of this family of

sensors (lacquer, crayons) do not change color3 the mar%s made by them simply disappear.

The pellet version becomes visually deformed or melts away completely.

6imitations include a relatively slow response time. Therefore, if you have a temperature

spi%e going up and then down very quic%ly, there may be no visible response. "ccuracy also

is not as high as with most of the other devices more commonly used in industry. 2owever,

within their realm of application where you need a nonreversing indication that does not

require electrical power, they are very practical.

#ther labels which are reversible operate on quite a different principle using a liquid crystal

display. The display changes from blac% color to a tint of brown or blue or green, depending

on the temperature achieved.

or eample, a typical label is all blac% when below the temperatures that are sensed. "s the

temperature rises, a color will appear at, say, the 77- spot ! first as blue, then green, and

finally brown as it passes through the designated temperature. n any particular liquid crystal

device, you usually will see two color spots ad5acent to each other ! the blue one slightly below the temperature indicator, and the brown one slightly above. This lets you estimate the

temperature as being, say, between 8- and 9/-.

"lthough it is not perfectly precise, it does have the advantages of being a small, rugged,

nonelectrical indicator that continuously updates temperature.

ilicon Diode

The silicon diode sensor is a device that has been developed specifically for the cryogenic

temperature range. &ssentially, they are linear devices where the conductivity of the diode

increases linearly in the low cryogenic regions.



1hatever sensor you select, it will not li%ely be operating by itself. ince most sensor choices

overlap in temperature range and accuracy, selection of the sensor will depend on how it will

be integrated into a system.

auto1 hand out part 2

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