tweng - accuracy_pressure_transm - copy

7/31/2019 Tweng - Accuracy_pressure_transm - Copy

1/16

Tweng Engineering Services

Understanding accuracy withpressure transmitters


2/16

Sensor: part of a measuring instrument that

converts a physical value into an electrical signal that

is proportional to the physical value. The output

signal is normally mV (millivolts).

Transmitter: This is a sensor completed with a

signal conditioner that allows to convert the analog

signal into a standard signal on high level (example :mA or Volts). Example: transmitter E913

General Information

Terminology


3/16

Technologies of pressure sensors

Piezo-resistive

Capacitive

Piezo-electric

Fibre optic

LVDT

Vacuum gauges

Strain Gauges

Silicon

Ceramic

Silicon

Thin film

Thick film

Bonded Strain Gauges


4/16

Accuracy of pressure transmitters

Summary

Characteristics of pressure transmitters

1. Linearity, Hysteresis, repeatability2. Factor of influence on accuracy

3. Accuracy according BFSL

4. Accuracy according straight line 0 to 100% FS

5. Total error band

Calibration of a pressure transmitters

1. Principle

2. Example of calibration in real time:


5/16

General information

Transfer curve of a pressure transmitter at 20C

mA

20

Ideal transmitter

4

10

16

P max. P


6/16

General informationDefinition of linearity

e L= e max. = x% EMS max.-S0

P

mA

Best fit straight line

S 0

S max.

e max.

e max.

FS

4

10

16

20


7/16

General informationDefinition of Hysteresis

P max.

P

S 0

S max.

e H= e max = x% EM2 (S max..-S0)

e max.

mA

4

10

16

20


8/16

e R = d0S average

General informationDefinition of Repeatability

14.6

14.7

14.8

14.9

15

15.1

15.2

t1 t2 t3

Output value

t

d0S average


9/16

BSL: best-fit straight line

Adjustment of a curve

y = a * x + b by least square method .

The measuring points are following the

initial (red) curve and are affected by

errors .

The BSL (green curve) is the bestestimation of the slope and intercept

according the quantity of information

contained in the measuring points.

This curve is a calculation it doesnt

take in account the error on zero and

the error on full scale

Output (mA)

FS Input

20 mABSL

Max linearity error

4mA


10/16

Full scale is straight line 0 ~ 100%

FS P Input

20mA

4mA

Output


11/16

Mathematical formula

Errorglobal = eL2+ eH2+ eR2

Namely the Global error of the transmitter at 20C

Global error at 20C


12/16

Factors which influence on accuracy

Global error

eg = eL2

+ eH2

+ eR2

Namely ;

- Zero point deviation

- Span deviation

- Thermal drift of zero point- Thermal drift of span

- ???(guess)


13/16

Total error band

Example: A total error band: 0,5% between -10C and +60C


14/16

Calibration of a pressure transmitterBasic principle

output electrical

calibrator

Transmitter to becalibrated

Pressure calibrator

PRESSURE

GENERATOR


15/16

Calibration of a pressure transmitterPractical installation

Pressure:Bar

Output:mA

Pressuregenerator:

Manual pump

Multifunctioncalibrator

Transmitter


16/16

Thank You

Questions?

tweng - accuracy_pressure_transm - copy

Documents