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How to Calculate Transmitter Performance and Calibration Frequency?

The overall performance of the transmitter is defined by the Total Probable Error (TPE).

TPE is the accuracy of the instrument in installed/actual conditions.

Transmitter Calibration Frequency

Transmitter Performance and Calibration Frequency

TPE is made up of four things:

1. Reference accuracy

This is to be calculated at calibrated Span.

2. Ambient temperature effects

Transmitters are factory calibrated at room temperature. If a transmitter operates at a different ambient temperature, the electronics perform differently, and a measurement error may result.)

3. Line Pressure Effects

Line pressure effect errors occur when diaphragms are altered under static pressure. Normally Manufacturers will define the performance affected due to every 1000 PSIG pressure changes.

There is zero and span effect due to static Line pressure. some vendor claims that zero effect can be eliminated by Zero Trim when it is performed under static pressure condition.

4. Some manufacturer defines Transmitter performance is affected by Overpressure up to 250 bar.

If the performance does not meet the minimum requirements, then the transmitter might not be a good fit for the application.

The below sample calculation performed to arrive Total probable error (0.1%FS) in the transmitter which is around 2.5 times degraded than reference accuracy (0.04% FS).

Another important term, Stability, is a transmitter’s change in output given at fixed input as a function of time. Better long-term stability results in the reduction of periodic Calibration /maintenance and repairs,  which leads to the reduced total cost of ownership.

Case study on Transmitter Accuracy and its stability which will ultimately guide to required calibration frequency.

The intention of this study is not to demonstrate which brand is better, but to educate the fundamental calculation, transmitter performance, and ultimately calibration frequency.

Example:

We use the below specifications for three different make transmitters to calculate their overall performance and calibration frequency interval.

  • Differential Pressure Range: 0-100 inH2O
  • Ambient Temperature range: 40 to 90 oF, Delta is 50 oF
  • Line Static Pressure: 1000 PSIG
  • Acceptable performance: 0.3% of Span

We have to calculate five factors to determine transmitters’ performance, they are.

1. Reference Accuracy of Calibrated Span (E1)

2. Ambient Temperature Effects on Transmitter (E2)

3. Static Pressure Effects (E3)

4. OverPressure Effects (E4)

5. Stability (E5)

So, we have to calculate these five factors for each transmitter make then only we can find their performance.

Yokogawa Transmitter [EJX110A]

Yokogawa Transmitter Span and Range

Calibrated Span 100 inH2O

URL is 400 inH2O

Reference Accuracy of Calibrated Span

Reference Accuracy of Calibrated Span

E1 = 0.04*100%

E1 = 0.04 inH2O

Ambient Temperature Effects on Transmitter

Ambient Temperature Effects on Transmitter

E2 = ± (0.04*100% + 0.009*400%)

E2 = ± 0.076 inH2O

Static Pressure Effects

Static Pressure Effects on Transmitter

E3 = ± 0.075*100%

E3 = ± 0.075 inH2O

OverPressure Effects

OverPressure Effects

E5 = ± 0.03*400%

E5 = ± 0.12 inH2O

Stability

Stability Factor of Transmitter

Stability per month = 0.1%*400/(15*12)

Stability per month = ± 0.002222 inH2O

Stability per month = ± 0.002222 %of Span

Total Probable Error (TPE)

Total Probable Error

TPE = SQRT((0.04)^2+(0.076)^2+(0.075)^2)

TPE = 0.114022 inH2O

TPE = 0.114022 %of Span

Zero Error on static line pressure effect is not included.

Calibration Frequency

Calibration frequency

Calibration frequency = (0.3 – 0.114022) / 0.002222

Calibration frequency = 83.69847 Months

Calibration frequency = 6.974872 Years

Total Accuracy

Total Accuracy of Transmitter

TA = SQRT((0.04)^2+(0.076)^2+(0.075+0.08)^2+(0.12)^2)

TA = 0.214012 inH2O

TA = 0.214012 %of Span

Zero Error on static line pressure effect & Overpressure effects is included.

Similarly, calculate these factors for another two different transmitters.

Emerson Transmitter [3051CD]

Emerson Transmitter Range

Calibrated Span 100 inH2O

URL is 250 inH2O

Measurement Span of Emerson Transmitter

E1 = 0.04 inH2O

E1 = 0.04*100%

Ambient Temperature Effects of Emerson Transmitter

E2 = 0.0125*250% + 0.0625*100%

E2 = 0.09375 inH2O

Line Pressure Effects on Emerson Transmitter

E3 = 0.1*100%

E3 = 0.1 inH2O

E4 = 0.05*250%

E4 = 0.125 inH2O

Long Term Stability of Emerson Transmitter

Stability per month  = 0.2%*250/(10*12)

Stability per month  = 0.004167 inH2O

Stability per month  = 0.004167 %of Span

TPE of Emerson Transmitter

TPE = SQRT((0.04)^2+(0.09375)^2+(0.1)^2)

TPE = 0.14279 inH2O

TPE = 0.14279 %of Span

Zero Error on static line pressure effect is not included.

Calibration frequency = (0.3 – 0.14279) / 0.004167

Calibration frequency = 37.72738 Months

Calibration frequency = 3.143948 Years

There is no data available for an Overpressure effect on Transmitter performance.

3051S model has better accuracy and stability than 3051 Models

Honeywell Transmitter [STD800]

Honeywell Transmitter Range

Calibrated Span 100 inH2O

URL is 400 inH2O

Honeywell Transmitter Accuracy

E1 = 0.0375*100%

E1 = 0.0375 inH2O

Honeywell Transmitter Temperature and Pressure Effects

E2 = (0.025 + 0.007(400/100))

E2 = 0.053 inH2O

E3 = (0.08 + 0.007(400/100))

E3 = 0.108 inH2O

Honeywell Transmitter Stability

Stability per month = 0.01%*400/(15*12)

Stability per month = 0.000222 inH2O

Stability per month = 0.000222 %of Span

Honeywell Transmitter Performance

TPE = SQRT((0.0375)^2+(0.053)^2+(0.108)^2)

TPE = 0.126013 inH2O

TPE = 0.126013 %of Span

It seems that Zero Error on static line pressure effect is not included.

Calibration frequency = (0.3 – 0.126) / 0.0002

Calibration frequency = 870 Months

Calibration frequency = 72.5 Years

Recommended frequency 4 Years

There is no data available for Overpressure effect on Transmitter performance

STD700 model has degraded accuracy and stability than STD800 Models

STD700 series model recommended calibration frequency is 2 Years

Conclusion:

We determined the transmitter performance factors for different makes, and we can choose based on the results.

Author: Jatin Katrodiya

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How to Calculate Transmitter Performance and Calibration Frequency?

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