Calculating the accuracy of industrial instrumentation loops?

[tbakos]

I need to determine the accuracy of some instrumentation loops (purity, flow, pressure, and temperature). My plan is to use the spec sheets to find the accuracy of each component in the loop and simply add them to get the total accuracy(inaccuracy). I was also going to assume no inaccuracies due to the installation. Is this valid?

Thanks and Regards.

 

[nbucska]

No.
You may have noise between the sensor and the amplifier.

A loop with an integrator zeroes out the static errors
but not the dinamic ones.

The whole control loop has to be analyzed.




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[IRstuff]

Adding results in the worst case.

Typically, RSS is used, i.e., root of sum of squares.

More precise approach is to develop the entire transfer function and use the partial derivatives as coefficients in the RSS.

TTFN

FAQ731-376

 

[dcasto]

Square root sum of the square as I learned it or RSS. Other wise measure it directly with a calibrator front the primary to rhe end devices. Put a NIST input to the first device and record what the last device reports over a range.

 

[HEC]

One to rember that may be significant, is the analogue to "digital" conversion in the PLC or other indicating/recording device. Analogue inputs are generally limited to 14 bit words i.e. 4095 count, hence a field range of 0-500 will have a maximum of +/-0.1 accuracy.

Mark Hutton

 

[jmw]

I think there is also a standard on sample loops like IP54 or IP55.... this makes recommendations about cross checking with lab instruments for example... (some time since I saw these docs so I might be wrong).
Note that most manufacturers declare the instrument accuracy as based on calibration in their own laboratories under reference conditions and may not perform with the same accuracy under process conditions.

JMW

http://www.ViscoAnalyser.com

 

[JLSeagull]

Inaccuracies normally exist due to installation issues.

Orifice flanges require a precise distance between flange taps. Use the wrong gasket and the distance is wrong. Head type meters require a fully developed flow profile. This is usually accommodated by adequate upstream and downstream straight pipe per standards. This may not eliminate swirl.

Impulse lines change the temperature of the fluid. Pressure instruments in gas service are sometimes mounted below the tap and collect condensate. The condensate provides extra pressure. This is significant if measuring 10 psig and less significant if measuring 1000 psig.