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Setting accuracy limit for a read out

#1
Hi all,

we always calibrate the whole measuring chain (logic, we think)

But for a new process we need very dry air with a dew point of -60 °C
All the suppliers of measurement systems for this kind of dry air only calibrate the sensor and its mA output.
They claim that a readout never drift !

How do we set the limit for the readout that we will be calibration using a mA loop calibrator?
(We normally only use those for trouble shooting)

Do we split the entire tolerance in two parts, half of the tolerance for the sensor and half tolerance for the the readout?
Or
Do we use the output that is written on the certificates and supply that to the readout
Than check the readout display against the reference value that was applied to the sensor?
 
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#2
They claim that a readout never drift !
Hello Johanna,
What type of "readout" are you working with?
If it is a PLC with a ROM lookup table...it doesn't drift. If it's an electronic display with a conversion algorithm...it doesn't drift.
It can break, but the ROM data won't drift.

I work(ed) a lot with thermocouples, tied to a readout for either monitoring or control (single display, PID or PLC based).
We attributed ALL errors to the thermocouple, since the readout is simply a computer translating mV to a degree value based on a ROM lookup table.

My guess is that your sensor and display work the same way (but I used the word "guess", so don't stop there...)

HTH
 

dwperron

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#3
All the suppliers of measurement systems for this kind of dry air only calibrate the sensor and its mA output.
They claim that a readout never drift !


The readouts do not usually drift, but they can and should be calibrated.
The probes for low dewpoint sensors definitely do drift! One thing you need to be sure off is that when you get the sensor recalibrated that they give you the as found measurements. There is a really good chance that it will be found to be out of tolerance at one of the extremes of its range.
 
#4
If it is a PLC with a ROM lookup table...it doesn't drift. If it's an electronic display with a conversion algorithm...it doesn't drift.
It can break, but the ROM data won't drift.
The LUT indeed is part of the ROM and wont change.
but the input cards (the electronics that are reading the signal) can drift.


I work(ed) a lot with thermocouples, tied to a readout for either monitoring or control (single display, PID or PLC based).
We attributed ALL errors to the thermocouple, since the readout is simply a computer translating mV to a degree value based on a ROM lookup table.
In this case the mV input card can show drift.
But compared to the drift expected from thermocouples, it is indeed a small factor.

The probes for low dewpoint sensors definitely do drift! There is a really good chance that it will be found to be out of tolerance at one of the extremes of its range.
We received from one sensor manufacturer a white paper claiming a drift lower than 3 °C dp over 12 months.
We will anticipate by drying the air to a level lower than -65 °C dp and start with a calibration interval of 6 months


One thing you need to be sure off is that when you get the sensor recalibrated that they give you the as found measurements.
As found and as left measurements are mandatory in our quality manual.
We consider it the only correct way of calibration.

The readouts do not usually drift, but they can and should be calibrated.
Do you split the tolerance between the sensor and the readout?
Or
Do you use the outputs mentioned on the certificate to simulate the readout and check if the complete loop is in tolerance?

Both approaches should have pros and cons,
but we have no practical experience in breaking the loop in several part for calibration (only for troubleshooting purposes)
 

dwperron

Trusted Information Resource
#5
Do you split the tolerance between the sensor and the readout?
Or
Do you use the outputs mentioned on the certificate to simulate the readout and check if the complete loop is in tolerance?

Both approaches should have pros and cons,
but we have no practical experience in breaking the loop in several part for calibration (only for troubleshooting purposes)


I have handled these two ways.
Normally I just send the sensor off for calibration and use a process calibrator to calibrate the readout. The sensor calibration results will expect that you are using a calibrated instrument as a readout, and you are correct that errors in the readout are typically very small compared to the sensor.

if the sensor and readout are being calibrated as a set I would typically combine their tolerances using the RSS method.
 
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