compound refrigerant gauge calibration best suggested method

[Kathleen]

I would like comments on the subjet:
We have an intern discussion at the office on the reliability of our compound gauges calibration between two methods use for refrigeration gauge calibration.
The first method is to first use the atmospheric reference, adjust the needle using the adjustment screw, and then take a room temperature cylinder of R134a (containing two phases fluids: gas and liquid) and use the calibrated gauge to verify if the pressure of the cylinder correspond to the chart pressure-temperature of the R134a
The second method is to use a “death weight” calibration tool.
I think to first one is quite reliable, because it do not need calibration. The death weight should be calibrated, to maintain reliability.
Please comments?

Best regards

[Brian_UK]

The dead weight is the best.

The cylinder test introduces another variable which needs calibration namely the means of measuring the refrigerant temperature accurately.

[Kathleen]

:) even if you are not sure, how often the dead weight was calibrated…the one used is quite old. Would you still trust the dead weight;)

[Brian_UK]

Yes, it is generally a requirement of 'testing' that the test instrument has a current and valid calibration certificate.

If this is not maintained then calibration should be outsourced.

Saying that your tester is 'old' does not qualify how accurate it is. It is also not an excuse for sloppy workmanship.

[Kathleen]

I wonder (as a contractor) if good refrigeration servicing and contracting firm are generally using the death weight, or they trust the cylinder testing for calibrating there gauge. To insure good test our death weight tool should be in control environment room, and maintain. Outsourcing the gauge calibration can be expansive.
I also know that the cylinder should be at least in a temperature control room.

I do not want to be sloppy :rolleyes:

[Brian_UK]

To be practical the refrigerant bottle calibration should be good enough when you consider that the newly calibrated manifold gauge set is going to be bouncing around in the back of the service technicians van for hours/days/weeks before it is used. What accuracy then?

A calibration certificate is only honestly valid until the equipment is disconnected from the tester.

What are the calibration requirements in your state/country?

[RANGER1]

:) even if you are not sure, how often the dead weight was calibrated…the one used is quite old. Would you still trust the dead weight;)

I would trust dead weight measurement.
We use a calibrated, certified MASTER gauge to test through the range of our test guages on a manifold, then log to keep with the gauge showing error, if any, as well as a log book for QA
Master gauge kept in workshop & checked every 5 years, or whatever you set up in QA.
You might pick certain pressures that have to be very accurate for high pressure switch settings, freezing points etc.
Digital gauges are very accurate, with less error, which a lot of techs would have already with so many different refrigerants out there.
Measuring bottle of refrigerant would not be accurate in my opinion as depends on how accurate thermometer is, changing ambient temperature & only one pressure point, not through range of gauge for overall calibration.

[Kathleen]

"What are the calibration requirements in your state/country?"

There is no specify standards in our province for calibration equipment for refrigeration contractor, but we have to mention on our contractor quality plan, how we insure our starting pressure test are done correctly according to the Canadian refrigeration code. The refrigeration Canadian standard mostly base on ashrae standard 15 and 34. Our standard only specify the pressure setting (high/low side) from different refrigerant and time laps, but do not refer to calibration procedure or procedure standard.
But the legislation want us to mention how we insure our test are perform, so in a part of the quality plan we identify that we calibrate the gauge from the office before performing our start up pressure test. If we go for the cylinder I have to annotate my control quality plan.

[al]

If you are pressure testing then i would recommend seperate gauges purely for pressure testing, calibrated in a lab annually, this way you can sign off on leak and pressure tests with confidence, these could also be used to set up pressure switches in the workshop. Other wise as brian says, manifold gauges will drift with time and use.