Evacuation Procedure Electronic Vacuum Gauge

[Refrigerologist]

Refrigerologist, is there any residual refrigerant in the system? Is it an all new system?

New system, installed by me,nitrogen bled through during brazing etc. Good practice followed.

Ultimately I was just asking if anyone else had come across this phenomena. I quite understand about pressure drop throught the vaccing lines etc. It is just that in this case the vac gauge is so far away from the vac pump, and installed on a 7/8" liquid line, I just never expected to see such a drop in pressure when the pump was switched off. When I have used an analogue torr gauge I would expect to see the opposite with a gradual rise in pressure until it equalizes to the final atmosperic influence that has been left in the system.

[nike123]

New system, installed by me,nitrogen bled through during brazing etc. Good practice followed.

Ultimately I was just asking if anyone else had come across this phenomena. I quite understand about pressure drop throught the vaccing lines etc. It is just that in this case the vac gauge is so far away from the vac pump, and installed on a 7/8" liquid line, I just never expected to see such a drop in pressure when the pump was switched off. When I have used an analogue torr gauge I would expect to see the opposite with a gradual rise in pressure until it equalizes to the final atmosperic influence that has been left in the system.

Bare in mind that electronic vacuum gauge measures temperature of gases and that convert in pressures, whilst torr gauge measure pressure directly. In that facts, I think, is answer to our little mastery.

[SteinarN]

As I understand it, a thermistor vacuum gauge reads the thermal conductivity of the gasses and convert that conductivity to pressure. That thermal conductivity is dependent on the pressure of course, but also on the molecular weight of the gas. The higher the molecular weight is, the lower the thermal conductivity is. Imo, a high molecular weight gas like hfc refrigerant will cause the thermistor gauge to indicate significantly lower pressure than is the actual pressure in the system. I suppose thermistor vacuum gauges is calibrated in normal air which have low weight gas molecules.

I have found thermistor vacuum gauge to be useless on systems containing refrigerant, that is systems which have been in operation already. In such cases the thermistor gauge will indicate good vacuum (less than 1000 micron) even with several psi left in the system. At least does my expensive thermistor gauge do that. I have not seen what you describe here when vacuuming new systems without any trace of refrigerant in it.

Can it be the compressor oil releasing vapour that flows by the thermistor when the pump is stopped and is replaced by nitrogen/air when the pump is on?

[nike123]

As I understand it, a thermistor vacuum gauge reads the thermal conductivity of the gasses and convert that conductivity to pressure.

Yep, you are right here and I was give wrong information!
http://www.belljar.net/tcgauge.htm

[Refrigerologist]

As I understand it, a thermistor vacuum gauge reads the thermal conductivity of the gasses and convert that conductivity to pressure. That thermal conductivity is dependent on the pressure of course, but also on the molecular weight of the gas. The higher the molecular weight is, the lower the thermal conductivity is. Imo, a high molecular weight gas like hfc refrigerant will cause the thermistor gauge to indicate significantly lower pressure than is the actual pressure in the system. I suppose thermistor vacuum gauges is calibrated in normal air which have low weight gas molecules.

I have found thermistor vacuum gauge to be useless on systems containing refrigerant, that is systems which have been in operation already. In such cases the thermistor gauge will indicate good vacuum (less than 1000 micron) even with several psi left in the system. At least does my expensive thermistor gauge do that. I have not seen what you describe here when vacuuming new systems without any trace of refrigerant in it.

Can it be the compressor oil releasing vapour that flows by the thermistor when the pump is stopped and is replaced by nitrogen/air when the pump is on?

I believe the vac gauge I use utilises a heated thermistor (I could be wrong as I have lost the manual) and relies on the temperature drop caused by the evaporation of gases over the sensor to determine the depth of vacuum. I have not had a problem on systems that had already been in commission, the vacuum inicated appears to be correct, apart from when the sensor has needed cleaning due to oil contamination of the sensor. I use the CPS gauge and would recommend cleaning the sensor after each use on a system that has been in operation.

Anyway, maybe it is time to close the thread as the vacuum I required was achieved and so the consultant is happy and I will get paid Afterall this is the most important part of our trade