Troubleshooting a vacuum pump and gauge set.

[Old Codger]

Hi,

My basic question is: Should a vacuum pump hold a vacuum after it has been turned off or is an in-line valve required?

My next question is: Is there a troubleshooting procedure to isolate a problem with the vacuum pump and gauge set with just those two tools?

I just acquired an inexpensive 7 cfm / 100 micron vacuum pump (looks new but no label or manufacturer - kind of like Master Cool) and a 134a gauge set (aluminum body but like above - no ID).

Here are two photos but since I can't post active URLs you'll need to paste these to your browser's address line and remove the spaces between h t t p.

Label on vacuum pump: h t t p://s225.photobucket.com/albums/dd163/Keps-Photos/Heater%20-%20AC/AC%20Tools/?action=view&current=VPTag.jpg

Picture of setup that I used: h t t p://s225.photobucket.com/albums/dd163/Keps-Photos/Heater%20-%20AC/AC%20Tools/?action=view&current=VPTest.jpg

To test the vacuum pump and gauge set out I filled up the oil, hooked the yellow gauge line directly to the vacuum pump and turned it on, opened the LO gauge valve, closed the HI valve and evacuated for about 10 mins (the gauge pegged out immediately). When I turned the pump off (but left the LO gauge valve open) the "system" seemed to hold the vacuum for a couple of minutes and then began to loose it at a faster and faster rate until I was back to ambient pressure in less than a couple of minutes . (I presume that whatever was leaking was doing so immediately but since my gauges don’t show very fine changes it just didn’t read right away.) I tightened all the screw fittings tightly by hand and ran the test with the LO hose both connected to and disconnected from the “dead head” fitting on the side of the gauge set.

Sorry to bother you folks with such a basic question but I haven’t found any reference to this yet and need to determine if I should return the pump. ( I know that I have an issue with the high side snap fitting.)

Kep

[The Viking]

No, a vacuum pump will not hold the "pressure" after it been turned off, unless it got a NRV fitted.

To test your setup, or any vacuum you will require a vacuum gauge.

[Peter_1]

TheVikin, you're right but it must be a special NRV with a very low DP over it

[Old Codger]

Thanks guys, I'll plumb in a valve so I can determine if the vacuum is holding in the system.

Kep

[The Viking]

No, a vacuum pump will not hold the "pressure" after it been turned off, unless it got a NRV fitted.

To test your setup, or any vacuum you will require a vacuum gauge.
..........

[BritCit_Juve]

Vacuum pumps usually do not have any means of isolation so when the pump is turned off the vacuum will slowly be refilled. You can fit a non return valve but this can cause its own problems due to the pressure difference. Some newer models of vacuum pump are arriving on the market fitted with a solenoid valve which is de-energised on power off giving a stronger isolation of the vacuum circuit.
To properly check how the unit is working you need to use a Torr or vacuum gauge. Standard refrigerant guages don't go low enough.
Brit

[Brian_UK]

When you had evacuated the LO gauge and hose did you close the LO valve to check your guage.

Bear in mind that the hoses could be leaking.

[icecube51]

I have a vacuum pump and has it as a gift from a friend,ho was demolishing some classrooms at the university.its an alcatel and something,an was about 15 years old wen i got it. after some remodeling and testing i came to now wat it can and cant do. just added a simple valve at the intake,and wen i reach the vacuum amount i want,i just close the valve and switch of the motor. works fine for me,....for the last 15 years.
tip;once and a wile you have to open your tools ore equipment,so you understand better how its works.than clean and grease,put it nicely back together and don't bother wat any one else has to say.;)

Ice