Automotive Air Conditioning Information Forum

rafjr64 wrote: ↑Mon Jun 17, 2024 6:34 pm Here's one for you. Prepping to charge a new Vintage Air system on a hot rod. I attached gauges to both high and low side. Pulled a vacuum for an hour. Question is, only the high side gauge would drawn down to 30hg. The low side gauge didn't move at all when applying vacuum. Is that a problem? Should both gauges give a reading or just one? Overnight, high side gauge remained at 30hg. Appreciate your help.

Should've started your own thread/topic......

Any, the only service gauges I've seen only have a vacuum scale on the low pressure gauge, the high side only goes down to zero. If at positive 30 psi, you have a leak or aren't getting enough vacuum to continue....

I did. 19 views. no replies. thank you for replying. interesting...this gauge set has it reversed;; vacuum scale is on high side...down to negative 30hg. I borrowed this from a neighbor who used it on his home AC system. are home AC and auto AC manifold gauge sets different?

In the photo, I see the vacuum scale on the low-side or left gauge, as it should appear.

However, a Manifold Gauge Set for R-134a will be more granular in its graduations and also have a corresponding refrigerant scale in degrees Fahrenheit (°F).

JohnHere wrote: ↑Tue Jun 18, 2024 6:52 pm In the photo, I see the vacuum scale on the low-side or left gauge, as it should appear.

However, a Manifold Gauge Set for R-134a will be more granular in its graduations and also have a corresponding refrigerant scale in degrees Fahrenheit (°F).

Cusser posted that image. The Amazon special I borrowed from a neighbor has -30hg on both gauges.
I just replaced both valve cores and was able to get vacuum immediately on both gauges. Lo reads -25Hg and Hi reads -30Hg after 15 minutes. Is this what I am after? It's running now. Will go out in 30 minutes see if the Lo side dropped any further. Here is the gauge set I am using ->
https://www.flickr.com/photos/19642395@ ... ed-public/

Unusual. I never saw vacuum indicated on the HS gauge before now.

It's normally the LS gauge that you should watch while evacuating a system. A reading of 25 inHg is inadequate. It should be ~29.92 inHg at or near sea level to boil off all the moisture and remove all the air.

I don't understand why both gauges are not reading the same unless something is haywire.

What kind of vacuum pump are you using?

These are the gauges:https://www.flickr.com/photos/19642395@ ... ed-public/
This is the 1/4 hp, 3cfm rotary vane pump: https://www.flickr.com/photos/19642395@ ... ed-public/
xtreme power brand - amazon. borrowed from neighbor

Probably just junky gauges, you think? Can they be calibrated?

You can calibrate or "zero" them by carefully turning the adjusting screw on each gauge's face. However, the Manifold Gauge Set pictured seems to have both gauges perfectly zeroed already. This MGS looks like an international version with the primary pressure scales in "bar" and the secondary scales in PSI, and they aren't as granular (at least not on the PSI scales) as I would prefer to see. They're still usable, though.

The vacuum pump looks good, but did you change the oil beforehand? Only specific vacuum pump oil should be used. If the oil has been sitting in the pump for an undetermined time and is contaminated from previous evacuations, the pump won't be able to draw-down a system to the desired inches of mercury or microns.