Can a vacuum pump be "bad" despite pulling to proper Hg vacuum?

[james89dx]

Is it at all possible that a vacuum pump could still pull down to ~28 Hg on the manifold gauge set (as mine does) but yet somehow NOT properly vacuum/remove all the moisture in the system?

Trying to figure out why my past couple of projects are all spitting out ~60 degree vent temps for no good reason when I had no such issues in the past. My vacuum pump is probably 10 years old now.

[Tim]

Best way to know you have a proper vacuum. Is to use a Micron gauge in the ac gauge set-up. 1000 microns or less with 500 microns being the best number to achieve.

Replacing vacuum pump oil is recommended to achieve best micron number.

[james89dx]

Well, I don't have a micron gauge and looks like they are not cheap enough that I'd care to get one just as a test. I mean for the cost of a micron gauge I can just buy a whole new good quality pump.

Let's say I did have a micron gauge though, and let's say my pulls were coming out to 500 microns on the gauge - does that number in and of itself guarantee a properly working vacuum pump and that a 'deep' vacuum has been achieved if it's running for the customary hour? I have always assumed yes until just very recently questioning the effectiveness of my vacuum pump despite the gauge reading looking correct during vacuum.

Hope that makes sense.

[bohica2xo]

A Supco VG60 micron gauge is around $150.00

The dial type vacuum gage on your manifold set can read "0" at 2" of vacuum.

Your pump oil should be changed regularly.

To use a micron gauge, pull a static vacuum against only the gauge to evaluate your pump. If your pump produces 300 micron results, that is the baseline.

Next, attach the manifold set with the micron gauge in the pump line. Close the handwheels, and check again. Any leaks in your manifold will show right up.

Finally, place the micron gauge in the low side line, and evacuate the system. When it achieves a steady reading below 1500 microns, close the valve so isolate the vacuum pump & turn the pump off. Walk away. Check the system in 15 to 20 minutes. If it is still in vacuum, it has no gross leaks.

Turn the pump back on, open the valve and pull the system down below 1000 microns. Let the pump run for 15 minutes, then isolate it again & shut it down.

Watch the micron gauge for 10 minutes - well walk away and get your charging equipment ready. If the vacuum is holding, it is ready to charge.

Running a pump for hours does not mean it got the job done. Oil and elastomers in the system outgas a little under vacuum. Pulling down to 1500 microns will start that process. Waiting during that first shutdown time will let the moisture / solvents / dissolved gasses break free & raise the pressure inside the system.

[james89dx]

Why do I only ever see micron gauges used in home HVAC work?

I'm not disputing the benefits for automotive work, but I've never seen them used, and even my professional mechanic friends don't use or own them for automotive AC work. It just seems like an expensive tool for me to add just to see if my pump is bad, only to then have to shell out for a new pump anyway.

What I'm mainly looking for here is a confirmation (or not) that it's a plausible scenario for my pump to be inadequate or damaged despite the gauge indicating otherwise by standard automotive rules of thumb (i.e., 28-29 Hg vacuum). It sounds like the answer is yes given the inability of the analog gauge to display values down to microns.

Are there any other "budget" micron gauges? The Supco mentioned has horrendous reviews about inaccuracies and build quality on Amazon.

[Tim]

Why do I only ever see micron gauges used in home HVAC work?

I'm not disputing the benefits for automotive work, but I've never seen them used, and even my professional mechanic friends don't use or own them for automotive AC work. It just seems like an expensive tool for me to add just to see if my pump is bad, only to then have to shell out for a new pump anyway.

This is an incorrect way to look at the cost of proper tools IMO. You said you did 6 other jobs. The micron gauge will stay attached to your gauge set. You will know every time if you have a proper vacuum or not. Help determine leaks also without using refrigerant.

I just answered your original post on how to tell if the pump was broke. I'm sure you know how to use Goggle. Many links for micron gauges. Also, you never provided the type of vacuum pump you are using. single or dual-stage, air-operated, or electric. What size is the pump? All are factors as to how well of a vacuum you will achieve. On top of that, your gauge set could have a leak. Then again the issues you are having might not be related to a vacuum pump or gauges.

We used Micron gauges daily in our shop back in the day.

[Cusser]

At higher altitudes too, the reading will not get as low.

You might have a leak in your hoses, connections, or manifold.

I'm not an expert, but from working in a laboratory the way I understand things is that each cycle/revolution of the vacuum pump does pump out some of the molecules in the system, then more on the next revolution. At 28 vacuum one is making it easier for water to change into water vapor at ambient temperatures, and so most of the air and water vapor are removed from the system. And at filling time the vacuum will draw enough refrigerant to get the pressure high enough to get compressor to engage.

Does your gauge start a zero or positive reading?

[bohica2xo]

It just seems like an expensive tool for me to add just to see if my pump is bad, only to then have to shell out for a new pump anyway.

So how do you propose to determine if your pump is junk?

Your original post mentions 28" of vacuum. That right there says there is a problem.

The compound gauge on an average manifold set goes from 30" HG to 150 PSIG. or 164.7 PSIA. A GOOD gauge will be +/- 2% of full scale. 3.3 PSIA error on a good day. The tin can & string inside the $20 manifold set from the local china junk store is about 6%

[james89dx]

This is an incorrect way to look at the cost of proper tools IMO. You said you did 6 other jobs. The micron gauge will stay attached to your gauge set. You will know every time if you have a proper vacuum or not. Help determine leaks also without using refrigerant.

I just answered your original post on how to tell if the pump was broke. I'm sure you know how to use Goggle. Many links for micron gauges. Also, you never provided the type of vacuum pump you are using. single or dual-stage, air-operated, or electric. What size is the pump? All are factors as to how well of a vacuum you will achieve. On top of that, your gauge set could have a leak. Then again the issues you are having might not be related to a vacuum pump or gauges.

We used Micron gauges daily in our shop back in the day.

Again, my question about the micron gauge being used in automotive applications was simply curiosity, not discrediting or questioning its value.

I mean if somebody told me I needed a framing square to do automotive mechanic work I would also be asking why (out of curiosity) as I've never seen it used before in auto mechanic work and I don't own one even though I've been working on cars for 20 years now. Even doing online research about micron gauges almost every last result is home HVAC. Why aren't they widely used by mechanics? At first thought I'd say "cost" but then on the other hand that doesn't seem valid because all these guys spend $20-40k in tools and boxes just because they have "Snap On" on them, so I can't imagine them blinking an eye at a $200 tool. Plus, even the $5k+ Robinair and Bendpack and Mastercool recovery/recharge machines don't have micron gauges.

My pump is so old the motor plate identifier deal fell off years ago but I do know it's an FJC. Looks like it's probably model 6909 - rotary vane, 3 CM, 1/3 hp, single stage with gimmicky "twin port technology for performance like a 2 stage pump". Definitely nothing high end and again it's probably a good decade old. I have always achieved really good results with this and mid 40's vent temps until my last few projects the last couple of years. My gauge set is a CPS MA1234 which is pretty high quality and served me well.

I think I'll run the car to my buddy's shop and evacuate/vacuum/recharge it on their machine to see if my vent temps significantly improve.

In the meantime I'm trying to think of ways I could possibly be inadvertently adding air to the system during a vacuum/charge. My gauge set yellow/service hose has the schrader valve Y'd into the fitting at the manifold block. When I hook up my refrigerant canister and open it - with both high and low side valves still closed at the manifold gauge set after a vacuum - I have been using the schrader valve to purge the yellow line before opening the low side valve to charge. My assumption has been that this accomplishes the same thing as actually slightly unscrewing the yellow/service hose at the manifold and letting some refrigerant escape before tightening it back down. Is this correct?

At higher altitudes too, the reading will not get as low.

You might have a leak in your hoses, connections, or manifold.

I'm not an expert, but from working in a laboratory the way I understand things is that each cycle/revolution of the vacuum pump does pump out some of the molecules in the system, then more on the next revolution. At 28 vacuum one is making it easier for water to change into water vapor at ambient temperatures, and so most of the air and water vapor are removed from the system. And at filling time the vacuum will draw enough refrigerant to get the pressure high enough to get compressor to engage.

Does your gauge start a zero or positive reading?

I'm in Texas so I wouldn't think the altitude would have any impact. I calibrate my gauges to 0 with the little adjustment screw (although that's only been necessary a few times over the years).

So how do you propose to determine if your pump is junk?

Your original post mentions 28" of vacuum. That right there says there is a problem.

The compound gauge on an average manifold set goes from 30" HG to 150 PSIG. or 164.7 PSIA. A GOOD gauge will be +/- 2% of full scale. 3.3 PSIA error on a good day. The tin can & string inside the $20 manifold set from the local china junk store is about 6%

Well, to be fair, my question was not how to determine whether or not my vacuum pump is junk per se. I have no intent of going through much effort or expense to determine whether or not a decade old $100 pump is bad.

Like I said, I am going to try to vacuum/recharge with a "professional" machine at a shop hopefully this weekend and see what results I get. If significantly better, then I think that will deem my vacuum pump ineffective.

To satisfy my curiosity though let me ask it this way - suppose I have a micron gauge, and suppose it reads a steady micron reading of 500 prior to recharging. Is it at all possible that the vacuum pump could be "bad" or ineffective despite the micron reading indicating otherwise?

[Tim]

1000 or lower microns. It would indicate a working pump in my opinion.

It's okay to ask questions. All answers are simply suggestions by those that may have knowledge of repairing a system. Not all of us will have the same opinion all the time.