Vacuum range on HVAC gauge

[tarantino]

I am trying to pull vacuum on a newly installed minisplit. I can't get to - 25 " Hg no matter how hard I try. The vacuum holds however including when still connected to the pump.
I understand that this is not a micron gauge but not getting even close to -30?! Is it the gauge or is it the pump?
The pump is indeed a cheapo Amazon set.

 

[American Locomotive]

The vacuum reading on the gauge isn't really useful for much, honestly.

You need a micron gauge if you want to do it right.

 

[Jim greengo]

28" of vacuum is about all you'll ever get with a regular gauge.
Make sure it's at zero before you start.

 

[fitter30]

A manifold gauge to read a vacuum is like using a yard stick to measure mm. You have a leak did you pressurised the system first to 100 lbs of nitrogen let it sit for 30 minutes then 500 - 600 lbs for 8 hours or longer. Then pull vacuum with a micron gauge? Does the pump have fresh oil? Skip a step u will pay for it.

 

[tarantino]

A manifold gauge to read a vacuum is like using a yard stick to measure mm. You have a leak did you pressurised the system first to 100 lbs of nitrogen let it sit for 30 minutes then 500 - 600 lbs for 8 hours or longer. Then pull vacuum with a micron gauge? Does the pump have fresh oil? Skip a step u will pay for it.

I doubt I have a leak as the system holds the vacuum for 24h.
I get it it's not a micron gauge but the question is if the low pressure gauge is so inaccurate that it shows 23 mmHg when it's close to 30 " Hg.

 

[whateg01]

You'll never know if it's 23" or not without either measuring a known vacuum or checking the system with another gauge. At this point you are just guessing at whether you have a vacuum pump that isn't pulling a vacuum or if your gauges are wrong.

 

[tarantino]

You'll never know if it's 23" or not without either measuring a known vacuum or checking the system with another gauge. At this point you are just guessing at whether you have a vacuum pump that isn't pulling a vacuum or if your gauges are wrong.

Very correct. I am seeking advice as to which one is the more likely culprit. I wasn't sure a gauge like that is simply not very accurate (5-10% margin of error) or completely useless (25% margin) to measure vacuum.

 

[jkuro]

Pull your vacuum. Shut the pump off and let it sit overnight. If it's at the same inches of vacuum from when you shut it off your good, if it's less, you have a leak.

 

[tarantino]

Pull your vacuum. Shut the pump off and let it sit overnight. If it's at the same inches of vacuum from when you shut it off your good, if it's less, you have a leak.

I may do that. I don't want to invest in a $150 micron gauge for a single minisplit install. Especially after buying manifold gauges and a pump. I really don't think it's a leak but rather a question if the vacuum is good enough.

 

[danski0224]

Vacuum isn't good for leak testing.

 

[tarantino]

Obviously there is a perfect/professional way of doing that. However, many DIY minisplits are sold these days and the manufacturers do not require you to do pressure testing. From Mr. Cool to the ones that require line vacuuming only. My manual (Tosot) says vacuum to -0.1 MPa and make sure it holds. Doing N2 pressurizing defeats the purpose of DIY.

 

[bonneyman]

If your pump is in good condition, it will pull down to the necessary vacuum.

I have a 3CFM JB vacuum pump I bought like 30 years ago. The local warehouse had a shin-dig every spring (free food, tool vendors, demonstrations, discounts on certain tools, etc.) - you know, all the enticements to get techs to show up. And they also had a "vacuum pump clinic". They'd drain and refill your oil, and test what vacuum it would pull with their fancy electronic gauge. (Hoping to find all the ones that sucked poorly so they could sell you a new one). Mine pulled down to almost 28" within a minute or so. Guy commented my pump must be pretty new to do that. You should have seen the look on his face when I told him I bought it like 20 years earlier! He confirmed the serial number - and was just shocked. I told him that's what happens when you take care of your stuff.

I'd run the pump 30 minutes on an evacuation, and the thing would start this strange pulsating noise after awhile. Got used to it - that told me the vacuum was getting deep. (Of course this was way back before the prevalence of electronic digital equiptment). If it got to 28" and held it for 10 minutes I'd say it was good to go. Maybe 3 or 4 times over my career I had an actual small leak when I thought it was tight. I started pressure testing early on - never had an issue after that.

 

[danski0224]

Obviously there is a perfect/professional way of doing that. However, many DIY minisplits are sold these days and the manufacturers do not require you to do pressure testing. From Mr. Cool to the ones that require line vacuuming only. My manual (Tosot) says vacuum to -0.1 MPa and make sure it holds. Doing N2 pressurizing defeats the purpose of DIY.

That's because the manufacturer wants to sell boxes.

And they don't care if it leaks.

The only useful vacuum gauge is a micron gauge. There's like 17,000 microns in that last inch of mercury, IIRC.

 

[tarantino]

That's because the manufacturer wants to sell boxes.

And they don't care if it leaks.

The only useful vacuum gauge is a micron gauge. There's like 17,000 microns in that last inch of mercury, IIRC.

25,000+. Being 1" = 25.4 mm.
It's a trade-off like many other things. $700 with self-install and higher risk of failure vs $3,000 or more for a professional install and some assurance. It's a fairly simple proposition.

 

[American Locomotive]

I'm really not sure how "N2 pressurizing defeats the purpose of DIY". You can rent a bottle of nitrogen for like $100 from a welding supply store, and a regulator is like $70. You don't need any special licenses.

Anyways, what we're ultimately getting at, is that it doesn't even matter if your manifold's low pressure gauge is faulty. Getting another manifold gauge to compare it against is pointless, because it's either way its the wrong tool for the job. You can get a basic TPI 605 micron gauge for about $110. It's the only way to truly tell if your vacuum is good enough.

When you're done, put the gauge on eBay for $80 and get most of your money back. That simple.

 

[engineer2]

The old standard is less than 500 microns before you recharge. I just did my car with new parts and was able to pull 200 microns but it took overnight. Dad was a refrigeration engineer and he said to pull it down overnight if you can to get the moisture out. It was 500 when I went to bed and 200 in the morning. I have a cheap Supco Y4 micron gauge with the LEDs. Nowadays I would get a digital one. Flush the gauge with alcohol before every use for best accuracy.

 

[BillK]

Is it the gauge or is it the pump?
The pump is indeed a cheapo Amazon set.

Hook the gauge directly to the pump and see how much vacuum it pulls. Use that as your baseline.

 

[tarantino]

Hook the gauge directly to the pump and see how much vacuum it pulls. Use that as your baseline.

Well, I did do that. It shows 23 "Hg or so but unfortunately the dilemma remains-i s it the gauge or is it the pump.

 

[danski0224]

Well, I did do that. It shows 23 "Hg or so but unfortunately the dilemma remains-i s it the gauge or is it the pump.

Without a micron gauge, you won't know.



Oh, and the standard analog compound HVAC gauge is typically +/- 2% accuracy, at half scale.

 

[BillK]

Well, I did do that. It shows 23 "Hg or so but unfortunately the dilemma remains-i s it the gauge or is it the pump.

Doesnt really matter for what you are doing. If the pump can only pull to 23 with your gauge then if you can vac the system down to 23 with your pump and gauge then you are good to go.

The only way to know for sure if it is the pump or the gauge is to get a known good calibrated gauge to test with.

 

[tarantino]

Without a micron gauge, you won't know.



Oh, and the standard analog compound HVAC gauge is typically +/- 2% accuracy, at half scale.

So I can't explain my findings purely by the poor resolution of the gauge. SOmething is faulty.

 

[tarantino]

Doesnt really matter for what you are doing. If the pump can only pull to 23 with your gauge then if you can vac the system down to 23 with your pump and gauge then you are good to go.

The only way to know for sure if it is the pump or the gauge is to get a known good calibrated gauge to test with.

OK but if I only get down to 23 and the number is accurate, that's too low by all standards., isn't it?

 

[Dagny]

There is always a triple evacuation.

 

[BillK]

OK but if I only get down to 23 and the number is accurate, that's too low by all standards., isn't it?

Not sure what you are trying to say. If all you are doing is checking the system for leaks then the actual amount of vacuum is not that important within reason. Put a vacuum on it, shut the valve and let it sit. If it holds vacuum than it is ok. If it doesn't then you need to find the leak.

 

[tarantino]

Not sure what you are trying to say. If all you are doing is checking the system for leaks then the actual amount of vacuum is not that important within reason. Put a vacuum on it, shut the valve and let it sit. If it holds vacuum than it is ok. If it doesn't then you need to find the leak.

I am evacuating that line set prior to releasing the refrigerant. It’s not just about checking for leaks.

 

[tarantino]

There is always a triple evacuation.

That involves n2 flushes, right?

 

[BillK]

I am evacuating that line set prior to releasing the refrigerant. It’s not just about checking for leaks.

Then I guess the only way you will know for sure is to find somebody with a known good pump and gauge and test yours to see which is wrong ? Cant help you much more than that.

 

[mcbane]

I am trying to pull vacuum on a newly installed minisplit. I can't get to - 25 " Hg no matter how hard I try. The vacuum holds however including when still connected to the pump.
I understand that this is not a micron gauge but not getting even close to -30?! Is it the gauge or is it the pump?
The pump is indeed a cheapo Amazon set.

You dont state your location, so this may or may not be pertinent: the typical analog gauge set is reading pressures relative to ambient. So while a full vacuum can be indicated at sea level, that same vacuum will read around 26" Hg at 5000 ft.

Micron gauges read absolute pressure and will reliably indicate vacuum regardless of elevation.

 

[rlitman]

You dont state your location, so this may or may not be pertinent: the typical analog gauge set is reading pressures relative to ambient. So while a full vacuum can be indicated at sea level, that same vacuum will read around 26" Hg at 5000 ft.

Micron gauges read absolute pressure and will reliably indicate vacuum regardless of elevation.

That's assuming you zero out the gauges.

Anyway, from what I've seen, the vacuum side of a pressure/vacuum gauge is never all that accurate. It may be +/-2% at mid-scale, but vacuum is likely +/-10% or worse.

 

[TurnipTruck]

I have a cheesy vacuum pump that can only **** down to 23” Hg, but have access to a lab vac pump that can get down to 4 torr.
Industrially, any gauge 2-1/2” in diameter or less is not worth repairing and gets replaced if questionable.

 

[tarantino]

You dont state your location, so this may or may not be pertinent: the typical analog gauge set is reading pressures relative to ambient. So while a full vacuum can be indicated at sea level, that same vacuum will read around 26" Hg at 5000 ft.

Micron gauges read absolute pressure and will reliably indicate vacuum regardless of elevation.

NYS, less than 200 ft.

 

[American Locomotive]

No matter what, you're not getting out of this without spending $100. Either a new $100 vacuum pump, $100 manifold gauge, or a $100 micron gauge.

You might as well just get the correct tool for the job, and not worry about it.

 

[tarantino]

No matter what, you're not getting out of this without spending $100. Either a new $100 vacuum pump, $100 manifold gauge, or a $100 micron gauge.

You might as well just get the correct tool for the job, and not worry about it.

Can you recommend a micron gauge?

 

[American Locomotive]

TPI 605 is basically the cheapest option and works well.

 

[rlitman]

TPI 605 is basically the cheapest option and works well.

I have a CPS VG200. Currently $152 https://www.amazon.com/gp/product/B009AXGCSC/?tag=atomicindus08-20

The TPI is a little cheaper, but I know the CPS has a cleanable sensor.

 

[tarantino]

THe TPI605 has a barber fitting. It that OK?

NVM: Not necessarily the case.

 

[mbunimog]

Tested the vacuum pump before using and was able to get down to 8 microns

 

[American Locomotive]

THe TPI605 has a barber fitting. It that OK?

NVM: Not necessarily the case.

It has a male fitting. You can either just attach it with a hose, or get a double female adapter.

 

[danski0224]

I would borrow at least 2 more sets of gauges, take the reading from the 3 of them and average it.

That might get you close.

Or not.

Might need a 4th.

 

[Dagny]

I could install a refrigeration system with no gauges or micron gauge.I would not do it without a vacumme pump. I have 5 sets of 4 valve manifolds in my truck and 3 different micron gauges. When I get done with a solder joint I look at it all the way around with a mirror and a small flashlight. I know there are no leaks when I start evacuating. The very best method of making sure a system has nothing but refrigerant and oil in it is triple evacuation but it is unlawful. Forget what the gauge says pull a vacumme for at least an hour then break the vac. with the refrigerant that the system uses till you have 4,5 psi you can leave it like this as long as you like then do it 2 more times you will have a very clean system with very very little non condensibles.