What kinda system are you working on and what is your evacuation setup? Are you pulling through a manifold or do you have an actual vacuum set up.

 

I've had issues with filter driers holding on to moisture or refrigerant and causing evacuations to take a lot longer. It could be slowly off gassing and raising your pressure while it's in a vacuum but you wouldn't see anything while it's pressurized. I would definitely change the driers and probably the oil too if the system had been open a while, especially if it's POE or another synthetic.

 

So unfortunately I can't divulge much about the system due the nature of what it is. What I can share is that no manifold is being used and vacuum and filling is being accessed via a core removal tool. This system does not include any oil since the refrigerant circulation method is 'non-traditional' and does not use a compressor.

We've been pulling vacuum down to the ~60 micron level for probably 24+ combined hours now, spread over a ~6 day period and we're still seeing the same issue. Honestly, due to the intended use and expected life of the system, I'm pretty close to just filling the darn thing with refrigerant. Not being an expert in the space, my limited understanding is that there can be a number of factors that can contribute to a rising vacuum pressure. I believe we've ruled out the presence of a leak due to the positive pressure testing, so I'm curious if anyone has experienced any 'gotcha' situations where a positive pressure test over a time scale of significance, yielded a false positive leak pass/fail result that a vacuum test was able to catch.

 

So..... you can actually pull a vacuum to fast....

It's rare but if you pull a vacuum to fast you can cause ice to form.

Your microns will rise as the ice melts.

[video=youtube;WNgoOp8T7RQ]https://www.youtube.com/watch?v=WNgoOp8T7RQ[/video]

 

“>60000 microns”. ? Not water or moisture at that level.
If that high, my guess would be a shaft seal or relief valve. System model & type would help……

 

Based on what hes talking this is something at a college or company.

non-compressor based system that still uses refrigerent.

Im guessing something using convection. There was a ton of noise around convection based CPU cooling a few years ago.

Thinking something like this....

Getting below 500 micro in 60 seconds indicates a very small volume to.

[video=youtube;R3RwoQt3m7M]https://www.youtube.com/watch?v=R3RwoQt3m7M[/video]

 

I would verify your “set-up” is not leaking.

Maybe take a pice of 3/4 tubing 10ft long, braze both ends closed, add a service port. Pull that down and make sure it will hold.

I’ve fought the same battle you are dealing with. Interested to hear if you get it resolved. We replace the little rubber thingys in the hoses as they leak under vacuum but not positive pressure sometimes.

 

For doing a long decay test you need to actually get all of the moisture out of the system.
It's virtually impossible to just vacuum to 500 microns and blank it off for a day without having an extremely deep vacuum pulled in the first place.
I have performed long blank off tests. Most of the time in order to legitimately pass the test my pump would need to pull that vacuum for days before blanking off.
So a legitimate 125 micron reading at blank off time would typically read 350 microns after 72 hours.
There is no such thing as a quick vacuum for a blank off test longer than 30 minutes

 

As stated, is this a small volume system?

If so, that could be the issue in itself.

 

I have seen this problem once. It was a pop off. Would seal under pressure, not so much under vacuum.
And a poorly brazed connection can seal under pressure, but leak under vacuum.

Sent from my phone, using that talk thingie.

 

I’ve never saw that, how does that happen? The braze hold under pressure but leak when not?

 

Just think about what happens to copper joints under stress. From vacuum to full pressure...and the expansion and contractions they go through

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You are doing something wrong! if you measure pressure this will vary with ambient temperature. You can't have it CONST!
By the gas equation pV = nRT your pressure will vary over the course of the day, as T does!

In contrast if you see vacuum decay you HAVE a leak!

 

Have you checked your hoses and core tool. Try a little nylog. Most usually my vacuum leaks are at my added connections and not my systems.

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Hmm well single access port is going to make it pretty hard to say. I have seen Schrader leak vac that would hold pressure but sounds like you have none.

Very low volume systems are hard to micron test because the tiny bit of leakage makes a huge impact.

Core removal tool might be a cause nylon the O rings, the vac gage itself, nylon it's o ring.

Sorry if it's insulting but if your team is qualified enough to be developing such a system they should be able to handle such a problem with ease.

 

Nylog not nylon

 

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The exact same leak will show quicker as a change in pressure under deep vacuum than it will at 100 psig pressure. The pressure difference is lower for the the deep vacuum but the number of molecules passing through that will show a change in pressure will be much smaller for the evacuated system. The micron guage has a significantly greater resolution as well.

 

Couple of things I would also consider. Hoses will outgas in a vacuum, even the non permeable type. On a small system, the pressure rise is much faster. Also, most core tools utilize a ball valve, you have to half cock the valve during pull down to evacuate the clearance space between the seals. Many core tools also have rubber gaskets, which can outgas. When we rebuild ultralow freezers, we braze on to the process tubes with 1/4" copper, flare the other end and connect that to our manifold. We even build our pump line with 3/8" copper, no hoses or gaskets, all brazed and metal to metal seals. Don't forget to put a small amount of oil on the flare sealing surface, the oil actually makes the seal. Once the system has been evacutad and passes (we have to hold under 200 microns for 24 hours, requires a triple evac to do it), we charge the system. We use 2 pinch off tools, cut the process tube, braze the stub over, release the pinch offs and fill the pinches with Staybrite 8 solder for strength. Be very careful with the torch, you are heating up tubing on a live system.

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hi, we have installed a little 7 kW mitsubishi split ductless unit r22 with a 1/4 and 1/2 inch pipe. there is only 4 flare nuts which we've manufacturer torque tightened. I am new to refrigeration but have passed a pressure test at 250 psi for 24hrs which holds on a testo digital gauge and a brand new testo micron gauge too.
When we try to vac down it goes down very quickly to under 200 microns, but as soon as I cut it off it doesn't hold and just keeps creeping up super slowly until its over 25k and unreadable. it only has one service valve.

I've tried putting nylog on every gasket/ i've opened and closed the ball valves several times during evacuation. We've tightened the service valves in case there a leak from the pregassed outdoor unit.. Help and ideas?

 

George- you need to create your own thread.

 

R-22 unit, has this been in service already?

 

New old stock 😃

 

We have installed a brand new little 7 kW Mitsubishi split ductless unit r32 (sorry i made a mistake earlier - not r22) with a 1/4 and 1/2 inch pipe. There are only 4 flare nuts which we've tightened to manufacturer recommended torque settings.

It passed a pressure test at 250 psi for 24hrs which holds on a testo 557 digital gauge and a brand new testo 552 testo micron gauge too.

I’ve now put it up to the max manufacturer max allowable pressure 45 bar (650 psi) for the indoor unit (35.2 bar x 1.3) overnight and it was holding when I left. I’ve tried two different types of professional bubble liquid over the manifolds, pipes, and anything that may leak can’t see a bubble anywhere.

I’ve also put trace gas (5% Helium) in and used an electronic sniffer but can’t find any obvious leaks (this is my first time sniffing but I’ve watched Youtube videos on it and seems pretty straightforward).

When we try to vac down it goes down very quickly to under 100 microns, I leave the vac pump running, then isolate past the micron gauge then turn the pump off. it doesn't hold and just keeps creeping one micron every 1 seconds or so slowly until its over 25k and unreadable. Mitsubishi only recommend going down to 500 microns and holding for 2 minutes. I have just bought a new javac vac gauge just to rule this out as a problem and will try this tomorrow. .


I've tried putting nylog on every gasket/ i've opened and closed the ball valves (on the hoses and Shrader valve removal tools) several times during evacuation in order to clear out any gas stuck in the valve. All the equipment is new and we've tightened the service valves in case there a leak from the pregassed outdoor unit.


As this is my first instal I’m sure I’ve made a mistake somewhere. I’ve even completely replaced all the copper with brand new just to rule out a micro leak in the pipe work.

On reading the forums my thoughts are now that we don’t have a leak, but have residual moisture in the system that I can’t get out and that is causing the slow micron rise. I read that for this issue you could try a ‘'triple evacuation' by flowing OFN through to purge, but there is only one service valve (confirmed by Mitsubishi) so there is no where for the OFN to exit on the liquid line. I have vacuumed down to 200 microns then flushed it with 10 bar of OFN three times to try and clear out any moisture. It still creeps up about 1 micron per second.

I have previously put another service valve on another unit to recover refrigerant (one that drives a pin into the copper to make a hole) but I didn’t want to do this as its another point of potential leak and may invalidate the warranty. If I was to do this I could also put the micron gauge here as it’s the furthest from the vac pump.. is this a good idea? I’m assuming no as Mitsubishi would have put on one if it was needed.

As it stands i've got the vac gauge isolated right next to the service valve and when I run the vac test everything else is isolated but it still rises.


The options left I can see are;

a) to fill the lines with a little recovered refrigerant from another unit and try the sniffer (although I’m sure the trace gas should pick this up and it is holding at pressure)
b) try injecting die and looking for a leak. I just bought a spectroline ez-jest fluorescent dye SPE-EZ5e in 15ml or 0.5 oz cartridges (my engineer recommended against this)….
c) assume the leak is in the brand new indoor unit itself – I keep thinking it must be operator error though!
d) just risk it and open the service valve to fill the unit and let it work. My reasoning is that the pressure is from the inside of the pipes out, and as it has passed that test (but failed the vac decay test) it should be ok operationally. However I really want to pass the vac test as this is my first instal and I have two more units to instal following this.


Any ideas please?
Hi apologies for no new thread – I’ve tried to find where to start one but can’t find it anywhere? i think i need 7 posts before I can do that. I am new to refrigeration but intend to instal lots of splits like this at a commercial site so want to get the fundamentals correct.

 

100 psi pressure hold for 24 hours can be reasonably well held to indicate a leak-free system for all practical purposes.

If the same system cannot maintain vacuum - either something external to the system is leaking into the system or something internal to the system is contributing to the gain-in-pressure / loss-of-vacuum.

PHM
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thanks PHM.
It's held pressure as you say for 24 hours but the vac test always rises despite triple evacuating. I will vac down to 200 microns and then open the service valve to gas it up from the external unit quickly before the vac rises then discconect everything?

 

I'm (obviously <g>) somewhat vague on the application and the criticality of it features and operation - but if the system is fairly standard overall I would add some leak detecting dye, charge and operate the system, and deal with anything - if it ever happens.

What is the longest period of time that the system was left under low vacuum with the vacuum pumps running? Hours? Days? Weeks?

PHM
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Just something to think about here. I have no idea how long the lines are for this system or where the access port is but . . .

I recently put in a 2 head ductless mini and installed 4 ball valves with access ports. I put my vaccum hoses on the large line ports and the micron gauge on the longest small line port. They required a deep vaccum held (blank off) for 1 hour after it got below 500 microns. What I noticed with this set up was that when I blanked off the VP the microns on the gauge would continue to go down after the blank off. That is because as I understand it the vacuum is deeper where the vacuum lines attach and higher at the furthest point which is where my gauge was.

Where you only have one port your gauge has to be on a CRT or pump manifold so close to the pump. The furthest point may not be any where close to as low as you think it is especially with an oversized pump for the system.

 

• There is moisture or some other contaminant in the system
• A system component is off-gassing under vacuum
• Vacuum rig is leaking between system and point of isolation
• Component of vacuum rig is off-gassing between system and point of isolation
• Faulty micron gauge - diode has been exposed to high pressure or contaminants