-
vacuum reduces when vac pump is isolated
Whilst evacuating a small hermetic system, consisting of a 1 hp condensing unit with hermetic compressor, 20 feet of tubing, an evaporator with txv and accumulator, I noticed the following: At a pressure of 2000 microns, when isolating the vacuum pump, which was being used through a manifold set, the pressure on the system reduced by over 200 microns. Initially the vacuum pump was connected to the suction and liquid valves at the exit of the condensing unit, ie after the liquid receiver and before the suction stub on the compressor. The micron guage was connected to the receiver valve which had been closed to make it the furthest point from the pump. After noticing this phenomenon several times, I used the vac pump in several different ways to try to determine the cause; I was unable to. What was even more curious was that upon restarting the pump, the vacuum actually rose to its pre-isolation value. I have been unable to determine the cause of this. Can anybody out there shed light on this:confused:?
-
Re: vacuum reduces when vac pump is isolated
If vacuum gauge is electronic, it is possibly that transducer is contaminated with oil,giving inconsistantancy, or else replace batteries as another possible cause.
-
Re: vacuum reduces when vac pump is isolated
gday bill.quite likely moisture.was 2000 micron the deepest it would pull ?
-
Re: vacuum reduces when vac pump is isolated
Hi Lowcool, Bill said it dropped 200 microns , not increased 200 microns. Me to as well confused, Bill may have developed a natural vacuum re-generation system... [ joke ] .
Normally only seen natural vacuums in the heads of clients, defying nature when it comes to paying for service, and they cannot remember you being there.
-
Re: vacuum reduces when vac pump is isolated
hi magoo, vac guage is electronic but sensor was cleaned before use.
lowcool, eventually pulled 300 micron total vac so moisture was removed totally.
thanksfor showing interest guys, keep thinking, there must be an answer.
how can a vacuum reading reduce when the vacuum pump is isolated, or how can you carry on removing moisture when the process is stopped?
is it possible that moisture emerging from the oil is being reabsorbed? the system was under a stable temperature of 32c to prevent the water from freezing.
-
Re: vacuum reduces when vac pump is isolated
Quote:
Originally Posted by
bill1983
At a pressure of 2000 microns, when isolating the vacuum pump, which was being used through a manifold set, the pressure on the system reduced by over 200 microns.
So... the pressure in the system was 2000 microns. Then you closed the valve on your gauge manifold, which isolated the pump from the system. Makes sense so far.
But then the pressure in the system either went to 2200 microns
OR
the pressure in the system went to 1800 microns.
Which is it?
Or is it the pressure in the pump that changed?
-
Re: vacuum reduces when vac pump is isolated
I have seen this too, only I had the electronic vac gauge mounted on the pump as I'm lazy....
Would be interesting to find out the cause.
Jon
-
Re: vacuum reduces when vac pump is isolated
gday magoo i did read it twice and three times today,confusion still reigns supreme,i only took it in the context that if a vaccum reduces,it is a loss in this addled mind still to me if i had a 27" vac and it reduced to 26" bloody micron values but i do find it the easiest to work with.
another natural vaccum ive found is with consultants and architects,the old told ya so phrase comes to mind.
have a good weekend fella
bottoms up!
-
Re: vacuum reduces when vac pump is isolated
thanks for joining y'all. to clarify. the vacuum reduced to 1800 microns from2000 when i isolated the pump. it took around 40 seconds to achieve this new level, which is why i thought that it must be due to moisture being absorbed. low cool you think you're confused. this was at 11 at night how do you think i felt even when i woke the next day?
-
Re: vacuum reduces when vac pump is isolated
I would say that possible reason for that behavior is puling vacuum from low and high side. Because of internal differences of restrictions in system, at point where you measuring vacuum there is lower vacuum ( higher absolute pressure) then in some other points in system.
When you close isolation valves, pressures in system equalize and you get higher vacuum (lower absolute pressure) then before equalization.
-
Re: vacuum reduces when vac pump is isolated
Quote:
Originally Posted by
nike123
I would say that possible reason for that behavior is puling vacuum from low and high side. Because of internal differences of restrictions in system, at point where you measuring vacuum there is lower vacuum then in some other points in system.
When you close isolation valves, pressures in system equalize and you get lower reading then before equalization.
Or perhaps a little refrigerant and/or moisture bubbling up out of the compressor oil. The oil absorbs both and will reluctantly and slowly release them under deep vacuum.
-
Re: vacuum reduces when vac pump is isolated
Quote:
Originally Posted by
Gary
Or perhaps a little refrigerant and/or moisture bubbling up out of the compressor oil. The oil absorbs both and will reluctantly and slowly release them under deep vacuum.
I think that will rise pressure on isolation and not lower it.
-
Re: vacuum reduces when vac pump is isolated
Quote:
Originally Posted by
nike123
I think that will rise pressure on isolation and not lower it.
Exactly so. Seems I am still confused... lol :confused:
-
Re: vacuum reduces when vac pump is isolated
Quote:
Originally Posted by
Gary
Exactly so. Seems I am still confused... lol :confused:
I edited little my post for some clarification. Maybe that helps.;)
-
Re: vacuum reduces when vac pump is isolated
Maybe it has smth to do with the working of the electronics in the gauge itself (not with the refr system). Its "kink" so to say.
-
Re: vacuum reduces when vac pump is isolated
Or rather of a particular vac gauge.
-
Re: vacuum reduces when vac pump is isolated
weird one all right ... my only suggestion is that the suction accum might have had a better vac and when you stopped the pump the negative pressure in that brought down the rest of the system.... or tranducer acting up... try a good old fashioned vac/torr guage and see if it's the same ... what make of vac guage are u using?
-
Re: vacuum reduces when vac pump is isolated
Quote:
Originally Posted by
bill1983
Whilst evacuating a small hermetic system, consisting of a 1 hp condensing unit with hermetic compressor, 20 feet of tubing, an evaporator with txv and accumulator, I noticed the following: At a pressure of 2000 microns, when isolating the vacuum pump, which was being used through a manifold set, the pressure on the system reduced by over 200 microns. Initially the vacuum pump was connected to the suction and liquid valves at the exit of the condensing unit, ie after the liquid receiver and before the suction stub on the compressor. The micron guage was connected to the receiver valve which had been closed to make it the furthest point from the pump. After noticing this phenomenon several times, I used the vac pump in several different ways to try to determine the cause; I was unable to. What was even more curious was that upon restarting the pump, the vacuum actually rose to its pre-isolation value. I have been unable to determine the cause of this. Can anybody out there shed light on this:confused:?
Just joining this thread and I'm happy this is brought up here: I'm an assessor for the F-gas exams in Belgium and I saw now +/- 400 candidates passing their exams from which most followed the pre-exam courses.
In this course, they all can do 2 complete recover cycles. So I saw at least 1000 vacuumcycles the last 6 months.
Well, I each time explain this phenomena and I promise each time a crate of Stella (finest Belgium beer :o) for the one who can explain this.
When they can't reach a certain vacuum, I advice them to isolate the pump and watch for 30 seconds. Then teh vacuum drops and rather fast. Once the vacuum has dropped with isolated pump and stays on a certain level, switch on the pump again and continue pulling deeper, starting from this new 'strange' deeper level.
This phenomena only happens with electronic meters (I tried 4 different brands of meters, Refco, Yellow Jacket, Inficon, ITE) but not with a precise absolute analog one. We use a very precise analog one to control all that electronic stuff.
-
Re: vacuum reduces when vac pump is isolated
BTW, we placed the sensor already everywhere on the system with the same strange result.:confused:
-
Re: vacuum reduces when vac pump is isolated
Quote:
Originally Posted by
Peter_1
BTW, we placed the sensor already everywhere on the system with the same strange result.:confused:
It seems that we need to analyze principle at which these electronic vacuum gauges working.
:confused:
-
Re: vacuum reduces when vac pump is isolated
Even this isn't true Nike: the Inficon works with a Wheatstone bridge, other uses a Piranni gauge as sensing element, others a piezoresistive sensing element,...
-
Re: vacuum reduces when vac pump is isolated
Piezoresistive is also resistor in Wheatstone bridge.
I am confused why pressure sensing piezo elements acting same as thermal conductivity sensing elements (pirani and thermistor).
Are you sure that both types are acting same?
Anyway, I am sure that there is no same vacuum level at all points in system during vacuum pump operation (otherwise there is no movement of contaminants in system), and that some pressure equalization is present when pump is isolated.
Since, in this case, vacuum pump has higher vacuum level than furthest point in system (where vacuum gauge is connected), it is normal for me that, at moment after pump is isolated, all points which are distanced from vacuum pump are at lower vacuum level (higher absolute pressure) than those closer to vacuum pump. And, after equalization of pressures in system, it is normal that at these distanced points vacuum level rise (absolute pressure fall). It could be checked by positioning two vacuum gauges (one at furthest point from vacuum pump and one at closest point to isolation valve. Then we could compare rise and fall of pressures during equalization.
-
Re: vacuum reduces when vac pump is isolated
Quote:
Originally Posted by
Peter_1
When they can't reach a certain vacuum, I advice them to isolate the pump and watch for 30 seconds. Then teh vacuum drops and rather fast. Once the vacuum has dropped with isolated pump and stays on a certain level, switch on the pump again and continue pulling deeper, starting from this new 'strange' deeper level.
This phenomena only happens with electronic meters (I tried 4 different brands of meters, Refco, Yellow Jacket, Inficon, ITE) but not with a precise absolute analog one. We use a very precise analog one to control all that electronic stuff.
I did not registered/noticed this phenomena, I will check this on my next few vacuum jobs. But I think that could also be explained by pressure equalization in system. Since pressure is equalized, there is no possibility for rise in absolute pressure at vacuum gauge point if you first turn on pump, than pump reach same or higher vacuum level like in system and after that you open isolation valve. Absolute pressure could only fall down further, but if you again isolate pump, it will fall again during new equalization.
How much is precise your analogue one?
-
Re: vacuum reduces when vac pump is isolated
i used yellow jacket vacuum guage for the procedure.
if the vacuum equalises when isolating the vacuum pump due to restrictions in the system, then would this cause an equal rise in vacuum (increased micron value)? if it is due to moisture/contaminents being reabsorbed then surely these would be released upon restarting the pump from the lower micron value, returning to the original pre isolation value. the time taken for both reactions is roughly the same.
peter, what level of vacuum were your students pulling when you saw the same phenomena? were the level rises and falls the same as mine or different at different levels of vacuum?
-
Re: vacuum reduces when vac pump is isolated
what was the name of the mercury job that was around during my apprentice days that got canned due to being not so enviroment friendly?
-
Re: vacuum reduces when vac pump is isolated
You draw the air at point A, vac gauge is set at point B. The space at the point A near the pump is more rarified than at the point B so the air flows from B to A.. You isolate the pump, the flow of air from the point B to the pint A continue to flow yet for some 40 sec due to inertia rarifying the space at the point B yet for some 40 sec. The vac gauge is showing it to you.
-
Re: vacuum reduces when vac pump is isolated
maybe the pump is to big for the application
-
Re: vacuum reduces when vac pump is isolated
Here is some PDF about book " Vacuum for Service Engineers " from nice guy named Rat on HVAC-PROTech Forums.
Credits to him.
-
Re: vacuum reduces when vac pump is isolated
Quote:
Originally Posted by
nike123
..
How much is precise your analogue one?
I don't know for the moment but in my opinion, much more precise than ALL that electronic stuff.
How I know this?
We once coupled all the vacuummeters we had on that moment (+/- 12 or 13 pieces) on a 7/8 tube with shraeders soldered around it and we vacuumed the 7/8 with a 3/8.
You can't believe the difference we measured and all those meters were only months old. We know now which are prone to failure, which ones has the best repeatability (very important),
Same comparable practical experience with electronic sniffers.
-
Re: vacuum reduces when vac pump is isolated
Quote:
Originally Posted by
bill1983
....
peter, what level of vacuum were your students pulling when you saw the same phenomena? were the level rises and falls the same as mine or different at different levels of vacuum?
We have to pull legally (EN378) lower than 290 Pa but we advise to lower than 100 Pa because you can vacuum the small systems for the exams easily with a tinny leak easily to 250 Pa. With a leak, you never can go lower than 100 to 150 Pa. So this phenomena happens lower than 150 Pa.
Stopping for example at 120 Pa, isolating the pump will initiate a drop of +/- 15 to 20 Pa and then stops.
Level stops on say 100 Pa, you re-engage the pump and you then can pump down further.
What I noticed is that this phenomena is the most
strong as soon as you notice the decrease of pressure is slowing down or even stopping.
You then can trigger a further pressure decrease with performing this technique (but not always)
-
Re: vacuum reduces when vac pump is isolated
Quote:
Originally Posted by
nike123
Here is some PDF about book " Vacuum for Service Engineers " from nice guy named Rat on HVAC-PROTech Forums.
Credits to him.
This link seems not working and I was interested in it
Not Found
The requested URL /forums/Vacuum for Service Engineers was not found on this server.
Apache Server at www.refrigeration-engineer.com Port 80
-
Re: vacuum reduces when vac pump is isolated
I have had this..several times.
1. Manifold Gauges connected to low side torr guage on manifold. Vac pump to guages. 3 way valves off as system not yet commissioned.
2. vacuum drops to say .7 torr and wont go lower.
3. shut guage valve...and amazingly the vacum slowly goes down to .5 torr.
-
Re: vacuum reduces when vac pump is isolated
We also use a technique I never saw before while performing leak checks on a pressurized system.
We can see within minutes (sometimes 30 seconds) on our small systems if there's a leak.
-
Re: vacuum reduces when vac pump is isolated
message to peter 1
May I ask if ur an fgas examiner where one could get study notes and lots sample questions to prepare for the 2079 test please.
-
Re: vacuum reduces when vac pump is isolated
Quote:
Originally Posted by
Peter_1
This link seems not working and I was interested in it
Not Found
The requested URL /forums/Vacuum for Service Engineers was not found on this server.
Apache Server at
www.refrigeration-engineer.com Port 80
It is fixed now!
-
Re: vacuum reduces when vac pump is isolated
I have the Belgium questions but those are completely different than those used in the UK.
And they're in Dutch and soon in French.
There was a poster some weeks ago - but this post seems erased - who tried to sell the questions but he then promised to post them here. We're still waiting.
-
Re: vacuum reduces when vac pump is isolated
Quote:
Originally Posted by
Peter_1
I don't know for the moment but in my opinion, much more precise than ALL that electronic stuff.
How I know this?
We once coupled all the vacuummeters we had on that moment (+/- 12 or 13 pieces) on a 7/8 tube with shraeders soldered around it and we vacuumed the 7/8 with a 3/8.
You can't believe the difference we measured and all those meters were only months old. We know now which are prone to failure, which ones has the best repeatability (very important),
Same comparable practical experience with electronic sniffers.
I measuring my vacuum with Refco VG64 and I wondered many times if it is acting on me or it is OK. With some experience I found that, if sensor is cleaned before use, it shows me always there is leak in system or at gauge and/hoses (if leak is present and I skipped leak test for some reason).
With +-10% (of reading) accuracy of this instrument I taught that these drops in reading after isolation are result of that and system pressure equalization (and maybe time needed for change of temperature of sensor). Now, after you presented that that phenomena is visible on all type of instruments except on analogue one, I am confused.
Usually, digital instruments reveal more than analogue if sampling rate is good.
-
Re: vacuum reduces when vac pump is isolated
Quote:
Originally Posted by
Peter_1
You can't believe the difference we measured and all those meters were only months old. We know now which are prone to failure, which ones has the best repeatability (very important),
Could you name best ones, and what you could say about performance of my Refco VG64 (if it is compared).
-
Re: vacuum reduces when vac pump is isolated
Quote:
Originally Posted by
lowcool
what was the name of the mercury job that was around during my apprentice days that got canned due to being not so enviroment friendly?
McLeod Gauge
-
Re: vacuum reduces when vac pump is isolated
If you are using valve core removing tools to isolate the system it may be there is a small amount of seepage around the valve shaft. When you rotate towards close it may be sealing off.
Next time you vacuum seal around the shaft with some vacuum grease and see if this phenomonon stops.