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Refrigerologist
26-03-2008, 08:18 PM
Hi guys!

I have been using an electronic vacuum gauge for the last 2 years and found something strange happening today!

With 2x 5cfm vac pump running on a remote condensing unit piped to an air handling unit coil, 6okw capacity, the vac gauge stopped at around 1300 -1400 microns. I closed all of the valves in the vaccing lines to prepare to add nitrogen for the final part of the triple vac procedure. When I looked at the vac gauge it had decreased to 350microns. I restarted the vac pumps opened the valves and the vac gauge pressure rose to 800 to 900microns. Closed the lines again and the gauge pressure dropped 350 to 400 microns.

Can anyone explain to me why this would occcur. I should state that the lines were checked for leaks under pressure and the gauge thermistor was cleaned with alcohol as per the manufacturer's instructions.

nike123
26-03-2008, 08:26 PM
Is your vacuum gauge close to vacuum pump?

frank
26-03-2008, 08:33 PM
the gauge thermistor was cleaned with alcohol as per the manufacturer's instructions.

What a waste !!!! ;)

Can't help with the electronic gauge though as I use a Torr gauge.

Refrigerologist
26-03-2008, 08:34 PM
Is your vacuum gauge close to vacuum pump?

The vacuum gauge is 12metres away!:)

Refrigerologist
26-03-2008, 08:36 PM
What a waste !!!! ;)

Can't help with the electronic gauge though as I use a Torr gauge.

Methinks I'll go back to a torr gauge, never seen this happen on an analogue gauge!

It was rubbing alcohol Frank:D

Refrigerologist
26-03-2008, 08:43 PM
Is your vacuum gauge close to vacuum pump?

Nike, from your query I guess this can happen if the gauge is to close to the vac pump, but I still don't understand why such a difference would occur. Also which reading should I work to? I like to think the lower reading as it is in my favour timewisehttp://www.refrigeration-engineer.com/forums/images/icons/icon12.gif

nike123
26-03-2008, 09:02 PM
Nike, from your query I guess this can happen if the gauge is to close to the vac pump, but I still don't understand why such a difference would occur. Also which reading should I work to? I like to think the lower reading as it is in my favour timewisehttp://www.refrigeration-engineer.com/forums/images/icons/icon12.gif

Difference is in pressure drop thru evacuation line. You say that this line is 12 m long. That is huge pressure drop for instrument like vacuum gauge. When you switch of vacuum pump pressure equalize and you get correct reading.
Keep vacuum pump close to vacuumed object and lines as short as possible you could? Mine are never longer than 1m in total.
Also, because electronic vacuum gauge is actually precise thermometer ( it measure temperature of gas inside vacuum gauge line) you should keep him away from heat sources who could possibly have influence on measurement.

For fast evacuation your lines should be short as possible, higher diameter and evacuate from both high and low side. Also, removing valve core and raising ambient temperature could significantly speed up evacuation.

Refrigerologist
26-03-2008, 09:09 PM
Difference is in pressure drop thru evacuation line. You say that this line is 12 m long. That is huge pressure drop for instrument like vacuum gauge. When you switch of vacuum pump pressure equalize and you get correct reading.
Keep vacuum pump close to vacuumed object and lines short as possible you could? Mine are never longer than 1m in total.

The vac gauge is connected on a port on the 7/8" liquid line 12 metres from the vac pump. I would never use 12 metre long vac lines, that would be rediculous Nike;) The vac pump is connected is connected via the compressor suction and liquid service valve ports.

Brian_UK
26-03-2008, 11:39 PM
This may be way off base but is it possible that there is a small vacuum leak at your gauge location which is active when the pump is running.

When the pump stops the leak stops ?

Leaking seal or thread on the gauge ?

Refrigerologist
27-03-2008, 12:59 AM
This may be way off base but is it possible that there is a small vacuum leak at your gauge location which is active when the pump is running.

When the pump stops the leak stops ?

Leaking seal or thread on the gauge ?

Pipe connections to vac pump were pressure tested, all except final two connections, the one on the pump inlet and the one on the shut off valve. New, flares, both smooth, both oiled. So unlikely, also if it were leaking I dont believe that less than 500 microns would be pulled. Certainly I never attained that depth of vac when using a torr gauge and had a small leak. The gauge would 'bounce' up and down about 1.5 torr.

All ideas are welcome. I don't suppose it is imortant as I have acheived a vacuum greater than 500microns. It just means for the last 2 years I have probably been vaccing for far longer than I needed to. No bad thing, apart from the wasted time. And time is money:eek:

Brian_UK
27-03-2008, 01:02 AM
Fair comment but I did mean the connection of your vacuum gauge and the guage itself.

US Iceman
27-03-2008, 01:19 AM
For fast evacuation your lines should be short as possible, higher diameter and evacuate from both high and low side. Also, removing valve core and raising ambient temperature could significantly speed up evacuation.


There is a lot of truth in this statement. Larger diameter evacuation lines and larger port connections help to speed up the process.

Just as a small liquid line will cause flashing of liquid due to pressure drop, a small diameter evacuation line is a restriction for the vacuum pump to pull against. The same applies to the port connection.

On large systems I have used copper tubing and connected multiple pumps in various locations of the system.

nike123
27-03-2008, 01:43 AM
New, flares, both smooth, both oiled.

Never oil flares when you work with vacuum pump. Oil tend to boil.

This is from Inficon Pilot vacuum gauge (one of the best):

OPERATING INSTRUCTIONS

2. Connect a dedicated or new charging hose to the Pilot’s refrigeration flare fitting.
a. A previously used hose may contain oil that could affect the sensor performance.
b. Normal refrigeration hoses do outgas (leak) which may make it difficult to
measure below 250 microns. We suggest using a metal braided hose for better
accuracy.
c. We recommend using a hose no longer than 3 feet. A longer hose may make the
gauge unable to read lower vacuum levels due to excessive outgassing.
3. Connect the other end of the hose directly to either the high or low side of the system
for most accurate readings. For convenience, you can also connect the hose to one of
the ports on the vacuum pump itself. To connect the gauge in-line with a manifold set or
the vacuum pump, you will need a separate ¼” male by ¼” male by ¼” female T
fitting which can be purchased through your distributor.I heaved similar problems as you before and my experience is, that best result is when gauge is in-line before manifold set.
Difference in reading during evacuation and after isolating vacuum pump are generally because of pressure drop thru lines or parts of system.

Refrigerologist
27-03-2008, 01:44 PM
Never oil flares when you work with vacuum pump. Oil tend to boil.

This is from Inficon Pilot vacuum gauge (one of the best):
I heaved similar problems as you before and my experience is, that best result is when gauge is in-line before manifold set.
Difference in reading during evacuation and after isolating vacuum pump are generally because of pressure drop thru lines or parts of system.

But not oiling the flares would make a mockery most equipment manufacturers guidelines and my own training, that is to say all flares should be oiled.

With regard to the position of the vac gauge it is as recommended by consultants and by the gauge manufacturer as far away from the vac pump as possible. I am vaccing at the condensing unit and the gauge is installed, on a port, at the ahu cooling coil, 12 metres away!

Brian, sorry I misunderstood your point. Yes, I suppose it could be that the gauge is actually leaking at the fitting point, which of course I have not leak tested, although I could pressure test this to 10bar as this being a thermistor the manufacturer says is OK.

Anyway, as the system was well below 500 microns after allowing the pressures to equalize I suppose it does not really matter. I had achieved the required vac and that is all we are required to do at the end of the day.

Anyway it looks like I have been the 'over vaccing' for some time, (I know it is not a bad thing, but it costs money, MY MONEY). Maybe at 1 torr I should have been isolating the vacuum pump from the system under vac to check the equalized pressures. Then continuing with vaccing if needed, or, as in this case, if the vac is low enough then it is time to stop and weigh some refrigerant in.

nike123
27-03-2008, 02:20 PM
But not oiling the flares would make a mockery most equipment manufacturers guidelines and my own training, that is to say all flares should be oiled.



From manuals of most manufacturers, I adopted oiling only at back side of flared pipe and not contact surface of flare/pipe. Oiling is only for ease of tightening of flare nut and to avoid slip of pipe at contact surface in last moments of tightening.
Softness of copper pipe is enough to make good seal and there is no need for oil.
Of course, you need to have good flaring tools and scratch free flared contact surface of pipe.

Refrigerologist
27-03-2008, 04:39 PM
From manuals of most manufacturers, I adopted oiling only at back side of flared pipe and not contact surface of flare/pipe. Oiling is only for ease of tightening of flare nut and to avoid slip of pipe at contact surface in last moments of tightening.
Softness of copper pipe is enough to make good seal and there is no need for oil.
Of course, you need to have good flaring tools and scratch free flared contact surface of pipe.

Nike, we could start a whole new discussion: Flares, to oil or not to oil, so I think we should agree to disagree as there will be no winnerhttp://www.refrigeration-engineer.com/forums/images/icons/icon10.gif otherwise this thread will be hijacked!

nike123
27-03-2008, 05:36 PM
Nike, we could start a whole new discussion: Flares, to oil or not to oil, so I think we should agree to disagree as there will be no winnerhttp://www.refrigeration-engineer.com/forums/images/icons/icon10.gif otherwise this thread will be hijacked!

Agree!:).......

SteinarN
27-03-2008, 06:14 PM
Refrigerologist, is there any residual refrigerant in the system? Is it an all new system?

Refrigerologist
29-03-2008, 01:34 AM
Refrigerologist, is there any residual refrigerant in the system? Is it an all new system?

New system, installed by me,nitrogen bled through during brazing etc. Good practice followed.

Ultimately I was just asking if anyone else had come across this phenomena. I quite understand about pressure drop throught the vaccing lines etc. It is just that in this case the vac gauge is so far away from the vac pump, and installed on a 7/8" liquid line, I just never expected to see such a drop in pressure when the pump was switched off. When I have used an analogue torr gauge I would expect to see the opposite with a gradual rise in pressure until it equalizes to the final atmosperic influence that has been left in the system.

Refrigerologist
29-03-2008, 01:36 AM
[quote=nike123;99016]Never oil flares when you work with vacuum pump. Oil tend to boil.

Nike, I just had a thought. How the hell do we pull any decent vacuums if we dont first remove the oil from the compressor. Also if it were a system that had been operating, surely we would need to blow the lines out:D

nike123
29-03-2008, 07:46 AM
[quote=nike123;99016]Never oil flares when you work with vacuum pump. Oil tend to boil.

Nike, I just had a thought. How the hell do we pull any decent vacuums if we dont first remove the oil from the compressor. Also if it were a system that had been operating, surely we would need to blow the lines out:D


I think that compressor oil start to boil at about 300 microns. If your pump is capable of achieving 50 microns (usual micron level of good double stage pump) than you could have oil boiled on parts of system and vacuum pump setup who could be below 300 microns. In order to have flow of gases toward pump there should be difference in pressures and that is how you could get some parts of system below 300 microns. Lowest pressure is at vacuum pump.
Therefore, I think that (and some literature said) we don't need to go below 500 microns in sisystem.

nike123
29-03-2008, 07:51 AM
New system, installed by me,nitrogen bled through during brazing etc. Good practice followed.

Ultimately I was just asking if anyone else had come across this phenomena. I quite understand about pressure drop throught the vaccing lines etc. It is just that in this case the vac gauge is so far away from the vac pump, and installed on a 7/8" liquid line, I just never expected to see such a drop in pressure when the pump was switched off. When I have used an analogue torr gauge I would expect to see the opposite with a gradual rise in pressure until it equalizes to the final atmosperic influence that has been left in the system.

Bare in mind that electronic vacuum gauge measures temperature of gases and that convert in pressures, whilst torr gauge measure pressure directly. In that facts, I think, is answer to our little mastery.

SteinarN
29-03-2008, 10:18 AM
As I understand it, a thermistor vacuum gauge reads the thermal conductivity of the gasses and convert that conductivity to pressure. That thermal conductivity is dependent on the pressure of course, but also on the molecular weight of the gas. The higher the molecular weight is, the lower the thermal conductivity is. Imo, a high molecular weight gas like hfc refrigerant will cause the thermistor gauge to indicate significantly lower pressure than is the actual pressure in the system. I suppose thermistor vacuum gauges is calibrated in normal air which have low weight gas molecules.

I have found thermistor vacuum gauge to be useless on systems containing refrigerant, that is systems which have been in operation already. In such cases the thermistor gauge will indicate good vacuum (less than 1000 micron) even with several psi left in the system. At least does my expensive thermistor gauge do that. I have not seen what you describe here when vacuuming new systems without any trace of refrigerant in it.

Can it be the compressor oil releasing vapour that flows by the thermistor when the pump is stopped and is replaced by nitrogen/air when the pump is on?

nike123
29-03-2008, 11:38 AM
As I understand it, a thermistor vacuum gauge reads the thermal conductivity of the gasses and convert that conductivity to pressure.
Yep, you are right here and I was give wrong information!
http://www.belljar.net/tcgauge.htm

Refrigerologist
29-03-2008, 04:26 PM
[quote=Refrigerologist;99356]


I think that compressor oil start to boil at about 300 microns. If your pump is capable of achieving 50 microns (usual micron level of good double stage pump) than you could have oil boiled on parts of system and vacuum pump setup who could be below 300 microns. In order to have flow of gases toward pump there should be difference in pressures and that is how you could get some parts of system below 300 microns. Lowest pressure is at vacuum pump.
Therefore, I think that (and some literature said) we don't need to go below 500 microns in sisystem.

It is a good point. I don't usually bother to go below much below 500microns. However, Liebert Hiross ask for a triple evacuation and a final vac of 0.3mbar/225 microns. Takes forever even with a good pump and 2 changes of oil.

The MG Pony
29-03-2008, 04:30 PM
My gauge does odd things occasionally, I just power cycle it in line and it behaves again, other then that just keep it clean and Use copper when ever possible as the rubber off gasses way to much making life annoying when Vacing a system.

But yes they do some times "stick" at a pressure until you isolate the pump for a moment.

Refrigerologist
29-03-2008, 04:39 PM
As I understand it, a thermistor vacuum gauge reads the thermal conductivity of the gasses and convert that conductivity to pressure. That thermal conductivity is dependent on the pressure of course, but also on the molecular weight of the gas. The higher the molecular weight is, the lower the thermal conductivity is. Imo, a high molecular weight gas like hfc refrigerant will cause the thermistor gauge to indicate significantly lower pressure than is the actual pressure in the system. I suppose thermistor vacuum gauges is calibrated in normal air which have low weight gas molecules.

I have found thermistor vacuum gauge to be useless on systems containing refrigerant, that is systems which have been in operation already. In such cases the thermistor gauge will indicate good vacuum (less than 1000 micron) even with several psi left in the system. At least does my expensive thermistor gauge do that. I have not seen what you describe here when vacuuming new systems without any trace of refrigerant in it.

Can it be the compressor oil releasing vapour that flows by the thermistor when the pump is stopped and is replaced by nitrogen/air when the pump is on?

I believe the vac gauge I use utilises a heated thermistor (I could be wrong as I have lost the manual) and relies on the temperature drop caused by the evaporation of gases over the sensor to determine the depth of vacuum. I have not had a problem on systems that had already been in commission, the vacuum inicated appears to be correct, apart from when the sensor has needed cleaning due to oil contamination of the sensor. I use the CPS gauge and would recommend cleaning the sensor after each use on a system that has been in operation.

Anyway, maybe it is time to close the thread as the vacuum I required was achieved and so the consultant is happy and I will get paid:) Afterall this is the most important part of our trade:D

praveen
02-04-2008, 09:37 AM
Sorry...have not used elctronic vaccumpumpso far..

praveen
02-04-2008, 10:41 AM
-30 psig equals to how much microns

nike123
03-04-2008, 05:56 PM
-30 psig equals to how much microns

You could have only -14,5037737730209215 psig! That is absolute vacuum (0 microns)
1 micron is 0,001 Torrs = 0,000019336774704623 Pounds per Sq Inch
1 atmosphere = 0psig = 762000 microns

bill1983
14-04-2008, 12:34 AM
just a thought, are both vacuum pumps pumping correctly and had oil changes with new unopened good quality oil. we recently had 2 new vac pumps tested before putting them into service and found that both would achieve 100 microns on our test rig, but that 1 took 40 minutes more than the other to reach this. no obvious cause for this was found, but if both these vac pumps were employed on the same system this may produce results similar to those that you experienced.