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Micron gauge help

92Cabby on Tue August 30, 2005 10:32 PM User is offlineView users profile

I need some advice on micron gauge reading. I recently purchased all new A/C equipment. A Robinair 14777 micron gauge, a 4cfm vacuum pump and a new manifold gauge set. I returned the micron gauge as it would only pull 500m after an hour on the pump, then it would drift up to 1500m and stabilize. The second gauge, connected directly to the pump, will pull down to 20m. However, after I closed the isolation valve, it, also, would drift up to 1500m. Next, I connected the vacuum gauge to the hose on the low side of the manifold and pulled vac through the center hose. Typically I was pulling 50m or so, maybe a 100m, and when I closed the valve and isolated the gauge on the low hose, it too, drifted up to 1500m and stabilized. I left it like that all night, came in the next morning and it was still 1500m. Both hard pipe and hose connections gave the same results, and so did two different gauges? Oh, by the way, I also cycled the vacuum off and on, to let it stabilize. I pulled vac for about 3 hours and it would still drift up to 1500m. Is this typical for thermistor technology?

Most people are familiar with “Murphy’s Law” which states: “Anything that can go wrong will go wrong. I subscribe to “O’Malley’s Law” which states: “Murphy is an optimist”.

jlh on Wed August 31, 2005 10:25 AM User is offlineView users profile

Got to say that I had a similar learning experience. After a certain age or not wanting to leave our comfort zone NEW does not always seem to be better. I used a large 8" dial analog vac gauge for years. Easy to quickly judge if the system was dry and tight. Then along come the LED units and I felt just like you.
Had to take a step back and look at what this new unit was offering me. Resolution was definitely better. gauge did not care if it was vertical or horiz. or in between. And it was repeatable with out interpretation. Haven't used a dial gauge since. Don't even need my reading glasses to see what the LED's tell me.
You'll like it too.
The rise you are seeing is probably vapor pressure - something is still cooking off. Indications are that the system is tight under vacuum if it holds as long as you say.
I have seen systems hold at 250 for up to a day when they are clean and dry. Add the oil and it seems that 500 is the best pull down but when isolated it also climbs 1 or 2 clicks then holds. The oil seems to perk off at a higher pressure than I would expect.
Don't forget to take into account the system size or volume. Most are pretty small so a little moisture or contaminant perking off will make a perceived large difference with the digital gauge.
Hope I made sense.

92Cabby on Wed August 31, 2005 1:59 PM User is offlineView users profile

Thanks for your reply. Well, at least I know I'm not alone. I have also been looking at a TIF 9450D, which is a non-thermistor technology. Any ideas' about non-thremistor gauges? My background is with Penning Gauges that read in the submicron range. I can't afford that technology, got to stick with something cheaper. Thanks, Greg

Most people are familiar with “Murphy’s Law” which states: “Anything that can go wrong will go wrong. I subscribe to “O’Malley’s Law” which states: “Murphy is an optimist”.

Theep Dinker on Wed August 31, 2005 9:53 PM User is offline

Greg, as jlh said you're probablly seeing outgassing. It really shows up on small volumes.

As you already know a cold cathode ionization (Penning) gage won't be much use. Neither will a hot filament ionization gage. If you want to do better than thermistor for these vacuum levels I suggest either a Pirani conductivity gage or a capacitance manometer ie; something like an MKS Baratron or a Vacuum General unit in the common 0-10 torr range. Try Ebay. Yes, I know Piranis are still thermistor based but they're better than a straight thermistor gage in several ways. All thermistor gages are gas species dependent but that's not an issue for AC work. Another problem with T/Cs is they're not very accurate down low because there just aren't enough molecules left to conduct the heat. Again, not much an issue for AC work.

I use a CM myself but Ihave pressure meseaurment needs that go beyond AC. CM's are kind of overkill for A/C but they beat thermistor technology any day of the week for accuracy and being an absolute device they're not species dependent. I see there are 3 brand new 100 torr units on ebay right now for $40 each. That's astonishingly cheap for something that normally sells for over a grand. You'll need to power it somehow but that's not a big deal and you can read it directly by using a DMM or a cheap digital DC voltmeter module. Be careful with it though, you can't dump a CM to ATM often or you'll tweak the diaphram. That's one nice thing about thermistor technology, it's pretty resistant to being dumped.

Edited: Wed August 31, 2005 at 10:21 PM by Theep Dinker

92Cabby on Thu September 01, 2005 5:40 PM User is offlineView users profile

Theep, Thanks for the input,

I’m just beginning to learn MVAC techniques and I’m sure that I’ll have a lot of future questions to ask. I haven’t been involved in vacuum work for the last 5 or 6 years. One of my primary jobs was to diagnose and repair electrical and mechanical problems in our vacuum systems. As I was typing the original post I forgot that the general consensus for good vacuum in MVAC seems to be 500 microns or lower. And, I had failed to recall the fact that the Penning Gauge turns on at about E-2 or -3 Torr. However, I called Robinair Tech Service about my second 14777 gauge and he said that the gauge shouldn’t jump like that. He told me to return it; maybe the third one’s a charm? Next I inquired about the TIF 9450D, asking what type of sensor it used. He had no idea and could only read from the manual. He then referred me to the TIF Tech.Service, with, basically, gave the same results. I’m not knocking these Tech’s, they’re hard working and they bent over backwards to help me. But, for some unknown reason, their parent company has not provided them with this particular information. When I was in their shoes, not having an answer, I would go up the ladder until I found someone with the answer; it’s good for business! Again, they were not provided with this resource. I wasn’t asking for a thesis, just a basic answer and I’ll do the homework. Bottom line: I don’t like buying a pig in the poke especially when I don’t have a lot of expertise in this area. I’m going to look at those gauges on e-bay. What kind of power are we talking about, 1 or 2kv? Regards, Greg

Most people are familiar with “Murphy’s Law” which states: “Anything that can go wrong will go wrong. I subscribe to “O’Malley’s Law” which states: “Murphy is an optimist”.

Theep Dinker on Fri September 02, 2005 1:09 AM User is offline


Yes, a Penning is a high vacuum gage that employs ionization. Useless for the low levels of vacuum dealt with in MVAC. If you tried to use it at those levels it would fail to ionize because the pressure is too high, at least for the 5 kv or so they use for power. It's why they need to be roughed down first.

A capacitance manometer works by having a chamber split by a (usually) stainless diaphram. One side of the chamber serves as a reference by being evacuated to around E-7 and then sealed. A getter is also added. The other side is open to the measuring port. The diaphram has mechanical stops and is connected to a variable capcitance. The signal is conditioned for an output that usually results in 1 millivot equaling 1 millitorr so you can easily read pressure using a DMM. The device requires + and - 15vdc to operate but 12 vdc will do. I have a rig I built around one using rechargable batteries and a small voltmeter module. Very sensitive, accurate, and isn't gas species dependent. Just don't be dumping it or the shock wave will tweak the diaphram.

That said, a T/C gage is really all that's needed for AC work. As I mentioned, the only problem with them is down low where, relatively speaking, there aren't enough molecules to transfer the heat for an accurate reading. But you really don't need accuracy as AC systems are very "dirty" in the vacuum sense and all you're trying to do is pump water vapor to a certain level. Being a polar molecule water vapor is one of the toughest gases to pump but real accuracy isn't needed because an AC system never gets much below the freezing point of water.

Now a cyro system that goes down to 10 Kelvin is a different story. There you have to worry about freezing nitrogen, oxygen, and other gases in the expansion area. It's why they use pure helium as a "refrigerant", because it freezes at about 4 Kelvin. What's interesting is how they can get to within a few degrees of absolute zero with the circulating helium never being in any other state than a gas ie; no phase change at all. Talk about cold. Too bad they won't work in a vehicle

92Cabby on Fri September 02, 2005 2:43 PM User is offlineView users profile

Good Read,

Well, that pretty much answers my gauge question. One of the kids has a 95 Neon that’s going to need AC service. The compressor squeals and throws belts. Sounds like I’m going to need a rebuilt compressor and other parts. And, if practical, I’d like to flush the evap and condenser in the car. I need a surefire, and cheap chemical process to facilitate the act. Once upon a time the process people, in my factory, were upset because they couldn’t get an E-5 or -6 in 12 or 14 minutes. I used some sort of granular detergent, smelled like pine, that I had found in the store room, and hot DI water for the initial cleaning. A good rinse with DI water followed by Acetone and maybe another DI rinse, then bake it out at around 250F. I was told, later, that this process brought their time down. Go Figure? Bottom line, we did a lot of things that Edwards said we couldn’t or shouldn’t do, with good results. I am thinking about doing this to the Neon. Any thoughts?

Most people are familiar with “Murphy’s Law” which states: “Anything that can go wrong will go wrong. I subscribe to “O’Malley’s Law” which states: “Murphy is an optimist”.

webbch on Sun August 29, 2010 3:28 PM User is offlineView users profile

Apologies for resurrecting an old thread, but just wanted to add that I too am experiencing similar drift problems with my Robinair 14777. Dead ended on the pump and it will draw down to 20 microns, but once the shutoff valve on the pump is closed, it drifts up to 1500-2000 microns in about a minute. My guage is just a little over 2 years old now, and sits on the shelf most of the time until my A/C goes out.

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