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Using micron gauge while charging

webbch on Mon July 21, 2008 11:00 AM User is offlineView users profile

Just a question to the folks who use a micron gauge. I have a Robinair 14777 and quite like using it to verify how low the system pumps down before recharging. However, it ends up being a bit of a mess, process wise, when trying to evacuate and recharge. Here's the steps I currently follow:
1) Hook up manifold gauge set to low and high side service ports (don't use micron gauge here because it says not to use it with a pressurized system as it may damage the sensor)
2) Evacuate system using center hose (and recovery machine)
3) disconnect the low side hose and install micron gauge on the low side service port
4) Hook up low side of gauge set to other side of micron gauge in preparation for pulling a deep vacuum
5) Pull vacuum through both low and high sides, using micron gauge to verify that system pulls down and holds adequately. I'll typically watch it for about an hour to make sure it's not leaking under vacuum.
...and here's the part that really bothers me
6) unscrew micron gauge from low side port and re-attach gauge set low side hose (again because the micron gauge is not supposed to be used in a pressurized environment when I recharge)
7) Pull a little bit more vacuum to compensate for the loss that ocurred when I disconnected the micron gauge (but now I have no idea what the micron level is)
8) recharge into the vacuum.

My only potential remedy to this problem is to tee off of the low side port and install the micron gauge on one side, separated by a ball valve that can be closed off during times when I don't need it. This would avoid having multiple removals. Assuming I could get the joints to seal properly (reasonable assumption?), do you think a standard ball valve would be adequate? Thanks.

I've mainly been working on R12 systems lately. I'd need to acquire several more couplers to do the same with R134a.
Any other recommendations on how to set this up? Thanks.


bohica2xo on Mon July 21, 2008 1:57 PM User is offline

Easier to just put the micron gauge in the vacuum pump line:

1) Convert to 4 port manifold as shown HERE

2) Install micron gauge in line between vacuum pump & manifold set.

3) Evacuate lines, system, manifold AND the line all the way back to the tank valve before you open the tank valve to begin charging.


"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

webbch on Mon July 21, 2008 3:13 PM User is offlineView users profile

Thanks B. That makes a lot more sense than the way I'm currently doing it.


boltz on Mon July 21, 2008 7:18 PM User is offline

boltz on Mon July 21, 2008 7:31 PM User is offline

Hi, I'm kinda new to this, but I have read that on the vacuum pump hose is maybe not the best place for the micron gauge.

I've been teeing mine in next to the high side port with a valve. I put the sensor up a 36" hose and hang it from the hood.
My experience so far has been that these things are pretty sensitive to contamination in the hoses, such as damp oil from
the vacuum pump.

When I got my micron gauge I hooked it to my vacuum pump to test it, and I wasn't getting a good reading (later turned out
to be a problem with the pump), so I went to the best shop I could find, to see if I could pay them to test it, and they said
"uh, what's a micron gauge?"

Sometimes I wonder if it's overkill for MVAC, but it does give me some peace of mind to see it get below 1000 and stay there.

-Jim Hart

webbch on Wed July 23, 2008 10:22 AM User is offlineView users profile

I read that you don't want to hook it directly to the vacuum pump connection because it could get oil on the sensor or something and that's a bad thing. I did it to my 14777 anyway and watched my vac pump pull down to about 20 microns. Not sure if it could've done lower, but I really didn't care as long as it pulled down below about 400, since I don't plan on going lower than that in any of my vehicles..

Also, the directions warn you to not hook it up to a full system, as the pressure may damage the sensor. However, in one of my first usages, I pulled a vacuum, then pressurized with about 130 psi of nitrogen, all while my micron gauge was still attached*. Once I realized what I had done, I was sweating bullets when I pulled the next vacuum, as I was hoping I hadn't done any damage. I don't have a reference gauge, so it may have thrown off the calibration a little, but so far it still seems to work as expected.

* FYI, when I pulled a vacuum on that particular system, I pulled down to 500 microns, and in 10 minutes it had only gone up to 750. However, when I pressurized with nitrogen, I realized I had a massive leak because I had damaged an evaporator o-ring while trying to get the lines hooked up. At that point, I was very glad I used the nitrogen, as this was an R12 system and all the refrigerant would've leaked out had I charged it.

bohica2xo on Wed July 23, 2008 1:29 PM User is offline

If the system is clean, and ready to be charged - where exactly would the "oil" come from? It can't get out of the system, and the flow is to the vacuum pump so the oil can't climb out of there either. The usual way a gauge gets contaminated is shutting down the vacuum pump with the gauge connected - and the oil boils out of the pump back into the gauges & hoses.

My old Supco micron gauge has been contaminated by a hose that had oil in it once, and by some crap that boiled out of a big system that was allegedly "clean" a few times over the years. No big deal, just clean it out with some trichlor, R11, etc. Contamination is obvious and easily fixed.

Every Supco micron gauge I have ever seen is built as a line Tee and while the instructions tell you not to install it in the pump line, many people do anyway. The reason they tell you not to use it as a "T" is because it does provide some flow restriction - meaningless on MVAC systems. On a 15cfm pump, using it as a "T" would be an issue - for a few minutes at least. Vaccum is vacuum even at reduced flow rates.

I have sent my VG60 out every two years for calibration, and after years of use / contamination / cleaning it is still just fine. Perhaps the Robinaire is just not built as tough.

FYI, I have found several hoses lately that would outgas for long periods of time. One of the things you should do periodically is micron test only your gauge set. Check it with the valves closed, one line at a time. The dead end the charging hoses, and open the valves to check the valve stem seals.


"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

webbch on Fri July 25, 2008 3:34 AM User is offlineView users profile

Thanks again for the info, bohica! I converted to a 4 port manifold tonight using that reference as a guide. Should make for a better setup the next time I go to charge a system. Thus far, none of the joints appear to be leaking substantially. I pullled a vacuum on the main line (one that will go to the tank under real use) down to 150 micron, and it went right up to 500 microns and held for 10 minutes (I didn't wait any longer). Not a thorough test I suspect, I may test it for longer a little later.

Now I'm wondering why I didn't just get the VG200 or similar. I guess I had it in my mind that I would be hooking the micron gauge up to the high side port on the vehicle at some time and having a remote readout seemed like a good idea at the time (I have a minivan where the high side port is in an inconvenient location for reading a display that's attached to the sensor). Now that feature is virtually useless to me I think. Oh well, live and learn. Another annoying part about the 14777 is that it likes to turn off automatically every 5-10 minutes or so, which is really ridiculous IMO, given the usage for which it was supposedly designed.

Just as an FYI to others, I ended up spending about $30 for all the fittings needed to do the conversion, which included a $10 ball valve (1/8" NPT), all from the local Ace Hardware store. Not proud of the fact that I used teflon tape (had some on hand), but I figured I could change that if it wasn't sealing properly. Thanks again for all the pointers.


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