Micron gauge comparisons

webbch on Sat September 18, 2010 1:40 AM

How many of you own multiple micron gauges? I bring this up because in deep vacuum discussions, we frequently refer to a "good" vacuum for MVAC purposes to be in the 500-1000 micron range. Compare this to HVAC guys that have to pull down to at LEAST 500 just to pass as "acceptable" work, and the conscientious ones pull down to 300 or better under normal conditions.

I have a Robinair 14777 and a CPS VG-200, and their readings vary dramatically. After a LOT of testing, I'm starting to believe that the VG-200 is the "correct" one, but when my Robinair gauge reads 750 microns on the digital display, the VG-200 reads about 5500. Once the Robinair gauge gets down to 300, the readings match to within about ÃÂ±50 microns. Unfortunately, the range of 300-750 microns is the range I care most about when doing MVAC. I was wondering if anyone else has run into this?

FYI - I learned that the 4-port conversion on my manifold gauge set was leaking through all this testing....after chasing what I thought was a system leak for about a day or two.

TRB on Sat September 18, 2010 2:20 PM

I have a CPS VG-200 and it works well enough for me and my line of work. Also carry the 98061 but have not done a side to side comparison.

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NickD on Sat September 18, 2010 5:17 PM

Quote
Originally posted by: webbch

FYI - I learned that the 4-port conversion on my manifold gauge set was leaking through all this testing....after chasing what I thought was a system leak for about a day or two.

Did that have any affect on your comparative testing? Also on drawing deep vacuums on MVAC systems, designed for good sealing under pressure, under vacuum, works against both the compressor seal and service ports and these results may not be repeatable either from gauge to gauge, or just repeating the same test with the same gauge.

These gauges typically have a sealed vacuum diaphragm with a theoretic perfect vacuum with a strain gauge variable with stress sensor with very minute changes of resistance translate via an A to D converter for a digital display. Making measurements in micrometers or microns that are exposed to atmospheric pressure in the range of an astronomical 760,000 micros where precise readings in the 50 micron range is but a mere fraction of that. Just more room for error, temperature also plays an important role unless they are compensated for that.

Using a standard mercury manometer for displacement would show only a displacement of 0.000020" per micron, just a tad bit small for the human eye to detect.

webbch on Sat September 18, 2010 11:32 PM

I didn't use the manifold in the loop for my testing. It was merely one of the contributing factors for why I was initially chasing my tail when diagnosing the original system. For testing, I hooked a copper line from vacuum pump directly to one port on an empty recovery cylinder, then hooked both micron gauges to the other port on the cylinder. With this setup, I was able to pull down below 50 microns on both gauges. Substituting the copper line for a charging hose let me pull down to about 150 microns.

NickD on Sun September 19, 2010 7:15 AM

In regards to test equipment in that these gauges fall under, in high quality units, accuracy specifications are given along with the period of time those specifications should hold and when recalibration is necessary. Just looking at some high end vacuum measuring equipment, how is this for a specification?

"The sensor specification is 30% of reading and 5% repeatability. " In other words, an error of +/- 30% at any giving reading.

CPS VG-200 isn't too bad with a +/- 10% reading error, no repeatability, but an additional 0.5% error for each *C outside of its rated temperature range. Perhaps the owners manual goes into greater detail, doubt if there is a service manual, can find the specs at http://www.cpsproducts.com/site/elements/PDF/CPSLTNV2008.pdf.

Can't seem to find too much about the Robinair 14777:

"
Ã¢ÂÂ¢Compact,pocket-size Ã¢ÂÂ 2.5""x 3". Ã¢ÂÂ¢Takes guesswork out of evacuation Ã¢ÂÂ accurately indicates when system is ready for recharge. Ã¢ÂÂ¢Designed for use under high vacuum; reads down to 10 microns. Ã¢ÂÂ¢Easy-to-read LCD automatically registers smaller numeric segments as system pulls down. Ã¢ÂÂ¢6'long lead and magnetic mounting strip Ã¢ÂÂ allows for convenient,easy-to-read, positioning. 14777 Specifications Range: 25,000 to 10 microns Vacuum Fitting: 1/4"MFL Power Supply: One 9V cell battery (included) "

But when doubt occurs, can always give the manufacturer a call and question what can be done about it. Limited when only testing one unit, when in my evaluation days, would test a hundred. Did get standard gauges, from the old NBS that I compare new gauges with, my vacuum gauge is in inches of mercury, but at least lets me know if I am even close to the ballpark. Last thing I tested, granted only one pressure gauge was one of those kit low side gauges, only showed 20 psi with 30 psi applied. That will only overcharge a system by 33.3% if what instructions supplied are followed.

NBS is now the NIST, one of the nicer government agencies to work with, can contact them for vacuum measuring standards if you are really interested, but not sure how that would work out as an individual. Wish the EPA was that nice.

Tennessee_Timmy on Sun September 26, 2010 8:23 PM

Just because I'm a big engineering nerd, I tried this with 4 micron gages, - a VG200 I just got back from CPS, a CPS VG100, and 2 Yellow Jacket models.

At ~400 microns, I had about a + 30 micron spread between the 4, and the VG200 was pretty much in the middle. I tried it at ~1200 microns, 3 of them still read within about 100 microns or so, with the VG200 again in the middle.

[One of these days, I'm going to use the correct reply button and quit posting blank replies that have to be edited]

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Good Luck 2 ya,
Timmy

Edited: Sun September 26, 2010 at 8:33 PM by Tennessee_Timmy

TRB on Sun September 26, 2010 9:37 PM

Just make sure you're logged on before hitting reply.

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