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I found a 'calibrated reference gauge' on Ebay for $200, verified it by my great 'Digi-Cool' tool.
Now all the techs have a good test spot for the dial gauges, untill I can graduate them.
luck dan

You could also use a certified reference gauge.

Use a T fitting and attach your gauge to one end, certified reference gauge to the other.

Observe both gauges and slowly pressurize an empty recovery tank with dry N2 (I wouldn't use air if you don't want to let moisture into the tank) to the maximum allowed pressure. Verify that gauge under test sweeps up smoothly with no sticking spot.

Let the N2 out of liquid side. Compare readings at approximately every 50 psi.

I don't recommend one point calibration as you can miss sticky spots, which can happen from abuse. The speedo on my car works ok, except in 50-70mph range. It has a response problem. If i'm at 50 and gun it to 70, it will read like 55. If I leave it on cruise control, it will eventually creep up to 70 though.

If I only tested it at 40mph, speedometer passes with flying color. Fortunately tachometer works, so I have stickers that correspond to speed in 5th gear on my tachometer for 50-80 range. In this case, my master gauge was speedometer on GPS unit.

Any gauge with dead spot should be discarded.

Attachment 263371

This is how I calibrate my gauges at the start of every season and you can do it with analogs as well. The temp meter is calibrated in ice water first and then taped to a bottle with insulation. Hook up your gauges and open the bottle and valves, remember to purge, compare the pressure read to the temp of the bottle it should match your p/t chart. If not make adjustments as needed. This pic is outside but it should all be left in your conditioned house or shop for a couple of hours to be consistent. In your case make calibration and then change temp of bottle a good deal and make sure the gauges still read correctly. Start saving up money for new gauges and digital ones if you do a lot of service work because you always need at least one backup pair.

Originally Posted by Reeferman

If you are going to stick with analogs I really suggest you replace them if they have been knocked around like that.

Also, try the rubber 'boots' for the analogs. They help, FWIW.

Originally Posted by jamessnevets

I was hoping someone could share some techniques for recalibrating manifold gauges. My R-22 and R-410A sets were in storage for a few years, not pressurized, while I was out of work. Recently I used them on job sites only to discover both sets are way off. While they were hung in storage, they were repeatedly knocked off their hooks by careless "helpers" (aka: friends of mine that are clueless to the sensitivity of gauges). Any help would be greatly appreciated. I am not permitted to join the Professional Member ranks as of yet and cannot access a link to another thread which provides the details I seek. Thanks!

www.digi-cool.com You're welcome.

If you are going to stick with analogs I really suggest you replace them if they have been knocked around like that.

Gauge calibration

I was hoping someone could share some techniques for recalibrating manifold gauges. My R-22 and R-410A sets were in storage for a few years, not pressurized, while I was out of work. Recently I used them on job sites only to discover both sets are way off. While they were hung in storage, they were repeatedly knocked off their hooks by careless "helpers" (aka: friends of mine that are clueless to the sensitivity of gauges). Any help would be greatly appreciated. I am not permitted to join the Professional Member ranks as of yet and cannot access a link to another thread which provides the details I seek. Thanks!