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A Manifold gauge set is the minimum requirement for testing, diagnosing, servicing or recharging your auto A/C system. Of course, gauge sets are available in countless different configurations, but here's a close up look at what's important.
Auto A/C Pressure Gauges and Vacuum:
HConventional manifold gauge set includes both high (red) and low (blue) pressure gauges. The low pressure gauge is known as a compound gauge. It provides readings for both 'pressure' and 'vacuum'. The pressure reading is the outer scale in black while vacuum is read on the green scale (which is to the right of the needle).
Good gauge sets will usually include a low pressure gauge with a 'RETARD' at the high end of the pressure scale. That prevents damaging the gauge should it be subject to excess pressures. It's also important to have a 'calibration screw' (silver screw on the face of each of the gauges). That allows you to 'zero in' the gauge. For the purpose of this photo, we purposely adjusted the HIGH pressure gauge so it would be 'uncalibrated'. You can see that although there is no pressure in the gauge, it's not reading '0'. That's easily corrected by removing the gauge cover and adjusting the calibration screw until the needle points to '0'. You should check the calibration of BOTH gauges on a regular basis.
A close up look at the vacuum scale:
The problem with relying on the compound gauge (low pressure gauge) for accurate vacuum readings is that you're reading vacuum on a scale that is usually less than an inch in width! That means that any gauge that is not properly calibrated could be providing you a false vacuum reading.

That's the reason for using a micron vacuum gauge for testing the performance of a vacuum pump. Looking at this chart you can quickly see how inaccurate your gauge can really be.

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