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Conventional manifold gauge sets used to service and test auto A/C systems include a low pressure 'compound' gauge. It's called a compound gauge because it provides a pressure reading as well as a vacuum reading. When you take a close look at the gauge, it's easy to see that the 'level' of vacuum is really not that accurate... no matter how much you spend on the gauge set.
Can you rely on your low side manifold gauge?
This is a close up look at the vacuum scale of a conventional low side manifold gauge. We've added the 'red lines' to show just how difficult it really is to tell what level of vacuum you're reaching.

The scale on the gauge has a green line for every 2" of vacuum. When you compare that to the thickness of the needle, it's easy to see that the needle itself is at least the same thickness as 1" of vacuum. In short, you can't rely on the compound gauge for accurate vacuum tests.  It's better detailed in the micron vacuum chart below.

A close up look at 'micron' vacuum readings
Reviewing the chart below, we can quickly see that using the manifold gauge for an accurate vacuum reading is simply not reliable. Consider that 29.14" of vacuum (as shown on your manifold gauge) would only be 20,000 microns. 29.89" of vacuum would be 750 microns. With a conventional manifold gauge only, you simply can not see the difference. After all, the gauge needle itself is as thick as 1" of vacuum!

Using the micron vacuum gauge for our tests allowed us to clearly see how each vacuum pump performed before and after the vacuum pump service.

% Vacuum (Percent)

Microns

Inches Mercury Gauge

0

760,000

0

97.4

20,000

29.14"

98.7

10,000

29.53"

99

7,600

29.62"

99.9

1,000

29.88"

99.9

750

29.89"

99.99

100

29.916"

99.999

10

29.9196"

100

0

29.92"


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