# ACtion Magazine - July/August 2017 - 18

```cepted at 29.92" Hg (mercury). Get up close and personal
with the low side (combination) gauge on your machine
or gauge set. Notice how close the 29" Hg and 30" Hg
lines are. Now look at how "fat" that needle really is. It
is impossible to determine just how deep your pump is
pulling using a mechanical gauge. Come to think of it,
pressure? Has anyone taken the time to "zero" the gauges

If we attach a vacuum pump to one
side of the manometer tube and turn
it on, the mercury (Hg) will be drawn
up the tube. The mercury on the other
side of the tube will drop the exact
same amount as the side the pump is
pulling on rises. If we attach a gauge,
our reading in "inches of Hg" will
correspond to the distance the column
of mercury has fallen on the other
side. As the column approaches the
"theoretically perfect" vacuum level
of 29.92" of mercury (at sea level), we
switch to measuring in MICRONS to
determine vacuum quality. The closer
we come to ZERO microns (29.92"
Hg), the better the achieved vacuum.
A thermistor vacuum gauge is the
tool required to read vacuum at these
levels (Figure 2). Over the past few
years they have greatly reduced in
size and cost. When installed in-line
with your vacuum pump, these gauges
provide accuracy at a level which is

Ruben Castelnuovo

HD and OTR vehicle service: Keep it clean

Figure 1: The
difference in
ď¬‚uid height in
a liquid column
manometer is
proportional
to the pressure
difference.

Mike Bailey

Looking at the very slight differences in vacuum on the chart,
it's evident that an accurate vacuum reading is critical. Enter
the micron vacuum gauge. And what exactly is a "MICRON"
anyway?

While we typically check vacuum using a low side
combination gauge, a MICRON is a linear measurement. A
SINGLE MICRON MEASURES ONE ONE-MILLIONTH
OF A METER - that's 0.00004 inches (rounded up).
Figure 2: A thermistor vacuum gauge is not very expensive
and helps ensure a quality repair.

These readings all stem from using a U-shaped instrument called a manometer. The "inches of mercury" gauge
reading we're familiar with represents a manometer, using
mercury (Hg) as the liquid to measure vacuum. See Figure 1.

18

ACTION * July/August 2017

impossible using a mechanical gauge. When installed directly on your vacuum pump, you can determine the condition of your pump or the oil inside.
What readings should you expect to see when using a
thermistor vacuum gauge? This depends on your vacuum
pump, its condition, the moisture in the system, and how
"tight" the system is.

```

Outlook
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HD and OTR vehicle Service: Keep it Clean
Positive Pressure dor operator cabins
Industry News
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Last Watch
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