Just Better Deep Vacuum Principles and Applications User Manual

Page 2

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Last Inch

Of Pressure


The electronic vacuum gauge

is the least purchased deep

vacuum tool. Yet, without

this instrument you might

just as well forget about

deep vacuum


dry ambient air is admitted to help prevent the moisture

vapor from condensing in the oil.

So far, we have defined our pump requirement as

follows: 2-stage, rotary vane; blankoff valve; gas ballast

valve; 1-1/2 to 10 CFM. A system is evacuated to

between 300 and 400 microns so obviously these

pumps should be able to produce vacuum in the low

micron range with a safety factor of at least 25 microns

total absolute. Thus, the pump should be able to achieve

vacuum readings of at least 25 microns total absolute.

We should also look for light weight and rugged con-

struction because we all know the vacuum pump will be

at our side as we climb those ladders to the roof top.

Finally, when checking out pumps, look at safety.

Belt driven units should never be used without belt

guards—if you don’t give a darn about your own fingers,

etc., give children and others exposed a chance.

Hospitals and court rooms around the world are full

because of this negligence.

The Electronic Vacuum Gauge

Coupled with good procedures which we will get

into later, the electronic gauge tells

us positively that we have a

noncondensable and a leak free

system. In general these gauges are

heat sensing devices, in that the

sensing element which is mechani-

cally connected to the system being

evacuated generates heat. The rate

at which this heat is carried off

changes as the surrounding gases

and vapors are removed. Thus, the

output of the sensing element (either

thermocouple or thermistor)

changes as the heat dissipation rate

changes. This change in output is

indicated on a meter which is

calibrated in microns of mercury.

Evacuation is complete when a system holds at 500

microns. The compound gauge only indicates that a

vacuum is being produced. The vacuum gauge on the

other hand, is the only tool for accurately reading that

low pressure.

Vacuum Gauge Selection And Accuracy

The most important feature of all is range. If the

micron gauge only indicates from 50 to 1,000 microns,

you will not be able to determine whether you are

pumping against a leak or against moisture. Look for an

instrument that reads from 50 microns to at least

9,000 microns.

A digital display with easily read numbers gives you

instant and continuous readout, whereas a gauge with

color-coded lights, displays the reading “within a range”

of microns. You have a “wait” period to see whether the

system is going up or down in microns.

Portable micron gauges typically operate from

battery power and should have a low battery sensor.

Some models have AC adaptor capability so you won’t

run out of power on the job.

Another feature to look for is a sturdy case to

protect the instrument. Finally, when you buy instru-

ments of this type, remember that you are really only

buying answers, and the instrument should give you

these answers quickly and accurately. You get paid for

adjusting refrigeration systems, not your tools.

As already noted, we are talking about accuracy

when we talk about micron type gauges. Gauge accu-

racy is affected by two factors. Extreme temperatures

especially with exposure to the summer sun on a hot

roof top or pavement and sensor contamination.

The vacuum sensor is factory calibrated on air. If

refrigerant gas or oil is drawn into the vacuum sensor of

a remote reading unit or unit connected to the pump

during the system evacuation, the gas will cause an

erroneous reading. Any oil entering into the vacuum

sensor via hose will also affect gauge accuracy. Im-

proper shut down of pump after evacuation and loss of

power will suck back oil and contaminate the hose and

micron gauge.

A hose used for charging or testing will contain

droplets of system oil spurted into the hose when the

schrader valve is opened. If this same hose is used on

the hookup to the gauge, oil will collect in the gauge

sensor. This can be prevented by using a dedicated

hose, preferably O-Ring type, for evacuation.

Evacuate Through The Gauge Manifold

Evacuate through the gauge manifold if, and only if,

it is O-ring sealed, piston construction. Other types leak

under vacuum. Next look at the design of the center

port. In order to handle the full capacity of both the high

and low side, the center intake should have double size

flow path throughout its length. All J/B 1/4" manifolds

have this feature. We suggest the fitting be replaced with

a 3/8mf x 1/8mp. You will now have a fulll flow

3/8" to the vacuum pump. You also have the

option of using the M4-Series manifold which is

designed to evacuate, charge or test a system

without disconnecting hoses and features

a 3/8" fitting.

Leak-Proof Hook-up

Deep vacuum has it own unique properties

which requires leak-proof design not only in the

manifold but in all components. The only con-

necting lines that are absolutely vacuum tight are

soft copper tubing or flexible metal hose. Charg-

The last inch of

pressure, as

indicated on the


gauge is 25,400