Just Better Keeping the life of your pump - Manual (Page 5)

Information on Testing Vacuum Continued
stretches back as far as when Inches of Mercury (inHg) was the way measuring
of a vacuum on a system was taught. A charging line hose can be vacuumed to
50 microns if it is clean. New environmental hoses, fresh off of the shelf, will
only reach about 300 microns until they are cleaned out with alcohol and vacu-
umed out for a while. Why is this? First, the charging lines are mostly gaskets
made for positive pressure. Second, they are permeated. See page 5 for how
charging lines and permeation occur. The only vacuum tight hose is a fl exible
metal hose. Third, the compound of the hose inside will outgas when under a
vacuum until it is cleaned out, as discussed earlier.
Another source of leakage is the gasket seal in the valve and hose couplers. This
seal is designed for charging and will not give a perfect seal required in deep
vacuum service. An o-ring seal coupler, like the ones JB makes, forms around
any irregularities in the fl are fi tting. When the coupler is screwed down, we get
a metal to metal seat and the o-ring lies around the lip of the fl are giving it a
positive seal.
If you are used to using a compound gauge when testing for a leak or holding
a vacuum, using a digital gauge will be a little tricky the fi rst time you use it.
Digital vacuum gauges, like JB’s DV-22N, will display microns jumping up and
down in measure. You might think that the gauge is erratic or that there is a leak
in the system. The reason for the changing microns is due to a whole other area
of understanding the environment inside a system being vacuumed. We will
discuss this event in the next section “Information on Digital Micron Gauges”.
To help show the difference of a digital and analog displays in microns, and a
compound gauge display in inches of mercury (inHg) as it relates to their dis-
plays of vacuum, we need to hook them up. Take a compound gauge, an analog
gauge, a digital micron gauge, and an empty refrigerant tank. This hook-up is
illustrated on the next page in Figure 18. This allows you to demonstrate the
four components in holding a vacuum: the connections, the volume, the depth
of vacuum, and the length of time that volume is in deep vacuum.
Link all three gauges together by solid brass adapters and o-ring couplers and
couple to the tank. The tank is connected by an o-ring coupler to one of the
intake ports of the pump by way of braided metal hose with o-ring connections.
Then, with the isolation valve in the open position, we can begin to vacuum this
hook-up and watch the readings on the various gauges move into deep vacuum.
Within seconds, the compound gauge’s needle should be nearing 27-29” while
the digital and analog gauge readings are still heading into deeper microns.
After the digital and analog gauges are at 500-600 microns, close the isolation
valve. You will see the digital and analog readings start a pretty rapid rise in
micron readings. Notice that the compound gauge’s needle has not moved.
Figure 18
(NOTE: If the compound gauge’s needle does move toward zero on the scale,
you have an air leak in your connections). Open the isolation valve again and
this time let the hook-up vacuum for 5 minutes. Then close the isolation valve
again and watch. Open the isolation valve for about a minute, then move the
valve to the pause position for about 5 seconds, then close the valve completely.
This removes that trapped air around the isolation valve. You will still see a rise
in pressure, but not as rapid. The readings will start to stabilize the longer this
hook-up is allowed to vacuum down and use the pause position of the isolation
valve the slower and lower the rise in pressure.
If you increase the volume of the cylinder and follow the same procedure, you
will notice a slower and lower rise. If you watch your compound gauge, you
will notice there is no movement.
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