Fluke RUSKA 2413, Руководство пользователя

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3-1
Chapter 3
Preparation for Use
Preparation for Use
Normally, when a transducer is shipped from the factory, it has been calibrated with
nitrogen and is dry in both cavities. Before installation, a quick performance tests may be
made by first connecting the box to the cell, with power on, adjusting the sensitivity to
maximum and the meter to zero. By pressing against the end of the open fittings with the
finger, the meter will be seen to deflect. The effect will be less when pressing on the
upper fitting, since the upper cavity has a volume some fifty times greater than the lower
cavity. At maximum sensitivity, it should be relatively easy to deflect the meter from zero
to full scale when pressing on the lower fitting.
Because of the small volume, it is of some advantage to connect the lower cavity to the
gas portion in a liquid-to-gas system. When so connected, less work will be done in
raising the gas pressure.
All fluids should be filtered before their introduction into the pressure system. A small,
hard particle, such as metal chip, in the diaphragm cavity will perforate the diaphragm
when the cell is over-ranged. Every effort should be made to keep contaminating particles
out of the transducer. In charging the upper cavity with a liquid, it is important to displace
most of the air with the liquid. There are many traps in the cavity which may retain small
air bubbles. If these bubbles remain in contact with the diaphragm or stem which carries
the transformer core, the performance will be erratic. The fact that the air bubbles
dissolve in the liquid when the pressure is increased may be used to an advantage. With
the vent plug removed, the liquid is pumped into the upper chamber until it appears at the
vent port. The plug is replaced and the pumping continued until the pressure in the liquid
system reaches 150 bar (2175 psi). At this pressure, the entrapped bubbles dissolve in the
liquid, forming a concentrated solution in the vicinity of the trap. Some time should be
allowed for the solution to diffuse so that, when the pressure is released, the bubbles will
not reappear in the same trap. The bubbles must reappear at some new point where they
may rise to the top of the chamber and be expelled through the vent port. The presence of
a bubble in the top of the cell cavity does not affect the measurement significantly, but it
does affect the response. It is therefore convenient to work the air out of the cavity as
much as is practical.
The cavity may also be charged by first evacuating and then admitting the liquid to the
evacuated chamber. Usually, some small bubbles still remain because of the difficulty in
reducing the internal pressure sufficiently through the small-bore tubing.
The presence of remaining air in the cavity may be measured if the liquid pressure
generator is a screw-type displacement pump and the system contains a Bourdon-tube
reference gage. It is first necessary to measure the air that exists in the portion of the
liquid system other than the transducer. To make this measurement, it is necessary to
isolate the liquid system from the cell and the dead-weight gage (if one is used). If there