4-1
Chapter 4
Operation
Operating Instructions
In comparing the pressure of one system to that of another, it must first be established
that the comparator or indicating device is adjusted correctly. The adjustment must assure
the operator that all hydraulic and pneumatic heads have been accounted.
With the transducer connected between two systems and prepared for operation, the
power is turned on and the circuit allowed to warm up for ten minutes. A sequence of
operations must be adopted in which one of the systems is always at a higher pressure
than the other during the period of change from one pressure to another. If there is a
choice, it is of some advantage, in a liquid-to-gas system, to maintain the higher pressure
in the liquid system during the period of change. This procedure is not difficult to execute
for both increasing and decreasing changes in pressures. If it is intended to raise both
system pressures from one level to a higher one, the liquid pressure is raised first to a
value somewhat below the final one. The diaphragm of the differential pressure cell is
driven to the lower cavity surface where it supports the excess liquid pressure. The
operator is then free to concentrate on raising the gas pressure to, but not in excess of, the
liquid pressure. As the final pressure is approached, it is usually possible to raise both
systems simultaneously, while keeping them sufficiently balanced for the meter pointer to
remain on scale.
Before starting a measurement on a liquid-to-gas system, the differential pressure
transducer is intentionally over-ranged in the direction proposed by the adopted
procedure; i.e.; from the liquid side. The pressure is allowed to remain for a minute or so
and then released. In some manner, the liquid system must be opened to atmosphere at a
point level with the diaphragm. An open-tube manometer valve opened and the gas
system also opened to atmosphere, the liquid is adjusted to stand in the tube at the height
of the diaphragm. Under these conditions, the pressure across the diaphragm is zero. The
electrical circuit, with sensitivity set at maximum or whatever value has been chosen,
may then be adjusted for the meter to indicate zero
Δ
P. As the manometer valve is closed,
the pumping action of the stem causes the liquid to rise slightly in the tube and the meter
pointer to deflect. The deflection is a normal one which results from the disturbance of
the liquid in the tube.
Before the measurement is begun, the sensitivity is reduced by placing the shunt switch
in the ON position. The shunt switch reduces the gain of the circuit by a factor of
approximately 1000. First the liquid pressure and then the gas pressure is raised in the
manner described above. As the gas pressure becomes approximately equal to that of the
liquid, it will be observed that the two pressures will rise simultaneously as the increase
in gas pressure is continued. At this time, the diaphragm is being forced away from the
lower cavity surface by the gas. The displacement of the diaphragm increases the
pressure in the liquid system. Although the two pressures are approximately equal, a
