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Pressure Transducers
System pressures are sensed by Pressure Trans duc ers.
The Evaporator, Condenser, Pump Oil (high side) and
Sump Oil (low side) pressures are sensed. There are
dif fer ent trans duc ers used to sense the various sys tem
pres sures. The actual transducer used is de ter mined by
the required pressure range and refrigerant ap pli ca tion.
The operation of the various transducers is iden ti cal.
The dif fer ence between them is simply the pres sure
range over which they operate. Each of the dif fer ent
trans duc ers has a different YORK part number. Fig. 55
lists the trans duc ers and the application of each one.
The transducers output a 0.5 to 4.5VDC voltage that is
analogous to the pressure applied to the device. These
outputs are applied to the Microboard, where this volt-
age is interpreted as a pressure value in terms of PSIG
(pounds per square inch gauge) in English mode or
KpaG (Kilo Pascals) in Metric mode. The Program con-
verts the transducer output voltage to a pressure value
with the appropriate formula in Fig. 57. The pres sures
are displayed and used for Chiller control and Safety
shut downs.
The Evaporator and Condenser pressures are con vert ed
to Saturation Temperatures per the appropriate re frig -
er ant pressure/temperature conversion table contained
in the Program. These Saturation Temperatures are
dis played and used for Chiller control.
The outputs of the Sump and Pump oil Pressure trans duc ers
are displayed individually as PSIG values. How ev er, the
System Oil Pressure is displayed as a dif fer en tial value
in terms of PSID (pounds per square inch dif fer en tial in
gauge). This PSID value is arrived at by sub tract ing the
Sump Oil Pressure transducer value from the Pump Oil
Pressure transducer value. During the Sys tem Prelube
period, the outputs of the oil Pres sure trans duc ers are
compared in a process called Auto-Zeroing. The dif-
ferential between the Sump and Pump Oil Pres sure
transducer outputs during a 3 second pe ri od be gin ning
10 seconds after the start of the System Prelube period
100
SECTION 17
PRESSURE TRANSDUCERS
(REFER TO FIG. 57)
are compared to determine the off set between them.
During this period, since both of the trans duc ers are
sensing the same pressure, their out puts should in di cate
the same pressure. How ev er, due to accuracy tol er anc es
in transducer design, dif fer enc es can exist. There fore,
to compensate for differences be tween trans duc ers and
assure differential pressure ac cu ra cy, this offset is fac-
tored with the actual dif fer en tial pressure to pro duce the
displayed PSID value. When the oil Pump is turned on
following the Auto-zeroing period, the dis played dif-
fer en tial value then becomes the actual differential plus
or minus the offset that ex ist ed during the Auto-Zeroing
period. For example, if the Pump transducer indicates
1.0 PSIG greater than the Sump transducer during the
Auto-Zeroing period, then 1.0 PSIG will subtracted from
the displayed PSID value while the pump is running.
Sim i lar ly, if the Pump transducer in di cates 1.0 PSIG
less than the sump trans duc er during this period, then
1.0 PSIG would be sub tract ed from the displayed PSID
val ue while the pump is running. The Auto-zeroing will
not be per formed if either trans duc er is out of range.
The transducers operate from a +5VDC power source.
This supply voltage is provided from the Power supply
via the Microboard. Each transducer is connected to
the Microboard with three wires. Two wires provide
the +5VDC supply voltage and Ground (GND) and
the re main ing wire connects the transducer output to
the Microboard. The voltage output of each transducer
can be measured with a Voltmeter at the Microboard.
Mea sure ment should be made from the transducer out-
put to Ground (GND). For example, the output of the
Con dens er transducer would be read from Microboard
J8-21 (signal) to J8-22 (GND). To convert this output to
a pressure, refer to the appropriate formula in Fig. 57.
If the pressure is known, the trans duc er output can be
predicted with the appropriate formula in Fig. 57.
If any of the displayed pressures do not appear to be
correct, refer to the Diagnostics and Troubleshooting
section of this book.
FORM 160.54-M1(503)
YORK INTERNATIONAL
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