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FORM 160.54-M1
ISSUE DATE: 10/25/2019
SECTION 20 - PRESSURE TRANSDUCERS
System pressures are sensed by pressure transduc-
ers. The evaporator, condenser, pump oil (high side)
and sump oil (low side) pressures are sensed. If the
chiller is equipped with the Variable Geometry Dif-
fuser (VGD), a stall detection transducer is located in
the discharge scroll of the compressor (See SECTION
26 - VARIABLE GEOMETRY DIFFUSER for details).
There are different transducers used to sense these
various pressures. The actual transducer used is deter-
mined by the required pressure range and refrigerant
application. The operation of the various transducers is
identical. The difference between them is the pressure
range over which they operate. Each transducer has a
different YORK part number. Figure 78 on page 178
lists the transducers and the application of each one.
The transducers output a 0.5 to 4.5 VDC 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
converts the transducer output voltage to a pressure
value with the appropriate formula in Figure 78 on
page 178. The pressures are displayed and used for
chiller control and safety shutdowns.
The evaporator and condenser pressures are converted
to saturation temperatures per the appropriate refriger-
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-
ducers are displayed individually as psig values. How-
ever, the System Oil Pressure is displayed as a differ-
ential value in terms of psid (pounds per square inch
differential in gauge). This psid value is arrived at by
subtracting the Sump Oil Pressure transducer value
from the Pump Oil Pressure transducer value. During
the System Pre-lube period, the outputs of the Oil Pres-
sure transducers are compared in a process called auto-
zeroing. The differential between the sump and pump
oil pressure transducer outputs during a 3 second pe-
riod beginning 10 seconds after the start of the System
Pre-lube period are compared to determine the offset
JOHNSON CONTROLS
between them. During this period, since both of the
transducers are sensing the same pressure, their out-
puts should indicate the same pressure. However, due
to accuracy tolerances in transducer design, differences
can exist. Therefore, to compensate for differences be-
tween transducers and ensure differential pressure ac-
curacy, this offset is factored with the actual differen-
tial pressure to produce the displayed psid value.
When the oil Pump is turned ON following the Auto-
zeroing period, the displayed differential value then
becomes the actual differential plus or minus the offset
that existed during the Auto-zeroing period. For exam-
ple, if the Pump transducer indicates 1.0 psig greater
than the Sump transducer during the Auto-zeroing pe-
riod, then 1.0 psig will subtracted from the displayed
psid value while the pump is running. Similarly, if the
Pump transducer indicates 1.0 psig less than the sump
transducer during this period, then 1.0 psig would be
subtracted from the displayed psid value while the
pump is running. The Auto-zeroing will not be per-
formed if either transducer is out of range.
The transducers operate from a +5 VDC power source.
This supply voltage is provided from the Power sup-
ply via the microboard. Each transducer is connected
to the microboard with three wires. Two wires provide
the +5 VDC supply voltage and ground (GND) and the
remaining wire connects the transducer output to the
microboard. The voltage output of each transducer can
be measured with a Voltmeter at the microboard. Mea-
surement should be made from the transducer output
to ground (GND). For example, the output of the Con-
denser transducer would be read from Microboard J8-21
(signal) to J8-22 (GND). To convert this output to a pres-
sure, see the appropriate formula in Figure 78 on page
178. If the pressure is known, the transducer output can
be predicted with the appropriate formula in Figure 78
on page 178.
If any of the displayed pressures do not appear to
be correct, see SECTION 34 - DIAGNOSTICS AND
TROUBLESHOOTING of this manual.
JCI COMPANY CONFIDENTIAL
20
177
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