Checking Transducers And Temperature Sensors - Rheem P17 Series Installation Instructions Manual

9.10 Checking Transducers & Temperature Sensors
TRANSDUCERS:
Checking transducers for accuracy can be tricky
at best. A technician will be required to do some
voltage reading and algebraic math to validate the
transducer is functioning properly. Additionally,
comparing it to a reliable source can also be a
challenge. Unless the gauges have been calibrated
recently, the question remains, which one is right?
To make matters even slightly more challenging,
the technician will need to convert PSIA (Absolute
Pressure) to PSIG (Gauge Pressure) because the
latter is the one we work with in the field.
Knowing where to take the measurement, and
getting solid reading is crucial. Measurements are
done in the 0-5 volt DC range. One confusing point
may be that we will take our measurements OUT
and IN to the control, which are reversed when
we talk formulas because we want the IN and
OUT of the transducer. For instance, 5VDC out
of the control board, translates to 5VDC into the
transducer. And the lower variable voltage output
from the transducer will be the input to the control
board.
The transducer is not removed or disconnected
to make these checks. The technician's meter
leads need to be the smaller needle type, or actual
needles can be used to gain access to the points
of measurements.
Starting with the formula which results in the PSIA,
we can then convert to PSIG.
SUCTION LINE TRANSDUCER:
PSIA = 375 * (VDC out / VDC in) - 22.8
PSIG = PSIA - 14.7
LIQUID LINE TRANSDUCER:
PSIA = 812.5 * (VDC out / VDC in) - 66.55
PSIG = PSIA - 14.7
Formulas such as this require us to follow some
basic algebraic rules. The order of operations
is called PEMDAS. (Parentheses, Exponents,
Multiplication, Division, Addition, and Subtraction).
Our formula doesn't have any Exponents so we
can eliminate that. Also, there is no Addition.
Multiplication and Division, similar to Addition and
Subtraction priority comes to whichever is first
in the formula. In our case, this doesn't matter
because the Division is within the parentheses
which comes first anyway.
9.0 DIAGNOSTICS & TROUBLESHOOTING
Starting with some arbitrary numbers completion of
the formula should look like this:
SUCTION LINE TRANSDUCER:
VDC out = 2.4
VDC in = 5.1
So...
PSIA = 375 * (2.4 / 5.1) - 22.8
PSIA = 375 * (0.47) - 22.8
PSIA = 176.25 - 22.8
PSIA = 153.45
LIQUID LINE TRANSDUCER:
VDC out = 3.4
VDC in = 5.1
So...
PSIA = 812.50 * (3.4 / 5.1) - 66.55
PSIA = 812.50 * (0.667) - 66.55
PSIA = 541.67 - 66.55
PSIA = 475.1
In order to get to PSIG, we must subtract 14.7
(Atmospheric Pressure) from our PSIA. This is very
important, because if we forget to apply this, we
will certainly notice a difference and may otherwise
condemn, a properly working transducer.
PSIG = PSIA - 14.7
PSIG = 153.45 - 14.7
PSIG = 138.75
So when should we check this? Should the system
be running or static? Well, if we were looking at
the indoor transducer, it probably should be off and
stable. We may need to remove the transducer to
check the pressure at the same port. If it's outside,
our gauge port at the vapor service valve should
be dlose enough in proximity.
Where do we take our voltage measurements?
With the system powered, we will use our smaller
meter tips and get our voltage from the back of the
transducer harness where it plugs into the control
board. Voltage In will be measured from the Red
and Black wires. (Red is +, Black is -). This should
be very close to 5VDC, but may vary by a few
1/10ths. Our Voltage Out will be measured from
Green to Black. (Green is the variable +, Black
remains -).
Now we can apply the formula.
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