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Fujitsu FO20R Series Installation Instructions Manual page 63

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9.10 Checking Transducers & Temperature Sensors
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.
PSIA = 375 * (VDC out / VDC in) - 22.8
PSIG = PSIA - 14.7
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.
Starting with some arbitrary numbers completion
of the formula should look like this:
VDC out = 2.4
VDC in = 5.1
PSIA = 375 * (2.4 / 5.1) - 22.8
PSIA = 375 * (0.47) - 22.8
PSIA = 176.25 - 22.8
PSIA = 153.45
VDC out = 3.4
VDC in = 5.1
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 close 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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