Chapter 7 Servicing
7-15
An error in the zero setting of the pressure transducer does not introduce a measurable
error in the output concentration reading. Therefore, if only a barometer is available and
not a vacuum pump, only adjust the span setting. A rough check of the pressure accuracy
can be made by obtaining the current barometric pressure from the local weather station
or airport and comparing it to the pressure reading. However, since these pressures are
usually corrected to sea level, it may be necessary to correct the reading to local pressure
by subtracting 0.027 mm Hg per foot of altitude. Do not try to calibrate the pressure
transducer unless the pressure is known accurately.
TEMPERATURE SENSOR ADJUSTMENT
This procedure should only be performed by an instrument service technician.
Equipment Required:
Calibrated Thermometer or 10K
Ω
±
1% Resistor
Screwdriver
CAUTION:
Some internal components can be damaged by small amounts of static
electricity. A properly grounded antistatic wrist strap must be worn while handling any
internal component.
1. Wear an antistatic wrist strap that is properly connected to earth ground, see “Safety
Precautions,” earlier in this chapter for more information.
2. Remove the instrument cover.
3. Tape the thermistor plugged into the Motherboard to a calibrated thermometer.
4. Adjust the
GAIN
potentiometer on the Analog to Digital Board until the internal
temperature reading agrees with the value on the calibrated thermometer. Since the
thermistors used in the Model 48C are interchangeable to an accuracy of
±
0.2
°
C,
and have a value of 10K ohms at 25
°
C, an alternate procedure is to connect an
accurately known 10K resistor to the thermistor input on the Motherboard, and
adjust the
GAIN
potentiometer for an internal temperature reading of 25
°
C. Note that
a 1
°
C change corresponds to a
±
5% change in resistance, thus this alternative
procedure can be quite accurate as a check; however, it clearly is not NIST traceable.
5. Re-install the instrument cover.
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