Detcon TP-424C - Manual (Page 12)

Detcon Model TP-424C Hydrogen Sulfide Sensor PG.12
3.9 C ALIBRATION F REQUENCY
In most applications, monthly to quarterly calibration intervals will assure reliable detection. However,
industrial environments differ. Upon initial installation and commissioning, close frequency tests should
be performed weekly to monthly. Less frequent test schedules should be implemented based on analysis of
tests prior to adjustment.
3.10 H EATER V OLTAGE A DJUSTMENT
Each sensor is pre-calibrated and tested prior to shipment. The heater voltage set point is marked on the
bottom of the stainless steel sensor housing. This temperature is referenced to 72° F ambient. Slight varia-
tions in heater voltage will occur in higher or lower ambient temperatures. Typical variation from 72°F ref-
erence point is + or – 3% of original value between 32°F - 100°F. For a sensor set for 5.25 VDC at 72°F, the
heater voltage change at 100°F would typically be –.15 VDC; and at 32°F would typically be + .15 VDC.
The general effect of the heater function is such that a lower voltage will cause a slower, but more stable
long-term repeatability to gas, while a higher voltage will cause a slightly quicker response but a less stable
long-term repeatability. If it should become necessary to elevate the heater voltage to maintain response
and clearing characteristics, do not exceed 5.75 volts in 72°F temperature point. Note that any adjustment
to the heater voltage will require that the sensor be recalibrated with H2S gas after 24 hours of stabilization
time.
The heater voltage is adjusted via the “TEMP” potentiometer and is measured at the two test points
labeled “VOLT”.
Optional Heater Power Adjustment Instructions
A new feature included with the latest revision of the TP-424C allows for a more accurate adjustment to be
made to the sensor heater circuit. This feature is based on power rather than voltage. Setting the sensor
heater voltage, as measured at the “VOLT” test points, is an acceptable method. However, the new power
feature is an option that allows the user to calculate and implement a more precise method of temperature
control to the sensor over longer periods of service. After several years of service the sensor heater film
may degrade and require more precise power level adjustment.
The main objective of the sensor heater circuit is to elevate the temperature of the solid state sensor ele-
ment to between 500 and 527°F. This temperature provides the best combination of speed and stability for
detecting H2S gas. Because the sensor heater resistance increases with age, the power being applied to the
sensor decreases even if the heater voltage remains constant. This decrease in power also reduces the sen-
sor temperature. Thus, a constant power being applied to the sensor will render better performance than a
constant voltage.
The desired power setting which is measured in milliwatts, will vary depending on ambient temperatures.
If the ambient temperature is 72° F, the desired power setting should be 235 mW, ±10mW. On hotter days
when the ambient temperature is 100°F, the optimal power setting should be 225mW, ±10mW. A cold day
with an ambient temperature of 32°F, would call for a power setting of 250mW, ±10mW.
To set the sensor heater for power rather than voltage, follow the instructions below.
Note 1: Power (P) is the result of multiplying the heater voltage “VOLT” (V) by the heater current “CUR-
RENT” (I). P = V x I
Note 2: Because the heater current is routed through a 1 ohm resistor located between the test points
labeled “CURRENT”, it is not necessary to set your volt/ohm meter to measure current to obtain the prop-
er reading. Your meter should be set to measure millivolts and the reading is simply interpreted as heater
current in milliamps.
1 - Measure the heater voltage at the test points labeled “VOLT”. (example = 5.25V @ 72°F)
2 - Next, measure the heater current at the test points labeled “CURRENT”. (example .045 A)
Thus, 5.25 (volts) x .045 (amps) = .236 (watts or 236 milliwatts). This falls within 10mW of the target power