Instruction manual

10.0 OTHER FUNCTIONS SM425 INSTRUCTION MANUAL
10.1.1.3
10.1.1.4
10.1.1.5
10.1.1.6
Robustness of a Cell
The robustness of a cell is an indicator of the remaining life of a sensor. It is expressed as one of four
time periods the cell may still be used:
1. more than a year
2. more than six months
3. more than three months
4. less than one month
The above four time periods are for preventive maintenance only.
This cell’s robustness is a function of the response time, the cell’s internal resistance, and calibration
factor. The robustness indicator requires the zero and span points to be set during calibration.
Cell Temperature
This indicates the cell (sensor) temperature, usually 750°C. Cell temperature is a function of the
thermoelectromotive force and cold junction temperature described below.
C. J. Temperature
This indicates the detector terminal box temperature, which is the cold junction temperature of the
thermocouple measuring the cell temperature. The maximum C. J. temperature allowed in the SM425 is
150°C. If the terminal box temperature exceeds this, move it away from the heat source.
Cell Voltage
The cell (sensor) voltage is an indicator of the condition of the sensor. The cell voltage indicates the
oxygen concentration currently being measured. If the indicated voltage approximates the ideal value
(corresponding to the measured oxygen concentration), the sensor is assumed to be normal.
The ideal value of the cell voltage (E), when the oxygen concentration measurement temperature is
controlled at 750°C., is expressed mathematically by:
E = -50.74 log (Px/Pa) [mV]
where, Px: Oxygen concentration in the measured gas
Pa: Oxygen concentration in the reference gas, (21% O
2
)
Table 10-1 shows oxygen concentration versus cell voltage.
Table 10-1 Oxygen Concentration vs. Cell Voltage, (cell temperature: 750°C)
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