The sensor assembly also provides the means of maintaining the temperature of the sensor at 720°C (1320°F) by
surrounding the sensor tube with a heater element, and measuring the temperature of the zirconia disc with a
thermocouple inside the sensor. (See Figure 2.1)
2.3 THE CARBON DIOXIDE SENSOR ASSEMBLY
The carbon dioxide sensor assembly is mounted on a circuit board in the 9070 cabinet. The CO
2
sensor PCB has a
separate microprocessor to control the operation of the CO
2
cell, maintain calibration and provide a linear output to the
main 9070 PCB. It has been designed to read carbon dioxide concentrations within the temperature range of 5 to 45°C
(40°F to 110°F), with ambient humidity not exceeding 85% RH. A state of the art temperature compensated sensor is
employed to maintain accuracy and reduce the need for calibration. The range of the CO
2
sensor PCB is 0 to 100%.
The 4-20 (0-20) mA output can be scaled to cover other ranges with a minimum span of 10% carbon dioxide.
The principle of operation is that of absorption of the specially designed infra red light source which is passed through
an analysis cell and a thin film filter into a solid state detector. The filter is selective and passes radiation only in the
carbon dioxide absorption wave band. The detector output is amplified and, with no carbon dioxide present in the cell,
is balanced against a reference voltage to give a zero output voltage.
Absorption of infra red radiation by the gas in the cell reduces the detector signal, leading to a positive voltage
appearing at the sensor output. The gain of the amplifier is adjusted automatically, and the signal is digitally processed.
The carbon dioxide level is transferred to the main 9070 microprocessor via a digital link. Span and zero calibration can
be done from the keyboard of the 9070.
The response from the CO
2
cell is non-linear with respect to carbon dioxide percentage, but is linearised within the
microprocessor on the CO
2
PCB.
Calibrate to ZERO & SPAN once every two months for best performance.
2.4 THE ANALYZER
The 9070 oxygen Analyzer is a microprocessor based, auto–calibrating instrument with a liquid crystal display, two 4–
20 or 0–20 mA output signals, a printer/computer port and four alarm relays with a total of 24 alarm functions.
The display will read in either % oxygen or ppm, as selected in set–up step 27. It is capable of calculating the oxygen
volume from less than 0.1 ppm to 100%. The top line of the LCD is used to display the oxygen and the carbon dioxide
content.
The lower line is used to display nine other variables such as sensor temperature, sensor impedance, date/time etc. The
lower line is also used to display alarm messages such as sensor ‘OXYGEN NOT READY’ and ‘A/D CAL ERROR’,
‘HIGH O2’ etc.
Many of the functions are user variable (such as 4–20 mA output channel ranging), and are changed using a menu
system from the keyboard. Even the one–time calibration is performed using the keypad. (See Section 2.8). The
changes are then all stored in a battery-backed RAM module.
2.5 ALARMS
Refer to OPERATOR FUNCTIONS Section 4 for details on alarm functions.
2.6 HEATER SUPPLY FOR THE OXYGEN SENSOR
CAUTION
The oxygen sensor heater is supplied with mains voltage. This supply has electrical shock danger to maintenance
personnel. Always isolate the Analyzer before working with the oxygen sensor.
The sensor assembly must always be connected to earth.
The heater is supplied from the mains power directly, and the temperature is controlled initially at 720°C (1320°F) after
turn on.
Page 12 Teledyne Analytical Instruments 9070 Oxygen Analyzer
