TGS2442 Datasheet, PDF (17/25 Pages) List of Unclassifed Manufacturers – Detection of Carbon Monoxide

CALIBRATION OF TGS2442
In order to meet the various regulations to which
Power ON
CO␣ detectors are subject, circuit design and method
of calibration should be based on the performance of
Alarm delay for initial action
the gas sensor. Recommended procedures for usage
of the TGS2442 CO␣ sensor are set forth in this
V1~V6 sampling
document.
1. Basic Circuit Structure
Generate trouble ^Yes
signal
Sensor trouble
detection
Circuit structure needed to satisfy performance
No
standards should provide for complex signal process-
CO calculation
ing, necessitating usage of a microcomputer with
features such as those shown in Table 1. This
microcomputer will process multiple signals from the
sensor, a thermistor (for temperature compensation),
Display CO concentration *
Convert to COHb
concentration
Main routine
(1 sec. cycle)
and a potentiometer (for calibration). The microcom-
puter will output a control signal for alarm operation.
Yes
Generate alarm
Alarm
determination
The following six types of output signals should be
monitored (refer to the sample circuit diagrams and
No
Suppress alarm
timing charts in Figs. 1 and 2 on the facing page):
* if applicable
V1: Sampling voltage for gas detection
V2: Sampling voltage for sensor element trouble detection
V3: Sampling voltage for reference voltage
V4: Sampling voltage for temperature compensation
V5,V6: Sampling voltage for detecting heater trouble
Figure 3 - Basic flow of detector operation
V6 are used for detecting sensor-related trouble and
heater driving circuit malfunction while V1, V3 and
V4 are used to calculate CO gas concentration. This
CPU core
4 or 8 bit microcomputer
(with 2~8MHz clock)
concentration is subsequently converted by time-
weighting into a COHb concentration (the key
Memory size
2~4k byte ROM
128~256 nibble or byte RAM
measurement used in performance standards for
determining the generation of alarm signals).
Pin size
Option
Mode
selection port
28~32 pins
8-bit A/D converter
(more than 3 channels)
high current direct drive
(more than 6 ports)
select calibration or normal
operation modes
2-2. Detection of sensor-related trouble conditions
The sensor trouble mode should indicate that the
sensor’s heater has broken or that the sensor element
itself has been damaged. The trouble signal for heater
breakage can be detected by an abnormal rise in
Table 1 - Recommended microcomputer features
heater resistance or as a result of lead wire breakage,
transistor problems, and/or short of the heater. The
2. Method of Signal Processing
2-1. Basic flow
Figure 3 shows the basic flow of detector operation.
To avoid potential nuisance alarming during the
sensor’s initial action period, a warm-up period of
several minutes (alarm delay) should be utilized
sensor element trouble signal is generated by damage
to the sensing material or breakage of a lead wire.
These phenomena produce an extreme change to the
values of V5, V6 and V2; consequently monitoring
V5, V6 and V2 enables detection of sensor-related
trouble conditions. See Table 2 for recommended
conditions for monitoring sensor-related trouble.
upon initial powering of the detector. After this
period, the program in the microcomputer starts the
main gas detection routine. During the gas detection
Heater trouble
Sensor element damage
V5 ≥ 0.1V
V6 ≤ 4.5V
V2 ≤ 0.15V
routine, the above mentioned six output signals are
acquired during each one second interval. V2, V5 and
Table 2 - Conditions under which an trouble signal
should be generated
Revised 04/01
2