TGS2442 Datasheet, PDF (23/25 Pages) List of Unclassifed Manufacturers – Detection of Carbon Monoxide

CALIBRATION OF TGS2442
Appendix-Calibration at two or more concentrations Following these procedures, each sensor can be fully
Detectors which display CO concentration demand ^{compensated for temperature and sensitivity slope.}
more accurate calculation of CO concentrations. To
obtain sufficient accuracy, calibration should be ^{4. Ability of Sensor to Meet Performance Standards}
conducted at two or more CO concentrations accord- This section will show how TGS2442, if used in the
ing to the signal processing procedures in Section 2. manner described in this document, can satisfy
For illustration, an example of 300ppm and 100ppm CO␣ detector performance standards such as UL2034
of CO as calibration points is used in this section. and the CSA 6-96 standard. The following
The procedure of compensation according to β grade
in Section 3-2(b) may be omitted. Instead, ƒ(Rs^x) of a
second CO concentration (300ppm in this case) is
obtained by following Steps 1~ 4 in Section 3-3. By
this procedure, the sensor’s β value can be obtained
for each detector from the following equation:
β = ƒ(Rs^x)(300ppm) / ƒ(Rs^x)(100ppm)
assumptions are made for this purpose:
* no error exists in calibration gas concentration
* no adjustment error in VRadj exists
* the gas sensitivity curve is linear on a logarithmic scale between
60-600ppm of CO
* no error exists in temperature compensation
Under these conditions, accuracy of calibration will
depend upon the accuracy of ƒ(Rs) and β. When
calibration is done using two or more CO
In turn, this β value can be converted into an α value ^{concentrations (Section 3-Appendix), calibration}
by using the equation shown in Section 3-2(b). This α ^{without error can be done at the targeted gas}
value (αmem) is temporarily stored in the RAM␣ of the ^{concentration. However, if calibration is done only}
microcomputer (see Sec. 2-3(d)).
at one gas concentration and is based on one of the
combinations of Rs and β grades supplied for the
To compensate for the slope of the sensitivity curve, _{sensor, since β falls within a range for each grade,}
a slope compensation circuit shown in Figure 10a can _{verification that such variation in β falls within}
be used. Since the range of values for V7 is from 0 ~ _{acceptable limits is required.}
5V and the range of α values is from -1.4 ~ -0.6, a
linear relationship between the V7 and α can be ^{4-1 Requirements of performance standard}
established (see Figure 10b). Then, in order to ^{The most stringent performance standard (CSA 6-}
compensate for α value, the V7 port is used--adjust ^{96) requires that an alarm be generated at not less}
the potentiometer (VRadj2) so that the αcalb value is ^{than 5% COHb but less than 10%␣ COHb. The}
set as close to αmem as possible. This procedure can ^{specified CO␣ concentration and accumulation times}
be simplified by directing the microcomputer to
activate an LED when α is set to the correct level:
For example: If αcalb = αmem ± 3%,
then “LED TURN ON” else “LED TURN OFF”
-0.6
103.0
53.6
23.2
5% COHb
10% COHb
Center (7.07% COHb)
C(min) C(max)
αcalb = .16(V7)-1.4
14.9
10.9
7.14
-1.0
VRadj2
Figure 10a - Slope
compensation circuit
-1.4
0
1
2
3
4
5
V7 (V)
Figure 10b - Relationship of slope (a)
to V7
Revised 04/01
65 100
200 300 400 600
CO Concentration (ppm)
Figure 11 - Alarm times permitted by performance standard
8