TGS2442 Datasheet, PDF (19/25 Pages) List of Unclassifed Manufacturers – Detection of Carbon Monoxide

CALIBRATION OF TGS2442
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2-3. Calculation of CO concentration
Figure 4 illustrates the process for calculating CO
concentration from V1, V3 and V4 signals.
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Sample V1, V3, V4
40%RH
a. Calculation of ƒ(Rs) value
b. Temperature compensation of ƒ(Rs) value
c. Compensation for Rs grade variation
d. Compensation for β grade
e. Convert to CO concentration output
Figure 4 - Signal processing flow for
calculation of CO concentration
1
0.1
0.01
a. Sensor resistance (Rs) calculation:
To represent sensor resistance (Rs) value in the ₁₀₀
microcomputer, the expression ƒ(Rs) shall be used:
ƒ(Rs) = ( 5 - V1 ) / V1
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b. Temperature compensation of ƒ(Rs)
In order to compensate for the temperature
dependency of ƒ(Rs) value, a temperature compen-
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sation coefficient (KTemp) must be determined.
0˚C
15˚C
25˚C
35˚C
40˚C
50˚C
100
1000
CO concentration (ppm)
Figure 5a - Temperature dependency of Rs
(Ro = Rs at 100ppm CO, 25˚C/40%RH)
40%RH
KTemp = Rs/Ro, where
Rs = actual sensor resistance in various conditions
Ro = actual sensor resistance under standard conditions
Using the circuit condition and recommended
thermistor in Figs. 1 & 2, measure actual Rs values
at various ambient temperatures and sample
thermistor output (V4). The correlation between V4
and KTemp can be obtained by this procedure and
should be written as a table of coefficients into ROM
on the microcomputer. Then, by monitoring V4
output during normal operations, the proper KTemp
value can be selected from the table and used to
temperature compensate ƒ(Rs) values according to
the following formula:
ƒ(Rs^x) = ƒ(Rs) / KTemp
ƒ(Rs^x) represents temperature compensated ƒ(Rs).
Table 3 shows an example coefficient table for temp-
erature compensation where measuring 100ppm of
CO in 25°C/40%RH is used as a standard condition.
Figures 5a and 5b show typical values of ƒ(Rs) before
and after temperature compensation.
0.1
0~50˚C
0.01
100
1000
CO concentration (ppm)
Figure 5b - ƒ(Rs) value compensated for temperature [ƒ(Rs^x)]
(Ro = Rs at 100ppm CO, 25˚C/40%RH)
V4 Input Value
4.15
3.92
3.66
3.37
3.06
2.75
2.44
2.14
1.86
1.61
1.38
1.18
1.01
0.86
0.73
Temp (˚C)
-10
-5
0
5
10
15
20
25
30
35
40
45
50
55
60
KTemp Coefficient
2.72
2.34
1.99
1.70
1.46
1.27
1.12
1.00
0.909
0.839
0.786
0.744
0.712
0.687
0.668
Table 3 - Table of temperature compensation coefficients
Revised 04/01
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