Next, add the two true concentration values obtained from above and divide them by 2 (known as
proportional allocation). The resulting value shown in the shading area in the table on the next page is
0.425%. This is the true concentration factor at 35
º
C (95
º
F) at a reading of 0.5%
Correction for Humidity
If the tube reading requires humidity correction with the measurement temperature of 0
º
C to 40
º
C
(32
º
F to 104
º
F), read the ambient humidity at the sample point using the no.6 Gastec Water Vapour
detector tube.
Example of Humidity Correction
If the reading on the nitric acid detector tube (No. 15L) is 5ppm and the measured value on the water
vapour detector tube is 6mg/l, calculate the true concentration by multiplying the reading with the
appropriate correction factor in the table below.
True concentration = 5ppm x 0.68 =
3.4
(ppm)
