When the demarcation of the colour change layer is pale.
Read the value in the middle between the dark layer end and the pale layer end. In this exaggerated
example, the reading should be 5, which is in the middle between 4 and 6.
Correction for Temperatures
If the tube reading requires temperature correction within the measurement temperature of 0
º
C to
40
º
C (32
º
F to 104
º
F), read the temperature of the sample point at the increment of 5
º
C (9
º
F). The
measurement temperature refers to the tube temperature, and not the sample temperature, but this
is usually the ambient temperature, since the tube temperature must be assimilated of the ambient
temperature before measurement.
Example 1: Temperature correction using correction factor
To find the true concentration when the tube reading is 2.5ppm at 15
º
C (59
º
F), first using the table
below obtain the correction factors for the temperatures 10
º
C (50
º
F) and 20
º
C (68
º
F). Then add these
two factors and divide the result by 2 (known as proportional allocation). Finally, multiply the reading
by the calculated factor.
Proportional allocation: (1.20 + 1.00) ÷ 2 = 1.10
True concentration = 2.5 (ppm) x 1.10 =
2.75
(ppm)
Example 2: Temperature correction using the correction table
To find the true concentration when the tube reading is 0.5% at 35
º
C (95
º
F), first using the table
below, obtain the true concentration factors for the tube reading at 0.6% at the temperatures of 30
º
C
(86
º
F) and 40
º
C (104
º
F). Then add these two factors and divide the result by two (known as the
proportional allocation). Repeat the step again for the tube reading at 0.4%.
