Vaisala GM70 - Manual (Page 49)

Chapter 10 _____________________________ Calibrating and Adjusting the Probes
VAISALA __________________________________________________________ 49
Adjustment with Reference Gases
Equipment Needed
For the adjustment procedure with gases, you need the probe to be
adjusted, accurate reference gas(es), a pressure regulator, a flow meter, a
field check adapter (can be ordered from Vaisala) and flexible tubing
with 3 mm (1/8") inner diameter. See section Accessories on page 65.
Reference Gases
To achieve reliable results reference gases shall be traceable to
appropriate standards (for example NIST).
- If adjustment is done by using one reference gas (one-point
adjustment), concentration of the gas should be near to the
concentration values in which the device is used.
NOTE It must be noted that when adjustment is done only in one point, this
results high accuracy only near to the adjustment point, not necessarily
over the whole measurement range.
- When using two reference gases (two-point adjustment), the reference
gas concentrations should represent the low-end and the high-end of
the measuring range. The low-end concentration should be less than
20% of the measuring range of the probe. The zero gas can be N
2
gas
with a purity of N5.0 (or better). The high-end gas can be mixture of
CO
2
and N
2
with concentration near to the upper limit of the
measuring range, being 110% of the range at maximum. Accuracy of
the concentrations should be 1% (or better). Difference between the
reference gas concentrations should be more than 20% of the
measuring range.
Two-Point Adjustment Procedure
CAUTION Please take special care regarding the following when carrying out the
adjustment:
- Check that you give the correct reference concentrations in the correct
units.
- Check that the reading has really stabilized before accepting the
reading.
You cannot revert the factory settings after the adjustment!