Service and maintenance
9.1 Instrument verification
SITRANS SL
Operating Instructions, 12/2010, A5E01132948-04
153
8.
If you use ambient air, you can correct the verification set point according to the air
moisture. Look for the ambient temperature on the x-axis of Fig. 9-2. Go vertically up to
the line of the present ambient relative humidity. Go left horizontally and find your
verification set point on the y-axis. Note this value in the "Verification Sheet".
With pressurized air of unknown moisture content you there is a higher uncertainty for the
verification set point. A good estimate is ±0.3 Vol%. O
2
. Note this value in the verification
sheet.
CAUTION
Use a hygrometer and a thermometer at the site where you take the ambient air. The
humidity must be measured at the temperature at which the verification is performed. If
for example the outside temperature is 0 °C and the relative humidity close to 100 %,
the relative humidity indoors at normal room temperature might only be around 20 %.
9.
Enter the appropriate values for the verification errors: Instrument error (according to the
instrument specifications), temperature error and pressure error.
Temperature and pressure error depend on your measurement. If the system and the
purging gas used are well tempered you can use the error of the thermometer itself:
1 °C.For the pressure error refer to the reading of your barometer.
10.
Calculate the total uncertainty of the measurement by adding instrument error, pressure
error, and temperature error.
CAUTION
Temperature and pressure influence on the verification procedure
It is very important to enter the accurate values of ambient pressure, temperature and
humidity. As a rule of thumb you need the same accuracy of the pressure or
temperature values as you expect from verification tolerance.
The value of 20.95 % is the relative concentration of oxygen in the atmosphere but the
absolute value will vary due to the barometric pressure or temperature. As an example -
in recent years the barometric pressure in Gothenburg, Sweden, which is at sea level,
varied between a maximum of 1 052 hPa and a minimum of 950 hPa which corresponds
to uncorrected oxygen readings between 21.78 % and 19.67 %. Although these are
extreme values they point out the importance of having an accurate pressure value
inside the tube.