5.3 Multipoint Precision Check
The multipoint precision check involves supplying the instrument with span gas at multiple known
concentrations and recording the output of the instrument. Multipoint calibrations are used to
determine the linearity of the concentration readings over the range of the multipoint calibration.
The instrument gain should not be adjusted to each individual point.
1. Ensure a suitable span source is connected to the instrument from a gas calibrator (Ecotech
recommends the GasCal-1100 with GPT) through the Calibration Port.
2. Record the analyser’s instrument gain before performing a calibration (refer to Section 3.5.14).
3. Perform a precision check using zero air (refer to Section 5.4).
4. Perform a span calibration (refer to Section 5.2).
5. Set up a program for measuring the span concentration through 5 steps down from 80% of
required measurement range.
6. Example for measurement range of 500ppb:
a. Set the 1st concentration on the gas calibrator to 400ppb, allow the instrument to sample for
15 minutes, record the measurement.
b. Set the 2nd concentration on the gas calibrator to 200ppb, allow the instrument to sample
for 15 minutes, record the measurement.
c. Set the 3rd point at a concentration of 0ppm (zero air); allow the instrument to sample for 15
minutes and record the measurement.
d. Set the 4th concentration on the gas calibrator to 100ppb, allow the instrument to sample for
15 minutes, record the measurement.
e. Set the 5th concentration on the gas calibrator to 300ppb, allow the instrument to sample for
15 minutes, record the measurement.
7. The linearity and correlation can be calculated for each point manually or all points calculated
within an Excel spread sheet. Both options are detailed below.
Manual Calculations
Record the concentration measurement at each point and determine the percentage difference
between the instrument response and the supplied concentration using the following equation:
DifferencePercent 100
ionConcentrat Supplied
ionConcentrat Supplied - Response Instrument
Equation 3 – Instrument accuracy
If the difference between values is less than 1% of full scale then the instrument is within
specifications. If not, a leak check and/or service are required (refer to Section 6).
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