Serinus 44 User Manual 3.0
5.6 Multipoint Precision Check
A multipoint precision check is used to determine the linear response of the instrument across its
operating range. The instrument is supplied with span gas at multiple known concentrations,
typically a zero point and at least four up-scale points, spread across the operating range of the
instrument. The observed concentrations are compared to expected values and the linearity of the
instrument assessed against local applicable standards.
Note: The instrument is inherently linear and the instrument gain should not be adjusted at each
individual point. Non-linearity indicates a problem with the instrument (refer to Section 7). Ensure
that the instrument has been running for at least one hour before any calibration is performed to
ensure the instrument’s stability.
Several methods for producing multiple known concentrations are available to use, such as
connecting multiple certified gas cylinders at different concentrations. However, Ecotech strongly
recommends the use of a dilution calibrator and a certified cylinder of NO at an appropriate
concentration (typically a cylinder of 40 to 100 ppm NO, balance in Nitrogen).
1. Connect your calibration system to the Calibration Port of the instrument (Ecotech recommends
the Serinus Cal 2000, refer to Figure 45).
2. Generate and record the displayed span concentrations for (at least) five stepped points (of
known concentrations) evenly spaced across the instruments measurement range (see the
example below).
3. Then using a program such as MS Excel, create an X Y scatter plot of expected concentration
versus the recorded instrument response and use linear regression to calculate the line of best fit
and the correlation factor (R
2
) – refer to the users local applicable standards.
Example for an instrument measurement range of 500 ppb:
a. For the 1st concentration, set the gas dilution calibrator to supply 400 ppb NOx gas to the
instrument.
b. Allow the instrument to sample the calibration gas until a prolonged stable response is
achieved (the amount of time this takes is impacted by the calibration setup). Record the
instrument response.
c. Repeat the above steps using concentrations of 300ppb, 200ppb, 100ppb and a Zero point.
d. Graph the results and use linear regression to determine a pass or fail as per your applicable
local standards.
Note: To highlight hysteresis errors, it is advisable to run the multipoint check in both descending
and ascending order - refer to applicable local standards.
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