IE-WFS-KAL-WiFi-01
5
Calibration (accuracy verification)
To verify the measurement accuracy (calibration) of temperature, relative
humidity, CO
2
concentration and barometric pressure a specific equipment
for generating these quantities (calibration chamber) is required. Calibration
of the instrument is performed by placing sensing element of WiFi sensor
(DUT) and sensing element of standard into calibration chamber. During
calibration are values of DUT and values of standard compared.
Calibration recommendations
The calibration interval is determined by the user of the device. It is based
on the manufacturer's recommendations and according to the requirements
of the application in which is the WiFi sensor used. The manufacturer's
recommended calibration intervals are stated in the User Manual. In some
specific applications, calibration must be performed by an independent
accredited laboratory.
General calibration conditions
The equipment of the workplace must be able to assess the measuring
accuracy of the WiFi sensor by its parameters. As a rule, the instruments are
checked for accuracy specified by the manufacturer, however the user of the
WiFi sensor can determine his own parameters. For each measuring point it
is necessary to calculate the so-called extended measurement uncertainty,
which includes the properties of the whole measuring chain (accuracy of
standards, inhomogeneity of the field in the calibration chamber, resolution
of instruments, etc.). This uncertainty must be better than the required DUT
accuracy in order to evaluate the measurement accuracy of the DUT. The
determination of uncertainties is performed according to document EA-4/02.
For each measuring point, the standard value, the measured value of the
DUT and the measurement uncertainty shall be reported. The evaluation can
be added.
Used standards must be metrologically traceable to the valid higher
standards. A reference gas of known concentration (for CO
2
) or a calibration
solution (for relative humidity) may to be used as the standard. There is an
indicative rule which says that standard should have at least three times
better accuracy than DUT.
When calibrating physical quantities, the sensing element of standard and
sensing element of DUT in calibration chamber should be placed closest as
possible. The distribution of temperature and relative humidity in the
calibration chamber may not be completely homogeneous. Due to improper
placement of sensing elements may be subject to a large systematic error. If
the calibration is carried out at different temperature than ambient, it is
necessary to ensure, that the values are not affected by the heat transfer
