11
the signal. After this, the signal will stabilize with an initial rapid
and then a more slow decrease. Wait until the signal is
below
20 mV and stable
. This may take many hours, e.g. 12 hours (see
below). If the sensor signal does not reach this value, please go
to the Troubleshooting section.
In general the sensor should be polarized and allowed
to stabilize for as long as possible before calibration
and measurements to get maximum stability. Unisense
recommends that you perform the pre-activation the day
before measurements and then leave the sensor polarized
overnight. If the signal does not stabilize or is too high or too
low, see ‘Trouble-shooting’.
C
ALIBRATION
PRINCIPLES
Calibration must be performed after the sensor signal has
stabilized. The N
2
O sensors respond linearly to the N
2
O
concentration within their measuring range (see Table 1) and
signals can thus be linearly converted to partial pressure.
Check and repeat calibration at appropriate intervals.
Shortly after taking a sensor into use, the appropriate
interval may be 2 hours; when the sensor has been used
for some time, it may be 24 hours. To minimize the need
for calibrations, keep the sensor polarized between
measurements, unless the time between measurements
exceeds several days.
The membrane permeability of nitrous oxide microsensors
changes with time, so a change in signal of up to 50% may
occur over months.
C
ALIBRATION
It is recommended to use the Unisense N
2
O Sensor Calibration
Kit for calibrating the N
2
O sensors (
https://www.unisense.com/
calibration_kits/
). This kit ensures accurate and simple calibration
both in the lab and in the field. The calibration kit can be shipped
as normal cargo and does not require dangerous goods shipping.
NOTE
The lifetime of
the
N
2
O
sensor
is the same
whether the
sensor is contin-
uosly polarized
or store
un-polarized.
IMPORTANT
Calibration
must first be
performed after
the sensor signal
has stabilized.
