ZIROX XS22, Руководство пользователя

Страница: 22 / 27

Measuring system XS22-E2010 9 Use of ZrO
2
-probes
22
XS22_E2010
The following applies:




⋅ − ⋅
⋅
=
K T
mV U
p
p
L
O
/
/
42 . 46 exp
2064 . 0
2
ϕ
.
9.2 Instructions for calibration
When operating in vacuum differences in the electrodes’ temperatures
usually cause indeterminable thermoelectric voltage which depends on the
respective production unit. This thermoelectric voltage adds to the signal
voltage which corresponds to the Nernst equation.
For exact measuring of the oxygen partial
pressure the
measuring system
must be calibrated with
a pressure gauge at the
production unit filled
with a known gas
(preferably pure
oxygen).
The line represents the
signal voltage U which
is expected for the
oxygen pressure scale
(x-
coordinate) at the probe with air as reference gas. The measured values
represent the probe signals after the gradual evacuation with different pumps
over the pressures measured in a higher pressure range with a capacity
meter, in the medium pressure range with the Pirani device and in the lower
pressure range at a cold cathode. The jumps between the different ranges
show that the pressure gauges were not adjusted and provide faulty results
themselves. It shows, however, that the probe works as
expected in a range of 6 decimal powers. The displayed range extends from
approximately a hundredth of the normal pressure to 10 billionths of this
pressure (corresponding to 10 Vol.-ppb).
The maximum deviations from the theoretical run were approximately 25 mV
or half a decimal power of the oxygen partial pressure. Since the deviations
are both positive and negative, a distinct explanation is impossible. In the
lower pressure range the deviation points to the range which is expected
when oxygen moves from the reference air side to the measuring gas side.
Therefore, calibrations should only be conducted with reliable pressure
gauges in the field of probe operation.