Siemens ULTRAMAT 23, Manual, Page: 162 / 244

Application note
9.1 H2S sensor with 'small' measuring range
ULTRAMAT 23
162
Manual, 3/2016, A5E37100388-003
Sample switchover between raw gas / pure gas
The sensor only functions correctly if the values at the measuring point do not greatly differ
from one another. We cannot recommend switching over between measuring points on the
raw gas side (high H
2
S concentration) and the pure gas side (low H
2
S concentration), since
the difference between the H
2
S concentrations of the two gas flows is too large for a reliable
measurement.
Pressure influence
Abrupt changes in pressure must be avoided. Although the sensor compensates pressure
variations within approx. 20 seconds, it cannot handle pressure surges which may occur
when switching over between samples.
Flow
The sample gas should flow continuously and at a constant rate, also during an AUTOCAL.
While an AUTOCAL is being carried out, the sample gas flow must be diverted by means of
appropriate valve switching.
Reason: H
2
S is highly soluble in water and accumulates in the condensate. This
accumulation increases as the sample gas pressure increases. This effect can result in
significant delays in the response time.
Gas moisture
The calibration gas must have the same moisture as the sample gas.
If the sensor is used with a very dry gas for a longer period, e.g. when feeding biogas into
the natural gas network, it is necessary to carry out an AUTOCAL with ambient air every 60
minutes. The dew point of the air should be in the range of approx. 9 °C ... 12 °C
(48 °F ... 54 °F). The AUTOCAL purge time should be at least 5 minutes. This prevents
premature drying-out of the sensor.
H
2
influence
The internal design of the 5/50 ppm sensor means that it is immune to the influence of H
2
.
NH
3
influence
Loading of 300 vpm NH
3
results in destruction of the H
2
S sensor within 2 to 3 days.
AUTOCAL / zero
An AUTOCAL of the zero point should be carried out every 60 minutes. On the one hand,
this is used to protect the sensor, but it also compensates the influence of temperature
variations during the day.