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4.21 Dust in the Flue Gas
For unheated probes with no filter, entrained solids or dust in the flue gas does not present a problem unless
the dust, when settled, is not porous. Allow the dust in the process to build up on the probe. It will form a
porous layer slowing the response time. To avoid mechanical abrasion of the electrode material in
installations with unheated oxygen probes, pack 'SAFFIL' or equivalent alumina based ceramic fibre in the
sensing holes to protect the electrode. Do not use silica based ceramic fibres such as 'KAOWOOL', which
can attack the electrode at high temperatures. Once the dust has built up the response time of the probe will
be slower.
For heated probes the preferred method of mounting for dust-laden applications is facing vertically
downwards with the filter removed. Probes can also be mounted horizontally with no filter with some dusts.
An occasional automatic back purge is helpful in this case.
Normally heated probes are supplied with filters for applications with particulates in the flue gas. The probe
response time should be tested when the probe is first installed, and then regularly until it remains constant
for a significant period. Filter purging should be set up on the time periods determined by these tests. To
test the probe response time, use a stopwatch to obtain the time for a probe to achieve a 63 % change from
one reading to another. If a probe filter blocks completely in a short period of time, then there is no option
but to use the probe without the filter. A trial probe with filter can be installed to test whether a filter blockage
is likely to occur.
4.22 Stratification
If the transmitter and probe have been fully tested and the oxygen readings in the flue gas are incorrect, gas
stratification may be occurring. The phenomena cannot be anticipated for any particular installation.
Generally, large flues have oxygen differences of approximately one percent across the flue. Occasionally
an oxygen error of several percent may occur in a flue of any size. Moving the probe to a new location
normally solves this problem.
The effects of stratification can be reduced by using two probes and averaging the two oxygen readings.
This can be achieved within a Novatech 1732 transmitter controlling two probes.
4.23 Connecting a Pressure Transducer
If the process gas pressure varies more than 4" WG and therefore requires automatic compensation,
connect a pressure transducer as shown below.
Place a link across LK2, near to the input terminals 24 & 25. If the link is not populated, you can achieve
the same thing connecting a 120Ω resistor across the two terminals.
A pressure change of 4" WG will cause a change in the calculated oxygen reading of approximately 1%.
Pressure Transducer
4-20mA Output
The shield wire must be connected to the
EARTH screw in the 1735 transmitter case
There are no calibration adjustments for the zero or span of the pressure transducer input.
Set the transducer scale range in the Commissioning Menu - See chapter 7.2.9 Flue Pressure Input Units
and Value. The pressure can be displayed on the lower line by enabling the corresponding option. See
chapter 6.4.2 Lower Line Items
August 2018
1732 Oxygen Transmitter
+
-
1735 Transmitter
24 BFT+
Optional 120 ohm resistor
(use if LK2 is not populated)
25 BFT -
Technical Manual
27
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