AMMONIA User Manual
AMMONIA_User 9/09
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The characteristic curve that is stored in the transmitter is, of course, valid for certain specified conditions, such as
the composition of the liquid and the pH value, and individual sensor characteristics also play a part. If conditions
differ from those specified, or different electrodes are used, then this will affect the
displayed value for the concentration. The individual characteristics of the particular sensor are allowed for in the
zero point calibration. First of all, the sensor voltage is measured in the presence of ammonia, and the curve shown
in Figure 5. is shifted accordingly. The slope is calculated according to the Nernst equation. This requires an
automatic temperature measurement with a Pt 100 sensor.
Special aspects
Coloration or opacity of the liquid being measured does not affect the measurement. Most ionic substances in the
sample will also have no effect, since they cannot pass through the hydrophobic membrane. But volatile amines, for
instance, may affect measurement. These include various substances, e. g. methylamine, ethylamine and hydrazine.
Such substances also exhibit an alkaline reaction in the sensor electrolyte. The presence of such substances in
addition to ammonia leads to an erroneously high concentration reading.
If the liquid being measured contains solvents or tensides from cleaning agents or detergents that can affect the
hydrophobic properties of the membrane, then measurements should be made in a gas space above the liquid. This
“head space” method requires that the ammonia sensor is built into a completely gas-tight fitting. A head space
measurement is also advisable if oil or grease is present, since these would also block the membrane and thus
adversely affect the response time of the electrode.
The sensor membrane is very delicate and must not be touched in any circumstances. Cleaning by mechanical means
is not possible. Measurements cannot be performed under pressure, because the compression of the inevitable air
bubbles in the sensor electrolyte can cause distortion and damage to the inelastic membrane.