Sensors for measuring hydrogen
For the measurement of hydrogen concentration in gases,
the high thermal conductivity of hydrogen can be used. Hydrogen conducts heat much
better than air (factor 7), nitrogen and most other gases. Only Helium has also a
similar high heat conductivity (6 times higher than air).
In the measuring cell a metallic wire is heated by electric current. The surrounding gas cools the wire and after short time a temperature equilibrium is reached. The temperature of the metallic wire determines its resistivity, which is measured by a Wheatstone measurement bridge. A Wheatstone measurement bridge compares the resistor which should be measured (in this case, this is the measurement wire of the H2 measuring cell) with a reference resistor. The bridge can recognize very small changes in the resistance and measure it precisely. If the test gas contains hydrogen, then the temperature of the wire and therefore also the DC voltage output (4-20mA) of the H2 sensor changes.
The H2 sensor from Stange is such a thermal conductivity gas sensor.
The special feature of this hydrogen sensor is that the heating wires,
which change resistance when the hydrogen concentration changes, are located in closed quartz glass cuvettes.
In the cuvette, a gas movement takes place by convection.
However, there is no direct connection to the reactor or vessel or the gas line in which the gas to be measured is located.
The gas to be measured, the hydrogen H2, consists of very small molecules which easily diffuse through many materials -
also through the wall of the cuvette. In contrary to the very small hydrogen molecules, the other molecules which
are present in the test gas and which are always much larger, cannot succeed to diffuse into the cuvette.
This fact effectively protects the measuring wire from contamination and damage.
Therefore, hydrogen can be measured in atmospheres containing other gases,
which might damage the measuring wire in a normal measuring cell.
This is a big advantage of this patented measuring principle.
The lifetime of the sensors is very high and the measurement accuracy is maintained for a long time,
despite the difficult conditions in the gas to be measured.
However, the hydrogen measurement takes some time.
Due to the necessary H2 diffusion into the measuring cell, the sensor only responds with a delay of 6-20s.
If this circumstance is not a problem then the Stange H2 sensor is the perfect measurement method for hydrogen in foreign gases of all kinds.
This sensor is appropriate for example for usage in nitriding furnaces.
For the evaluation of the test signal and the recalculation to a display, showing hydrogen concentration, you can use a Stange controller, your own temperature controller which is installed already on your furnace or any other suitable display device.
Everywhere, where combustible gases are used, it is important to check the production area.
Especially in the ambience of
conveyer furnaces or
industrial furnaces, which are used for
hydrogen anneal it can happen that in case of a failure,
that critical gas concentration is reached in the ambiance. With the ambient air gas sensor such events can
be supervised and monitored effectively.
This sensor works with a catalytic bead in the measurement cell. The heated pellistor causes catalytic oxidation of available flammable gases by air on its surface. In the same manner as within the heat conductivity sensor the resistivity of the heating wire is measured by a wheatstone measurement bridge. However in this case, not the cooling of the wire is determined but the heating of the wire, cause by the chemical reaction of the hydrogen or inflammable gas with air at the surface of the pellistor.
The ambient air sensor reacts to
The sensor is calibrated on 60% of the lower explosion limit of hydrogen. This corresponds to a concentration of 2,5% hydrogen in air. This gas concentration is still harmless, but an indication for threatening danger. Hydrogen is always taken as reference gas, thus safe alarm limits are ensured also for other gases. Up to three sensors are attached to one base station. In the event of failure, a potential-free contact is closed. This signal can be further processed in the controller of the furnace.