 |
|
How Hydrogen Sensors Work
by Nate Speer
H2Scan, LLC
Hydrogen sensors have been around for many years with the ability
to detect hydrogen, yet with many limitations and drawbacks.
Companies have searched for a way to detect hydrogen reliably,
or monitor hydrogen concentrations for industrial applications
effectively. Worker safety, and process efficiency has forced
industry to develop hydrogen sensors which are specific to hydrogen.
As this industry grows and changes, so must its ability to monitor
H2 in an effective manner.
 The
Basics
The sensors developed by H2Scan detect hydrogen in a range of
concentrations from a few parts per million to 100% hydrogen.
To measure hydrogen in low and high ranges, the sensors utilize
Pd/Ni thin films. The low level sensor is a MOS capacitor with
a Pd/Ni plate for one side of the capacitor, and the high level
sensor is a meandering Pd/Ni thin film resistor. They operate
on hydrogen partial pressure and does not require oxygen. To
improve accuracy, the sensor also has no cross sensitivity to
other combustible gases, including natural gas, methane, propane,
and butane. (Photo right: H2Scan Hydrogen Sensor)
High Hydrogen Level Sensors
To measure high hydrogen levels, the H2Scan sensor uses a Pd/Ni
thin film resistor. The resistance of the H-resistor changes
as a function of the hydrogen concentration and operates up
to one atmosphere of hydrogen and down to 0.005 atmosphere of
hydrogen (0.5% hydrogen in air or nitrogen). Operation at hydrogen
partial pressures up to 35 atmospheres has been demonstrated.
The palladium catalyzes (breaks the molecular bond of the hydrogen
molecule) and the hydrogen atoms attach to sites (palladium
atoms) on the surface of the palladium-nickel thin film. The
hydrogen atoms then diffuse into the bulk of the thin film and
reside in interstitial sites in the metallic structure. These
interstitial hydrogen atoms increase electron scattering and
increase the electrical resistance of the thin film. The change
in resistance is measured to determine the level of hydrogen
that is detected.
Since the resistivity of metals increases with increasing temperature,
temperature sensitivity is managed by controlling the temperature
of the H-resistor. A resistive thin film heater and temperature
sensing resistor are manufactured on the silicon chip. The electronics
measure the resistance of the temperature sensing resistor and
control the temperature to a fraction of a degree centigrade
through the heater resistor. Then , the operating temperature
is chosen to be higher than ambient to provide control and prevent
condensation of water in high humidity operations.
Low-level Hydrogen Sensors
The second embodiment of this technology, the low-level hydrogen
sensor, is configured to measure from 10 ppm to 1% hydrogen
at one atmosphere of pressure. The palladium-nickel thin film
is used to form the MOS capacitor. The Pd/Ni thin film is deposited
on an insulating thin film that forms a dielectric between the
Pd/Ni metal plate and the n-silicon opposite plate. Some of
the absorbed hydrogen atoms in the Pd/Ni reside at the metal/dielectric
interface and the presence of this atom changes the electric
field of the capacitor, changing the capacitance. The change
in capacitance is proportional to the hydrogen concentration
in the sampled gas and sensed with specialized electronic systems.
Summary
For low- and high-level hydrogen sensing, these two implementations
of the catalytic effect of palladium on hydrogen complement
one another. Due to the different technologies, the sensing
ranges overlap and therefore have a wide range of capability.
In addition, each technique addresses its own range and can
stand-alone individually.
In summary, this system is:
* Hydrogen specific
* Insensitive to other combustible gases
* Stable, repeatable operation over a very wide range of hydrogen
concentrations
* Does not need oxygen to detect or monitor hydrogen
* Small, compact, low power.
Whether in today’s present applications or looking forward
to the hydrogen economy, the push towards a quality, hydrogen-specific
sensor is crucial. It will provide needed safety and the confidence
to know that our monitoring and detection issues are handled
in an effective and safe manner.
|
