Development of a Hydrogen Monitor for Use In Coal Mines

Transmission of light through a palladium film changes when the film is exposed to and absorbs hydrogen. The changing transmission can be related to the hydrogen concentration and forms the basis for a hydrogen sensor.    To be useful in coal mines, the hydrogen sensor must be capable of detecting percentage concentrations of hydrogen in the presence of approximately equal concentrations of carbon monoxide. Unfortunately, at room temperature, carbon monoxide is adsorbed onto the surface of palladium films reducing the rate of hydrogen absorption (and hence sensor response rate) to unacceptable levels.

It has been found that by heating the palladium film, carbon monoxide adsorption decreases and hydrogen absorption (sensor response) is again rapid and reproducible, though reduced in magnitude. At elevated temperatures the palladium film 'oxidises' with time and this alters its response characteristics. Coating of the palladium film with gold has been effective in preventing oxidation, however the benefits are only short-term and are lost because the gold capping film diffuses into the palladium, once more leaving the surface exposed to oxidation. A silica 'diffusion' barrier (~5Å) was placed between the gold and palladium films. This barrier slowed down the rate of 'oxidation' but was not able to completely prevent it.

In its final form, it was found that the film could be operated at approximately 55°C with an acceptably low level of 'oxidation' (baseline drift), while a temperature of 70°C was required to obtain an acceptable response rate in the presence of carbon monoxide.

Thus in its current configuration the sensor is not viable. However the research program has lead to the development of a multilayer film which is 'oxidation' resistant at near ambient temperatures and thus applicable to situations where carbon monoxide interference is not a problem.

Project Objectives

To build a viable hydrogen detector, it was necessary to pursue two different research objectives in parallel:

• Produce a palladium film which would reproducibly respond to hydrogen in the presence of carbon monoxide 
• Build detector electronics which could measure small changes in optical transmission and whose signal would show little to no drift over extended periods of time.

Conclusions

Palladium Film

The use of elevated temperatures to overcome carbon monoxide interference with hydrogen detection has not been successful due to the slow absorption of oxygen by the palladium film at these temperatures. Thus in its current configuration the sensor is not viable. However, the research program has lead to the development of a multilayer film which is 'oxidation' resistant at near ambient temperatures and thus applicable to situations where carbon monoxide interference is not a problem.

Work Program Description

In developing the hydrogen detector it was necessary to undertake research on two fronts simultaneously - development of a sensor film and development of the supporting electronics.

Sensor film development required optimisation of response rate and magnitude and elimination of the carbon monoxide interference. A suitable substrate for the film had to be found. Various thicknesses of palladium and gold films were examined. It was also necessary to develop calibration curves to enable conversion of the sensor's output (a voltage) to a hydrogen concentration. Stability tests were required to determine the detector's response characteristics after extended periods of use. It was also necessary to assess the effects of low-level impurity gases (e.g. sulphides) on the sensor's performance.

Successful commercialisation of the optoelectronic hydrogen sensor required optimisation of sensor head design not only in terms of signal stability but also with respect to simplicity of fabrication.

It was necessary to develop a detector circuit capable of accurately measuring the small change in transmission of the palladium film upon absorption of hydrogen. Moreover the output from this circuit had to be sufficiently stable so as to exhibit negligible drift over the period of a shift (8 hours). Since the palladium film was to be heated, it was necessary to develop a heating/temperature control circuit.

A signal conditioning circuit was necessary to accept the output from the detector circuit before feeding it to the microprocessor. The microprocessor had to convert the analogue signal from the signal conditioning circuit to a digital value which it then processed and displayed on an LCD display together with the appropriate alarms and warnings.

 The electronic circuitry and associated display and alarms had to conform to the Australian Standards 2380-1,7 and 2275-1,2 which cover the design criteria for this type of device. Selection of the packaging, which includes function buttons, for the detector was also important as it had to be robust, dust tight and thus suitable for use in coal mines.

Potential for Industrial Application

While the palladium film sensor is not suitable for use in the presence of percentage levels of carbon monoxide, it is suitable for hydrogen detection in 'normal' industrial situations, for example in petrochemical plants and battery rooms. With its unique triple film combination, this detector overcomes the limitations of current palladium film based hydrogen detectors, which have reproducibility problems, presumably as the result of palladium 'oxidation'. Crystalaid Pty Ltd is currently evaluating the market for such stand alone hydrogen detectors.

Technology from a previous project NERD&D project No. 1304 which dealt with a mine heat up monitor incorporating a commercial low level hydrogen detector has been transferred to Crystalaid and is the subject of continuing collaboration. A new agreement between Unisearch and Crystalaid Pty Ltd is being negotiated.

Recommendations

The multilayered film developed in this project is an outstanding basis for a hydrogen sensor - if a means other than elevated temperatures can be used to overcome the carbon monoxide interference. It is therefore recommended that development of the hydrogen detector be pursued using alternate means to eliminate carbon monoxide interference