The ownership of a gas detector is an investment in employee safety. It serves as both a safety and analytical tool that provides protection against the unseen gas dangers of hazardous working environments.

As a safety device a gas detector forewarns a worker with both visual and audible alarms at set points that are derived from exposure standards. It can also be utilised as an instrument that provides responsible personnel with data that can assist in the development of management programs that will eliminate or reduce the risk in areas such as confined spaces where a gas hazard could exist.

By playing such an important role in employee safety it is evident that there exists a duty of care to ensure both a high level of training in instrument operation and a comprehensive maintenance program. Unit reliability is paramount in ensuring confidence in the instrument's operation and the validity of results. This can only be achieved with regular 'challenge' tests of the unit and scheduled servicing and calibrations. App-Tek recommends that NATA calibrations or equivalent be performed at intervals specified by manufacturers of gas detectors as it is well documented that ageing of sensors can cause a detector's response to drift over time.

Single point calibrations are sometimes used in an attempt to save money but App-Tek believes it is not possible to completely verify correct operation of gas meters using this technique.

A single point calibration merely confirms that the detector will read accurately if exposed to a gas at exactly the same concentration as that at which the calibration adjustment was made.

A NATA certified calibration tests for linearity across the measuring range (exposure to three concentrations of gas at approximately 25%, 50% and 75% of the range) and incorporates tolerance levels, as nominated by Australian Standards, that must be achieved in order for a detector to pass.

When a detector does not meet these stringent tolerance levels it must be returned to a service workshop for sensor replacement and/or repair before another calibration attempt is made.

A service provider must be committed to ensuring the highest quality of service and that entails being technically competent in all aspects of the instrument and conformance to relevant legislation.

Any repairs or adjustments carried out on a gas detector must be in accordance with Australian Standards in relation to I.S.approvals.

Duty of Care obligations are best met when instruments are covered by a scheduling system to ensure that all instruments are challenged, calibrated and serviced at appropriate times.

A strong relationship with a service provider/NATA calibration facility will ensure your gas detectors perform reliably and the risks posed to personnel working in hazardous environments are minimised. The gases used for calibration should be NATA traceable and presented to the instrument via a regulator, flow meter and piping constructed of material that will not alter gas concentrations.

Gas must be applied to the gas detector at a predetermined set pressure and flow rate for a prescribed time duration. Conditions such as temperature and pressure must be controlled as changes may alter equipment behaviour during the calibration process.

An example of how a calibration is done on an OdaLog Monitor is to flow Hydrogen Sulphide (NATA Traceable) gas across the sensor at 0.5 litres/min for 10 minutes before attempting to adjust the instrument. This must be repeated for all three points to verify correct response over the measuring range. It is often found that incorrect readings are obtained either high or low in the range after calibration at the middle point and, when this happens, the sensor usually requires replacement.

A calibration of the H2S channel in the above example will thus consume 0.5 x 10 x 3 litres of calibration gas = 15 litres per calibration.

As calibration gas is extremely expensive great care must be taken to turn all gas lines off upon completion of calibrations. These calibration gases are often toxic and, where high concentrations are required, should ideally be used inside a fume cupboard.

Calibration gas ages (one or two years depending on the gas) and strict controls must be implemented to ensure gas is replaced when expiry date is reached. H2S is particularly reactive and can quickly decrease in concentration as it ages.

Challenge gas kits are meant for the end user to check that gas detectors work and when exposed to gas the alarms must be triggered. This type of testing is done under much less stringent conditions and for a shorter duration (until alarm is activated) than calibration.

Gas concentration accuracy is usually acceptable for "challenging" as long as it is sufficient to trigger the alarms and check the span reading. This lower level of control over the gas accuracy and process is not an acceptable substitute for NATA calibration (rather, it is used to complement regular calibrations).

Calibration/Challenge gas can be obtained by purchasing a range of dedicated gas mixes in disposable cylinders used with specialised stainless steel regulators. A small disposable H2S bottle holds 58 litres of gas whilst a small O2/CH4/CO bottle holds 103 litres.

When using App-Tek challenge gas test kits it is recommended that they be used for challenge/span check daily or prior to every use.