The devices used in industry, by the public and police forces to analyse the level of alcohol in breath require calibration with high precision gas mixtures to ensure that they measure accurately.
Breath alcohol concentration (BAC) testing is conducted in private and business situations and is also used by the Australian police. The police forces in Australia generally enforce the level of 0.05 grams (50 milligrams) of alcohol in every 100 millilitres (ml) of blood for most drivers. Beyond that, drivers in sensitive categories such as bus drivers or heavy truck drivers are subject to a 'Zero BAC' limit. When measured, this must be less than 0.02 grams (20 milligrams) of alcohol in every 100 millilitres of blood.
The instrumentation that is used for these tests may either be calibrated using wet gas which involves bubbling air through an alcohol solution containing a very precise amount of ethanol in water or by the use of dry specialty gas calibration mixtures in cylinders. The calibration gas cylinder mixtures are produced to contain the equivalent amount of ethanol in a balance of nitrogen gas that would represent the above blood alcohol levels.
In addition to selection of the appropriate ethanol concentration there are some additional product selection considerations.
These calibration gas mixtures should be traceable to NATA standards. This can be demonstrated, for example, with a calibration certificate which has an ISO/IEC 17025 accreditation. Analysis of these ethanol in nitrogen gas mixtures is within the Coregas ISO/IEC 17025 accreditation scope.
Where possible, a balance gas of nitrogen is selected in preference to the alternative balance gas of air. This simplifies the manufacturing process and enables more cost effective supply of these gas mixtures.
The shelf life of breath alcohol calibration gas mixtures is limited to three years by the requirement for ISO/IEC 17025 accreditation. The calibration gases are used in relatively small quantities and are often transported to the location where the breath alcohol analysers are being used. For these reasons, it is typical to select very small highly portable cylinders for these calibration gas mixtures.
In some BAC detector applications a zero gas will be used in addition to the calibration, or span, gas. Either Zero Air or Nitrogen 5.0 would be ideal for this purpose.
The cylinder regulator will typically be a chrome plated brass specialty gases cylinder regulator with single stage design. Use of a stainless steel regulator or two stage regulators would be possible but the additional purchase cost of these configurations are not required for this application.
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