In Western Canada, police use dry gas, sometimes referred to as air gas, as an alcohol standard. Instead of a wet-bath simulator containing liquid alcohol standard, police use a gas alcohol standard, attached to the approved instrument. Ambient atmospheric pressure is important to reliability of this gas alcohol standard. Temperature is not. Dry gas alcohol standard is purchased by police from the approved instrument manufacturer, from a Canadian distributor, or from a company such as "Air Gas". It is difficult to acquire, and independently test, from a defence perspective.