Our customers frequently ask for assistance in selecting an alcohol gas tank for use with an Alco-Sensor – should they use an .038 tank, a .082 tank, or a .100 tank? Can they calibrate with a .082 tank and do accuracy checks with a .038 tank? Here’s the scoop on how we answer these questions.
Any Tank Will Do
Answering the questions purely on technical and scientific grounds, the answer is that any tank will do the job. The reason is that Alco-Sensor instruments are “linear” from 0.00 to 0.400. Linear means that the results are directly proportional to the amount of alcohol in the sample.
To demonstrate that a device is linear one does a series of tests at varying alcohol concentrations. For example, an Alco-Sensor reads an alcohol sample of .05% alcohol accurately at .050.
- If we then double the alcohol concentration of the gas (2 x .05% = .100%), we expect the Alco-Sensor result to be .100.
- If we triple the alcohol concentration of the gas (3 x .05% = .150%), we expect the Alco-Sensor result to be .150.
- If we halve the alcohol concentration of the gas (.05% / 2 = .025%), we expect the Alco-Sensor result to be .025.
Intoximeters has performed tests similar to this using alcohol concentrations up to 0.400. If we plot the results on a graph they will form a straight line from .000 to .400. That’s why we say the Alco-Sensors are “linear” in the range from .000 to .400. If the Alco-Sensor reads accurately at any one point on the line between .000 and .400, it will read accurately at every other point on that line within that range.
Therefore, a user may perform Accuracy Checks with any alcohol gas tank to document the accuracy of test results. For example, if we document that the Alco-Sensor reads accurately at .038 using a .038 alcohol gas tank, we can be confident that the Alco-Sensor will give accurate readings at every other point between .000 and .400.
Convention Trumps Science?
In spite of the fact that any tank will do, many users feel more comfortable using an alcohol gas tank with an alcohol concentration that is close to the critical level for their testing application. For example:
- Many years ago when the DUI limit was .100 law enforcement preferred to do accuracy checks using a .100 standard. Some law enforcement agencies continue to prefer a .100 calibration standard.
- Now that every state in the U.S. has a legal DUI limit of .08 many law enforcement agencies prefer to use the .082 alcohol gas tanks.
- Federal DOT alcohol testing rules establish critical levels of .02 and .04 for safety sensitive employees. Workplace customers typically use a .038 alcohol gas tank.
- Substance abuse treatment programs and corrections agencies, who typically are implementing zero tolerance policies (interpreted as .01 or .02 cut-off level), usually use the .038 alcohol gas tanks.