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Figure 4


Ethanol Gas


Condensation Issue Compressed ethanol gas will condense into a liquid state if the tank gets too cold. The temperature at which ethanol condenses (the dew point) depends on the ethanol concentration and the pressure of the tank. Because higher tank pressure and higher ethanol concentrations increase the dew point, full tanks are most susceptible to ethanol condensation. Temperatures around 41˚F. will cause the ethanol gas in a full tank (1200 psi) with a .082 ethanol concentration to condense3


.


pressure decreases; as barometric pressure decreases, the expected ethanol vapor of gas standards also decreases. For example, when used at an elevation of 5,250 ſt., a tank labeled as .038 BrAC will give an ethanol vapor of .031 BrAC. Te operator corrects for the effects


of barometric pressure by referring to an altitude chart that can usually be found on the label of the gas standard. Elevation charts give a correction factor for each elevation range, which the operator uses to calculate the expected value, or the chart simply gives the expected value for each range of altitudes. Te most certain method for correcting for the effects of barometric pressure is to use a device that measures barometric pressure and calculates the expected ethanol vapor value. Tese devices display the adjusted ethanol value without the need for the operator to know the elevation. Tese devices can be stand-alone devices or they may be built into the breath testing device. The take-away: If you conduct breath


testing at an elevation of 500 ft. or higher, you must adjust the expected reading of ethanol gas standards to account for the effects of barometric pressure.


48 datia focus


As a tank is emptied and the pressure decreases, the dew point also decreases, which means that the tank must be subjected to even lower temperatures to condense the ethanol. If the operator releases gas from a


cold tank in which the ethanol gas has condensed, the readings will be lower than expected because the gas that is released will contain mostly nitrogen gas and very litle ethanol gas. Releasing gas from a tank in which the ethanol has condensed permanently alters the gas mixture in the tank. Once the tank warms and the condensed ethanol changes back to a gas, the tank will always give a higher alcohol concentration than stated on the label because the ppm proportion of ethanol to nitrogen has changed—there is now less nitrogen gas in proportion to ethanol gas. However, if no gas is released from a cold


tank with condensed ethanol, the tank will give accurate readings once it has warmed up. Studies have shown that tanks warm up in as litle as 15 minutes at room temperature4


,


and will then give accurate results—even without rolling the tanks on a table, which has sometimes been suggested as a preventive measure to avoid the condensation issue. The take-away: Don’t use a tank that


is very cold and allow a minimum of 15 minutes for a very cold tank to warm up before use.


Winter 2016


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