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Cross-examination about Henry's Law

Updated: Aug 3, 2022


To use the cross-examination of the CFS scientist to educate the Court about Henry's Law

Q. All right. In a nutshell, what’s Henry’s Law? I think – I think the Court needs to know that. A. All right. Henry’s Law is a principle in chemistry that looks at the relationship between the concentration of – I’ll use the simulator, essentially. So, the simulator contains an alcohol solution. There’s alcohol in the liquid that’s put into it. When it’s heated to 34 degrees Celsius, because alcohol is a volatile substance, what happens is, is that some of the alcohol starts to go into the air above the liquid. So, for example, when you have a glass of brandy, if you hold it in your hand, and – to warm it up, you can see that the alcohol starts to evaporate up the side of the glass, right? And so, what happens is, Henry’s Law is the relationship between the amount of alcohol that’s in the solution and how much is in the headspace, or the air above that liquid for a given temperature. Q. So, if we know the given temperature we can predict the ratio of concentration of ethyl alcohol in the water in the jar below, compared with the concentration of the ethyl alcohol in the air in the jar above the water. A. You could, theoretically, calculate it, yes. Q. All right. And knowing that that ratio – the fact that we know what that ratio is, then we know that if police buy a bottle of alcohol standard from a reliable source and one that’s got a certificate of an analyst, that’s – that comes with the purchase, and it’s put into a reliable

simulator that’s being used properly, then if it’s 100 standard, our expectation should be that we now have a target value, when pumped through the instrument during a calibration sequence, that the instrument should be perceiving that as being the equivalent of human breath at 34 degrees Celsius. A. Correct. Q. At 100 milligrams per 100 mils. A. Correct. Q. And that’s why we use wet bath simulators in Ontario. A. Yes.

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