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Linearity Studies are Done on New Instruments not Aging Instruments

Updated: Oct 21, 2022


To obtain admissions that the published studies dealing with the linearity of approved instruments assume a new instrument recently calibrated by the manufacturer.

Sample cross-examination of CFS expert on the inadequacy of studies relied upon by government scientists to maintain that approved instruments have good linearity - the studies were all done on new instruments, recently calibrated at the factory, not on aging instruments out in the field:

abstract from Can. Soc. Forensic Sci. J. Vol. 44

Q. Well, every study that we see having to do

with a new instrument, for example, Ms. Martin's, Terry

Martin's study, the published paper relating to the

Intoxilyzer 8000C...

A. Yes.

Q. ...she did linearity checks.

A. She did.

Q. She looked at the math and she did an actual

check to examine the linearity of the instrument?

A. Correct

Q. But my --

A. That's the purpose of an evaluation. An

evaluation is from the operational requirements of the


Excerpt respecting 8000C linearity from paper by T. L. Martin on the evaluation of the Intoxilyzer 8000 for the Alcohol Test Committee prior to type appoval as "approved instrument" in Canada

Q. But the evaluation is always with respect to a

new instrument, an instrument that's been recently

calibrated, that's been sent by the manufacturer out for


A. Yes.

Q. The problem is, when we've got instruments out

in the field, if no one is checking linearity, and on a

regular basis, if no one is sending the instrument out with

basic regular either inspection intervals or regular

calibration intervals, it is not at all safe to assume that

the linearity of the response is going to be maintained.

A. Some of that is addressed at the time of the

preventative maintenance inspection done either on an annual

or biannual basis.

Q. And so that's why it's so important that some

of that being addressed is being done properly. And so if

the individual doing the annual maintenance is not properly

responding and documenting a problem when they see, say, the

numbers when they do an inspection at 50 milligrams per 100

mills or 40 milligrams per 100 mills being way out of spec,

and then they do nothing about it, that is problematic, I

would suggest to you, and that's why it is that the CFS

policy that every test stands on its own doesn't work out in

the field.

A. That's your opinion, yes.

MR. BISS: Your Honour, should we mark those

documents as exhibits?


See also:

"Beer's Law and Spectrophotometer Linearity" by C.G. Cannon and I.S.C. Butterworth, 1953, Vol. 25, No. 1, Analytical Chemistry

"Does the Intoxilyzer 4011AS-A conform to the Beer-Lambert law?" by J Mack Cowan, Journal of the Forensic Science Society 1988: 28: 179-184

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