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What is Differential Voltage Monitoring (DVM) ?

Many years ago Ontario defence lawyers cross-examined breath techs on "how much room there was" on the particular Intoxilyzer 5000C to take a subject test. We weren't talking about historical or ADAMS data, but rather "room" in the electrical system/sample chamber system. Was the sample chamber dirty, such that the baseline electrical system signal was always much too high to permit a reliable breath sample?


Breath techs would sometimes, on the witness stand, tell us they knew their Intoxilyzer 5000Cs were reliable and ready to take samples, because "the DVM was rock-steady". What on earth did they mean by "DVM"? What was "rock-steady"? How did they know it was "rock-steady".


In Amy Ramsay's book, "A Guide to the Intoxilyzer 5000C", 1999, close to the introduction of the Intoxilyzer 5000C into Ontario, we were taught that the DVM test would allow a technician to check the signal's drift and stability.


Why were these issues of any significance to reliability of the subject test results?


Are these issues relevant to today's Intoxilyzer 8000Cs?



200 BAC is the difference between 550 and 350 and 250 BAC is the difference between 650 and 400
This diagram appeared at page 6-9 of the Intoxilyzer 5000C "Training Manual" published by the CFS and located in the reference section of the H. Ward Smith Library at the CFS in Toronto.


In the very old CFS 5000C "Training Manual", that I located at the CFS library many years ago, it said at page 6-8:


The DVM can be thought as the scale of the instrument. Rather than a visual scale graduated from 0.0 to 0.40 as it is for the Breathalyzer. The DVM scale is electronic, each 0.001 unit on the DVM scale is equivalent to approximately 0.001 g/lOOmL of alcohol. For simplicity the DVM scale will be refer to as whole number. The scale of the DVM is between 0 and 1023, which is 2(to the 8th), the binary equivalence of this number. The DVM is normally set at 350. If this was the DVM at the start of the breath test, and the BAC was 200 mg/1OOmL, the DVM would increase to 550; as each unit of DVM is equivalent to 1 mg/1OOmL BAC.


If the DVM was at 400 athe start of the breath test and the BAC was 250, the DVM would be 650, again BAC is added to the DVM to result in the final DVM. The actual number of the DVM is not important as long as the DVM is constant before each test. The instrument is looking at the change in DVM for a beath test not the actual number.


What can an Ontario defence lawyer learn from this:


1. We already know that zero, the "baseline", for any individual breath test, depends upon careful control by the breath tech of ambient conditions in the breath room, at the time or times zero is set. "Fudging" ambient fails with the breath tube under the instrument is not going to be helpful to reliability.

2. We already know that ambient conditions, are changing contantly in the breath room with the presence of ethyl alcohol in the room air and interferents in the breath room air (e.g. cleaning chemicals, fingerprinting chemicals).

3. If the DVM is wandering between the time zero is set and the actual control check or the actual subject test(s) then control checks and subject tests may not be reliable.

4. As noted in R. v. St. Onge, in evidence given to Parliament and in his report, Brian Hodgson tied reliability of the approved instrument to no significant drift in accuracy and precision over time. According to OIML R 126, drift can be short-term and long-term.


I learned the importance of wandering DVM when I bought old discarded Intoxilyzer 5000s and tried to use them to get reliable tests. Frequently there would be PROCESSOR ERROR messages on startup:


PROCESSOR ERROR 3: An unacceptable positive processor drift was found.


PROCESSOR ERROR 4: An unacceptable negative processor drift was found.


(See page 5-1 of the Intoxilyzer 5000C "Training Manual" September 1994)


The frightening thing about using these old instruments was that once you'd "turn it off and turn it on again" a few times, everything would appear normal. Please enjoy watching an endless number of my old Intoxilyzer 5000s "in-need-of-maintenance" videos at this site.


Sometimes my ancient machines would crash on startup and then shift to a display of DVM - wandering all over the place. Sometimes they would work better if you banged them or shook them.


The Intoxilyzer 5000C Training Manual September 1994 said at 6-9:


As dust and dirt enters the sample chamber, the DVM,increases, since some of the IR light is absorbed by the dust. Again the actual DVM value is not important, it is the change in DVM after the breath test. Therefore dust and dirt in the sample chamber will not affect the accuracy of the analysis but will affect the workload ·of the instrument. The highest value the DVM can be is 1023. Therefore, iif the DVM started at 900, any BAC greater than 123 mg/lOOmL would exceed the limit. To ensure that there is adequate workload, an ERROR message (PROCESSOR ERROR 5) occurs if the DVM exceeds 600. This allows a workload of at least 423 mg/lOOmL. New instruments and instruments that have been recently maintained or repaired should have a DVM of about 350, this allows a workload of 673 mg/lOOml.


Imagine how much dirt, spit, vomit, dust, old alcohol standard there is in the sample chamber after 10 years of use. It is very easy for a breath tech or an experimenter to accidentally switch the input and output wet-bath simulator tubes and thus flood the sample chamber. It is very easy during maintenance or experimental testing, using the ABA method of introducing simulator vapour into the the breath tube to accidentally flood the sample chamber. I've done it and I know other people who have done it. It messes up the machine. Sometimes you can air it out by continuous air blanks.


On the Intoxilyzer 8000C there is a keyboard function Esc Esc A which starts a continuous air blank. See page 143 of 238 in the CFS Intoxilyzer 8000C Training Aid December 2013. On an Intoxilyzer 5000 or 5000C there needs to be a more hands-on approach. In the 5000 and 5000C there was a "Procedure for Removing Solution from Intoxilyzer 5000 Cell" (CMI 1984 Intoxilyzer 5000 Breath Analysis Instrument, Operator's Maniual Rev H) that included, after opening up the cover:


4. Unplug the fan connector from the mother board. Find the socket labeled pump on the mother board.

5. Unplug the pump connector and plug it into the socket labeled fan. Be aware that when the instrument is turned on the pump will run continuously and will force the fluid in the cell to be exhausted out port on back right hand side of the instrument.

6. Plug the instrument in and turn on.

7. Once the fluid coming out the back has subsided, tilt the instrument to the left and forward slightly. This should bring all fluid to the outlet port of the cell. Be careful not to get any fluid get on the inside of the instrument.

Wow! Imagine if the aging instrument has a mix of vomit, dirt, dust, spit, and fluid on the inside of the sample chamber cell! How could the DVM possibly be stable? The bottom line is that it is not a good idea for police to be using aging approved instruments that have not been recently (in the last year) sent out to an independent service centre for annual maintenance and the kind of recent re-calibration contemplated in the facts of Gubbins and Vallentgoed.


So does any of this apply to the Intoxilyzer 8000C? Can signals become unstable as sample chambers accumulate dust, dirt, vomit, spit, and fluid? And does it make a difference that the sample chamber in the 8000C (29 mL) is a lot smaller than the one in the 5000C?



sample chamber of 8000
End of sample chamber in dismantled and inoperable Intoxilyzer 8000.

Please see the following excerpt from the Evaluation for the ATC by the CFS of the Intoxilyzer 8000C p. 3:


The two “reference checks” that occur after the first two air blanks in the breath test sequence are equivalent to the “>>>>>>>” chevrons on the Intoxilyzer® 5000C and correspond to a check of the stability of the differential voltage monitoring (DVM) as well as establishment of baseline prior to the calibration check and subject test.


A DVM Monitor can be accessed manually from the keyboard. The DVM Monitor is accessed as a menu item in the second password protected Esc Esc level:


Instructional slide 2:D by the author

The following is an excerpt from the CMI Intoxilyzer 8000C Ontario Operator's Guide p. 7 showing keyboard entries:


excerpt from the CMI Intoxilyzer 8000C Ontario Operator's Guide p. 7
excerpt from the CMI Intoxilyzer 8000C Ontario Operator's Guide p. 7


Please notice the reference to "voltage output". Please notice the reference to "ranges of 9000 to 16000".


Please notice the reference in the CFS Intoxilyzer 8000C Training Aid under "Exception Messages", to the exception "Unstable signal". Please notice the references in the Training Aid under "the Diagnostics Check" to the explanations for "Diagnostics Voltage/Current Test", "Diagnostics DSP Test" and "Diagnostics Analytical Test". Fortunately my own Intoxilyzer 8000 (unlike my 5000s) is not experiencing failed diagnostics under these categories - so I don't have any whacky 8000 DVM videos to show you. My hope is that I can keep its sample chamber clean as long as possible. Hopefully its DVM will stay low and stable.


It is apparent to me that we, as defence lawyers, need to pay close attention to DVM "room" and signal stability in aging Intoxilyzer 8000Cs in use by police in Ontario. Instruments need to be maintained annually! We need to understand that wandering DVM may be as much a factor in unreliable control tests, as changing ambient conditions and police fudging of ambient fails.



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