Problems with the Pressure Sensor on Florida’s Intoxilyzer 8000
During a recent hearing, we learned that a Departmental Inspector with the Florida Department of Law Enforcement (FDLE), Roger Skipper, claims that he “developed” Florida’s version of software used on the Intoxilyzer 8000.
Roger Skipper has conducted thousands of departmental inspections on an annual basis and also after the machine is “returned from” repair. Amazingly, despite his involvement with developing the Florida specific software and all of his trips to CMI, he claims not to know what type of “flow sensor” this machine uses. Although he has looked under the cover thousands of times, he said he never looked at the logo on the part of the machine that measures flow or pressure to calculate the estimated amount of air volume blown into the machine. He apparently has no idea how many times CMI may have changed the sensor or other hardware in the instrument.
The Intoxilyzer 8000 Doesn’t Have a “Flow Sensor”
After a little more research, I may have asked the wrong question. Apparently, the Intoxilyzer does not have a “flow sensor” at all. It can really only be described as a pressure sensor or pressure transducer since it doesn’t really measure flow directly.
We now believe that the Intoxilyzer 8000 machine currently use a Motorola 5010D “pressure sensor” to interpret flow. This device was designed for a wide range of applications, not specifically for breath testing instruments. It appears that CMI adopted this relatively inexpensive device for use in its machines. Although the device may work well in other settings, it was not intended to be used for human breath testing. We may never know the secret algorithm used by the software to convert the data that measures pressure output into a flow rate since CMI refuses to disclose that information. The top officials with FDLE claim not to even know whether the Motorola 5010D is used in the Intoxilyzer 8000.
Additionally, CMI has added a big polyester capacitor in the flow sensor circuit adjacent to the MOT5010 device. Perhaps this polyester capacitor is averaging or capping something coming off the device, but without the schematics for the Intoxilyzer 8000, it is impossible to tell why CMI had to add this capacitor. The polyester capacitor does not appear on the Motorola sample circuits, so it appears to be something that CMI had to add to the Intoxilyzer 8000.
Limitations of the Motorola 5010D on a Alcohol Breath Test Machine
The accuracy specifications for the Motorola 5010D boast an impressive 5.0% Maximum Error over 0° to 85°C. But turn to page 4 of the Data Sheet and you find that this error rate of +/- .5kPa for the device range of 0 – 10 kP. That error rate is apparently found under controlled testing conditions. Depending on how well the hardware is set up in the Intoxilyzer 8000, this error rate might be much higher in the field. Even more troubling, at the lower levels 1kPa of breath, the accuracy may only be +/- 50%, although it is difficult to tell from looking at the specs.
Without independent testing or looking at the source code for the Intoxilyzer 8000, we do not know what the machine is using as a threshold for the minimum breath flow. But we do know that the error range of +/- 5% can not be reached until the Intoxilyzer 8000 senses 10kPa, even under the best conditions.
Dry Air vs. Human Breath with 100% Relative Humidity
Another issue with the Motorola device is that it is only tested and warranted with dry media (air) and the specs themselves say that the use of “Media, other than dry air, may have adverse effects on sensor performance and long–term reliability.” This, by itself, a big concern since human breath is always 100% relative humidity.
How Often is the Pressure Sensor is Being Used Out of Range?
The other issue is whether the pressure sensor on the Intoxilyzer 8000 is being used outside of the intended range for the device. If a person blows as hard as 3.5 Liters per second, this may translates to about 11 or 12 kPa. The device range is only 0 to 10. The specs clearly state that: “The output will saturate outside of the specified pressure range.” The question then becomes “what happens when the output saturates?” “Saturation” generally means that the output goes full scale and stays there. If so, then a person blowing hard would not be registered correctly and, in fact, the indication of total volume would be lower than the actual volume delivered.
What Is the Difference Between a Pressure Sensor and a Flow Sensor?
The difference between a pressure sensor and a flow sensor is that a pressure sensor produces an output voltage that is proportional to the amount of pressure applied. This pressure must be then be translated into a flow indication of some sort. The difficulty is that there is no direct translation between the amount of air flowing and the degree of pressure, since the amount of pressure will be influenced by the overall design of the system, the size of the tubing, number of bends and a host of other factors. Since it does not interpret flow directly, to call it a flow sensor is really a misuse of terms.
A flow sensor, on the other hand, measures flow directly without the need for any interpretation. Usually it does this by taking a small fixed portion of the air being blown into the instrument and diverting it across a pair of thermistors that will cool in direct proportion to the volume of air passing over them. As the thermistors cool, the output voltage varies in direct proportion to the degree of cooling which is in direct proportion to the amount of air flowing.
Background Information on Why the Flow Measurement is Important
In most breath testing instruments there are a number of condition that must be met in order to have the software accept the sample as a valid one:
- Either of minimum time or minimum volume or a minimum flow rate that allows for slope detection to begin by insuring you have breath in the sample chamber. This calculation is a precondition to the other conditions listed below being evaluated.
- A rate of increase that has slowed to some specified rate, usually less than about .003 g/210L per second.
- Some indication that the person is running out of breath or the end the blow.
Without looking at the source code, we have no idea how the Intoxilyzer 8000 is calculating these conditions in the software. Having the source code would tell us how the Intoxilyzer 8000 is interpreting the electronic values being produced by the device and how it integrates that information into the algorithm used for sample acceptance.
Syringe Testing to Confirm the Pressure Measurement is Accurate in Calculating the Flow Rate
Although the FDLE ordered syringes to measure the accuracy of the machine in measuring volume, the syringes were never used for any testing in Florida. Those syringes are still sitting unopened somewhere on a shelf in Tallahassee.
The pressure sensor measures pressure (kPa) and somehow does an internal calculation to estimate the volume (Liters) of air blown into the machine. There is no direct correlation between pressure and volume. In other words, there is no scientific equations that convert kPa into Liters.
The only way to calibrate the Intoxilyzer 8000’s volume calculation may be by using a big syringe. Theoretically, the instrument could use some kind of potentiometer to check the calibration and a repair person could adjust it as needed by the software to interpret the amount of air delivered by the syringe correctly. Maybe there is no attempt to calibrate the instrument’s calculation from pressure to volume and only when the flaw becomes obvious does the machine get sent out for repairs to fix it. Without seeing the schematics and the software for the Intoxilyzer it is impossible to know.
Does the software calculate the voltage output of the pressure transducer factoring in the time at the given rates of voltage output in order to determine a value in Liters? If so, the volume calculation maybe inaccurate, but knowing how inaccurate is impossible to determine without looking at the software and doing independent testing.
Potential Problems Caused by the Pressure Sensor as Used in the Intoxilyzer 8000
Problems with the pressure sensor on the Intoxilyzer 8000 could be the main reason the machine misinterprets volume during subject testing. If the machine finds that the subject did not blow 1.1 liters of air into the machine, then the machine triggers a “volume not met” flag. Often this is interpreted by the breath test operator as a refusal.
The MOT5010 sensor may also contribute to other common problems with the Florida Intoxilyzer 8000 such as
- The total of indicated volume delivered may or may not be accurate. CMI has testified that the accuracy of the volume is +/- 10%, but there is absolutely no way to know how they arrived at that figure without looking at the source code.
- Since minimum volume is one of the criteria for sample acceptance, an inaccurate volume measurement would mean a sample acceptance parameter that allows acceptance too early or too late.
- An inaccurate minimum flow threshold assuming that the Intoxilyzer is even able to monitor a minimum flow threshold at all.
- Inability to detect mouth alcohol consistently.
These overall problems might also contribute to the high number of samples without .02 agreement, certain machines having higher than average refusal readings, and the inability to determine the presence of mouth alcohol. Without knowing the algorithm used in the software to determine when a valid sample is found, the exact extent of the problem is impossible to determine.
Read more about the “Volume Not Met” flag on the Intoxilyzer 8000.
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