All electrochemical sensors will have certain volatile compounds they react to other than the target gas (CO). These interfering gases will result in false positive readings which will result in the end user assuming that carbon monoxide is present when in fact it is not. These false positives readings can not only confuse the end-user, but create costly and embarrassing situations where a work place is shut down for a contaminant positive reading which later turns out to be a false positive. At a fill station if you are going to announce that a tank is potentially contaminated with CO one had better be sure it is a true positive and not from an interfering gas.
The sensor manufacturers try to minimize these cross-reacting compounds by placing an adsorptive filter such as activated charcoal over the sensor which will remove many of the interfering compounds in question but still allow the CO to pass through. Activated charcoal does not adsorb CO.
Common interfering gases with CO electrochemical sensors include the alcohols (methanol, ethanol, isopropyl alcohol), hydrogen, hydrogen sulfide, nitric oxide, nitrogen dioxide, ethylene, and other hydrocarbon solvents such as toluene.
Have a look at Table 3 in this paper and one can see the large variation between different detectors, and what interfering gases the sensors react to. The BW Gas Alert is one of the best sensors and only reacts to the alcohols whereas the Drager Datalogger reacts to just about everything under the sun making it a poor choice for trying to detect solely CO. There would be so many cross-sensitivities one would never know whether a positive reading was CO or something else.
Cross-sensitivities of electrochemical detectors used to monitor worker exposur..... (DOI: 10.1039/b510084d)
In this document sensors CO-B1 and CO-BF are identical except that CO-BF has an adsorptive filter on it to remove many of the interfering gases. While the filter is not perfect it does reduce many of the interfering gases to zero or close to zero.
http://www.alphasense.com/pdf/AAN_109.pdf
Probably the biggest disadvantage of the Pocket CO is the large number of interfering gases since the sensor is not filtered while the BW Tech Gas Alert and the CO Experts sensors are.
All three detectors will react to the alcohols (commonly found in permanent and non-permanent marker pens) with the Pocket CO showing very high false positive CO readings. When an Avery-Dennison Dryboard marker (ethanol/isopropanol) was held a the gas inlet of the three detectors for one minute the Pocket CO read 80 ppm and then took close to an hour to recover to 5 ppm. The BW Tech unit which has an alcohol filter read 7 ppm and also took an hour to recover. The CO Experts unit with its very large filter was not affected by the marker pen challenge.
For a two week period all three units were tested in my vehicle to see if they could discern CO in traffic from many of the other interfering gases such as NO and NO2. Nitric oxide (NO) is from vehicle exhaust but is a very unstable molecule and with ground level ozone formed on hot sunny days quickly becomes nitrogen dioxide (NO2). The Pocket CO unit particularly on hot sunny days (temp 80 to 85 F) would show 5 to 20 ppm while driving on an 8 lane highway which was full of traffic. The other two units would remain at 0 ppm indicating that the Pocket CO is likely reacting to the NO and NO2 in the air and not the CO. I suspect when you were walking around PDC or Tulum and reported CO at the 10 ppm level this was in fact NO or NO2.
On many occasions such as pulling up behind a truck or stopped in traffic all three units would register a CO hit and when this did occur they all read within a couple ppm of each other. Once away from the true CO source the reading would quickly decay back to zero.
One of the best ways to tell if one has a true CO reading with the Pocket CO or false positive from an interfering gas is to examine the recovery time. With a true CO reading if the detector is removed from the CO point source it will recover to zero within two minutes, whereas if it is sensing something like NO/NO2 the recovery time is prolonged often in the 10 to 15 minute range. If one removes the apparent CO point source and the Pocket CO does not quickly return to zero it is likely not CO, but an interfering gas.
One interesting very reproducible interfering gas occurred when I would wash my truck's windshield with methanol (used up here for cold weather) while driving. The Pocket CO would pick the methanol vapors up each time and read 5 to 10 ppm and then take 10 to 15 minutes to return to zero. Even the BW Tech unit would read a few ppm and then take 15 minutes for the interfering methanol to clear. The CO Experts unit did not react to the methanol vapors at this low concentration.
The multiple cross-sensitivities with the unfiltered Pocket CO sensor is a real problem if you can't measure your tanks in a nice cool clean room. From my experience to date if the unit is "stuck" and shows a reading which won't decay back to zero within a minute or two one is not dealing with CO but some other interfering gas. If I was to approach a fill station owner about CO in the tank I would want to see the display rise from zero to a positive reading and then return to zero within two minutes after removing the potentially contaminated gas. If the CO Pocket does not recover to zero I'd assume you are measuring something other than CO likely not in the tank but in the ambient air.
I think the Pocket CO is a great unit for what the manufacturer originally designed it for which was to detect potential lethal levels of CO in planes, vehicles, hotel rooms, etc. at one atmosphere. Without a filter on the sensor I would be hesitant to attribute low tank air/gas readings to carbon monoxide unless I was in an air conditioned room away from traffic NO/NO2, and solvents which could interfere with the sensor. It does make a reasonable portable "go/no-go" detector though for detecting potentially heavily CO contaminated tank air.
Peter
Gave it to my LDS that just lost their truck, trailer, compressor, etc in a horrible wreck. From looking over the manual, I think I'd want one of those on any boat with a generator, and the partner units in every room hooked up to the main unit.
