Forensic Detectors Carbon Monoxide (CO) Detector Review

[tmassey]

Like everything I write, this is long and detailed, and you may not need much of this detail. I use a CO monitor both for continuous CO monitoring of my compressor, as well as ad-hoc checking of tank fills I get elsewhere. This is divided into sections so that you can skip the sections you don't care about, or skip to the next post to get to the details of the unit -- of *course* this is more than 10,000 characters...

Compressor CO Monitoring​

For just about as long as I've had a compressor, I've had Carbon Monoxide (CO) monitoring. I use a Nitrox stick, and I wanted a way to continuously monitor my output O2 level, so plumbing in a CO monitor didn't add much to the process. I simply teed off my output just before the PMV and attached it to an old SCUBA first stage, and then used a BCD hose and plumbed it to a cheapo shop air regulator, then through an acrylic flow meter (I use 2lpm) and then across my O2 sensor and into the CO sensor. Easy peasy: I strongly recommend it to *any* compressor operator, Nitrox or otherwise. At the time, I spent no more than $300 or so in parts if you can scrounge any old SCUBA first stage. Well worth it, and it's run great for the past 5 years.

At the time I set this up (2017), there weren't a lot of choices for CO monitoring. I ended up with a Toxi RAE 3, which is a personal monitoring device designed to be worn by people such as miners, who are in a confined space with combustion engines. They have the advantage that they are designed to instantly alert with any exposure to CO, and to do so aggressively. They are also relatively inexpensive. They're not really designed to have air shoved into them from a tube, but if you point the tube at the sensor, it works *just* fine. We don't need precision, we need lights and sirens if there's *any* CO, and I've tested this many times: works great.

Fast forward to this month, when my CO monitor alerted while I was running my compressor. This had happened once before when someone was mowing the lawn, but this time, I couldn't find a likely culprit. After much examining of my intake, compressor temperatures, oil condition, and consulting with a couple of other compressor experts, the consensus was that it was *very* unlikely to be coming from the compressor, and to me it didn't seem likely that it would be coming from the air. Someone asked if it could be the sensor, and of course it could. So, I decided to see if I could get an affordable second sensor, not only to be a double-check but to also sit there and monitor the intake *before* I sucked it into my compressor.

Searching For a Detector: Most Won't Work!​

So, I went to the place that most of us go to when we want to buy something we're not going to get locally: Amazon. It turns out there's a lot more options today than 5 years ago. But unfortunately, most of them are useless for our purposes.

First, the obvious ones: Home CO detectors. These won't work: most won't alarm below a rather high number, and won't alarm unless they're at that level for a rather long amount of time. We want instant alert for any detectable level. So Home CO detectors are out.

Second, there are a couple of different designs of handheld monitors, that are sold under several dozen different names, but obviously came out of the exact same assembly line. These had a *very* attractive price ($30-$40) and seemed to do exactly what we want. One type had the sensor in the body (and an on/off switch on the side), and the other had a 14-inch or so metal cable with the sensor on the end. Unfortunately, if you read the writeups *very* carefully (including reviews, or comparing different vendors' listings for the obviously-same item), you find that both of them wont' alarm unless the CO is higher than 30 ppm, which is at least 25ppm too high for us. So they're out too.

Sensorcon: Might Work for Checking Tanks​

So, the cheapo detectors are out. Of course, we can go with the same type I'm already using: the PPE monitor. In fact, why not go with another Toxi RAE 3, seeing as it's worked so well? Well, Honeywell bought them, and it seems that they now want to charge a premium price: $300. And I'm cheap, so if I can find something else, I'd rather do so. The other brand (that I didn't know about until after I bought the TR3) is Sensorcon. They make a few different families of these sensors, which as far as I can tell are just relabeled and use different software. For manual use, the Sensorcon Inspector might work. I'm not sure if it will alarm at near-zero PPM, but I *know* that for me there is a definite shortcoming: you can't program the alarm to less than 35ppm. That's way too high, and seeing as I need it to actively alarm me to an issue when I'm using my compressor, it's out for me. Now, if you're just using it to examine a tank after it's filled, this would be just fine: most O2 sensors don't alert you, either. If you step up to the Pro models (for an extra $50 or so), you *can* change the alarm. If I couldn't find anything better that's what I would do, but I wanted to keep looking.

 

[tmassey]

Forensic Detector CO Monitor: Just What I Needed​

So after some more searching, I came across the Forensic Detector CO Monitor. It's designed to be a PPE monitor like the Seonsorcon or the Toxi RAE 3, but you can tell that this is something that's been put together with off-the-shelf components put into a very basic case. If I were buying PPE for hard-working miners, this would *not* make the cut, but for a device that will sit there and do its job, it seems sufficient. Most important, it alarms immediately, has configurable alarms and only costs $99 delivered next-day by Amazon:


The features of this device are a fair bit different than the Sensorcon and Toxi RAE. First, it has a rechargeable battery charged through a mini-USB (mini, *not* micro: the much less common angular one, not the rounded one used on every Android phone until 2020 or so). I have no idea how long the battery lasts, but given its role as PPE for workers, I think it'll hold enough charge to do a few dozen tanks at least. Second, it has a time and date calendar, which is nice for knowing *when* an alert happened. Third, it has a temperature sensor, for what that's worth.

How Well Does it Work?​

So, how do you use it? Very straightforward: Press the Power button and wait 5 or so seconds while it starts up. It will then show you the PPM of CO. If you're just using it to check tanks, that's it. Use it like an all-in-one O2 sensor: crack the tank and point the sensor hole at it. If you don't have CO in 10 seconds, you're not going to. You don't care about how accurate the number is, you just need to know zero or not-zero, and the device is ready to go out of the box for this.

For continuous use on a compressor, though, you may want to change a few settings. The alarms default to 50/150ppm (low/high), and I moved them to 5/10. Also, the time and date were off, and I set those. The menus are slightly awkward to navigate the first time, but at least they're consistent: all of the menus work exactly the same way, so it didn't take long to get used to it. And hopefully I won't have to do it again.

So much for the settings. Now, how well did it work? My first test was a simple 'bump test': I used my CO calibration gas cylinder and flowed a small amount of gas at the sensor. I got a reaction in easily under 2 seconds: immediate lights and sirens. Perfect! When it got above 10ppm it gave me even *more* lights and sirens! Just what I want. Remove the gas, and in about 3-4 seconds we're back to zero.

That is something that is common with all of the sensors I've seen: when the CO goes away, the alarm goes away. That means that if you walk away from your compressor, you can miss an alarm! Fortunately, all of the sensors also record alarms, so you want to make sure you review this when you're done with a compressor run. That's why I like the time/date stamps on this: I know if it is this run or a previous alert, and you don't have to make sure you clear the alerts to avoid confusion.

Next thing was to test the accuracy. Supposedly this thing is calibrated from the factory -- a calibration sheet is included. But I wanted to see how mine did, so I used the calibration cap to properly supply my 50ppm calibration gas. I was disappointed: it measured it as 30ppm, not 50. OK, so let's check out the calibration.

The unit is calibrated at 500 (five hundred) ppm, and I'm giving it 50 (fifty). That's like measuring 300 PSI on your pressure gauge: it's *way* at the end of its spectrum, and as we're taught in Open Water class, don't depend on the accuracy of your gauge way at the ends of the scale. So, I went ahead and recalibrated it using my 50ppm gas. Also straightforward, and now it measures 50ppm (of course). That should make the very low exposures I would expect to measure more accurately, and possibly a little more sensitively as well.

(Here's some more details about my calibration setup, which cost like $60 total: Source For Replacement Sensors in the US and Elsewhere )

Once that was done, I went ahead and compared it to my Toxi RAE 3. Turns out my TR3 was alerting me as soon as I turned it on, no matter where I moved it or what gas I blew over it. So it seems that my previous alerts might have been due to the sensor, not to an actual problem! So right there was a great reason to buy this second sensor! I ended up recalibrating the Toxi RAE 3 and was able to make it work reliably again: I think that was the source of my problems earlier. (Root cause analysis: redo the zero calibration when you redo the span calibration. I thought it would be better to *not* do so unless I had a reason to do so (if it ain't broke don't fix it?), but that function exists for a reason...)

But back to comparing the two I don't have a great way of doing A-B comparisons of CO monitors (it's not exactly my day job), but in my experimenting with the two at the same time, they both alarmed at the same point, and showed very similar numbers (within 5% at any time, which is literally only 2.5ppm or so!), so I'm pretty confident that they are both working as intended.

Forensic Detector: Near-perfect SCUBA monitor?​

I am very pleased with the Forensic Detector. It has all of the features I need, a few features that are nice, and the limitations are things I can live with.

Advantages
It has all of the features I need, whether you are just checking individual tanks or using it for continuous CO monitoring (or like I do, for both!). It alerts at any CO exposure, it records each alarm event (with time and date), it has loud and noticeable lights and sirens, and it's relatively small and portable. Those are the features I need no matter what.

Neutral
The rechargeable battery is a plus and a minus. It's easy to recharge, but the non-rechargeable lithium battery in my Toxi RAE 3 lasted 4 years before I had to replace it, which is pretty darn convenient on its own. The display is fancier and is backlit, but maybe not quite as easy to read as the watch-style displays on the Seonsorcon or TR3.

Disadvantages
The unit is noticeably larger than the better PPE devices, and does not look for feel *nearly* as rugged. But seeing as I won't be using this actually inside of a mine, it's *plenty* small and rugged enough for my purposes. In fact, it's probably as 'small' and 'rugged' as a Divesoft O2/Trimix analyzer. The listing states that it has a vibration alarm, but I did not notice it -- unlike my Toxi RAE 3, which is in danger of vibrating itself off the table when it goes off! But seeing as I won't actually be *wearing* it at any time, vibration isn't useful to alert me anyway, and the lights and siren work fine.

Summary​

If you're looking for a straightforward and affordable way to measure Carbon Monoxide in your breathing gas, consider the Forensic Defender CO Monitor. At under $100 delivered, it's less expensive than an O2 analyzer. It works and acts very much like an O2 analyzer in practice. And for those who might want to do continuous CO monitoring of their compressor, it has some really nice-to-have features (like time/date stamps and rechargeable battery/USB power) that make it really useful.