I assume you are looking for one to hang on your compressor and also to check tanks.
The Sensorcon unit above is certainly a good price point and I have one to do some testing with over the next year. Advantages are the sensor replacement cost is very reasonable and getting the unit calibrated back in the USA by the manufacturer is also very easy. What is not clear is what will be the optimal calibration interval for such an inexpensive monitor and how long will the sensor last. The company recommend a six month calibration interval for breathing air.
You might also have a look at the BW Tech Gas Alert Extreme which has a user replaceable battery and sensor although the replacement sensors are very expensive. My sensor lasted about 4 years though with intermittent use.
The Rae System ToxiRae 3 is a nice unit with a user replaceable battery but you must use the Rae Systems $30 battery. Mine lasted about 3 years but they could be difficult to find while traveling. Once the sensor goes on this unit though you toss it out.
If you are going to do both a compressor and tanks also have a look at the Oxycheq unit. User replaceable battery and sensor which is a reasonable price.
I would not purchase a recent Nuvair portable CO monitor as the power consumption is atrocious. You'll get about 30 minutes per 9 volt battery. Their units are made by De-Ox in Italy.
We have two Analox CO Clear fixed units on different compressors and they are great. You can't move them around though nor check individual tanks. They state the sensors are warrantied for 18 months I think but ours have lasted easily 3 years. Cost is about $800 for the units and another few hundred for the regulators to hook them up. Sensors are a couple hundred bucks.
CO Clear Carbon Monoxide Alarm: Analox - Looking after the air you breathe.
A couple things to be careful of if looking at other units.
1. Watch out for 'blankout' which is a feature for use in an occupation setting to reduce CO noise. On some of the previous BWT and Rae Systems units the display is blanked out from 0 ppm to 3 ppm or 4 ppm. In other words the unit will display zero if reading 2 ppm.
2. Some of the units will lock you out if they are not calibrated within the manufacturer's recommended interval. Some other ones let you adjust the calibration interval. If you get locked out and don't have calibration gas on hand you won't be able to use the unit.
