This question can actually be interpreted in two ways and the obvious one is not usually the interpretation that the user is looking for. In most cases, 0 – 1000 PPM is not the right answer.
The measuring range, or range of concentrations detected by the sensor, can generally be found on the specification sheet for any gas detector. But more often than not, when this question is asked the user is actually trying to determine the range or area of coverage that the monitor provides. They have the mistaken belief that the monitor will protect them within an area of X amount of square footage or within some fixed radius of the detector.
The true fact is that most portable gas monitors are designed to be passive or diffusion based detectors. They respond to gas that diffuses into the monitor through the sensor openings or membranes. They are only capable of detecting gas that is in immediate contact with the monitor. Yes, that means that you can be working just a few feet from a deadly cloud of gas and the air currents and other conditions in the area may leave it completely undetected.
This is why many people believe that a gas monitor must be equipped with a pump to be effective. Unfortunately, this is not the case and the instrument with an internal or attached pump will perform in the same manner as the diffusion instrument. It can only detect gas that is in the immediate vincinity of the end of the sample line or at the sample inlet. A simple 500cc/minute pump cannot draw enough gas to sample from the complete volume of a space of any significant size.
So pay attention to the air flow and how gas may diffuse around your work area. Keep your monitor close to you and don’t simply hang it up or leave it lay in the area you are working in and expect it to give you the protection that you need.
Keep it safe!