In industry, when purchasing sensors from an external manufacturers, are those sensors tested, and if so, how thoroughly? For example, I buy some accelerometers from Sparkfun and do some minimal testing using gravity. Is this a waste of time? What are the chances of getting a defective sensor?
Electronic – Testing sensors
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You might be able to make this work but there are some sticky issues. First, if the ground connections of the sensors are not connected to the same ground as the controller then the sensors will not be able to pull their outputs down to the controller ground. As a result the controller will never see a logic low level from the sensor. One way to correct for this is to add an optoisolator at the output of every sensor. The output of the sensor would then activate an LED in the optoisolator and the phototransistor in the optoisolator could activate the controller input. This is pretty easy and inexpensive.
The harder problem is providing a stable supply voltage to the three sensors. When circuit elements are connected in series we know that the current through them must be the same but the voltage across them can be different. You need a way to make sure that the voltage across each sensor remains relatively constant even though the current they draw varies over time. I'm thinking that it may be enough to put a 16V, 1W, zener diode and a sizable capacitor in parallel with each sensor's power connections.
If you're not comfortable doing this then I would suggest just creating a 12V or 15V power supply using a linear regulator (say a 7812) from the 48V supply. You only need about 30mA to operate the three sensors, and I can't think of any problem that would occur by doing this.
You are absolutely right, this sensor gives you a single output, and you don't get any hint if for example Rs/R0 = 0.9 is caused by 200ppm CH4 or 1000ppm H2.
If Rs/R0 < 1, you may say that the gas is CH4, but not CO: CO will never cause such small values within the measurement range of the sensor. But you don't know what happens for very high concentrations of CO...
If you have your CO and CH4 sensitive sensor plus a pure CH4 sensor, you can distinguish between air with CO and air with CH4, but you may not be able to identify air with both gases, because you do not know how the MQ-5 deals with mixtures. Do they sum up, or do you get the signal of the "strongest" gas only?
You definitively need sensors giving you signals sensitive to the one gas, but not the other and vice versa.
Best Answer
The chances of a bad sensor (if you're buying 1-10 units) is probably acceptably low. I think you'll notice if you do any testing of your device when the sensor is installed. But a dedicated method for testing just the sensor might be a little onerous.