MQ-2 Gas Sensor tests - Page 1
Forum section: electronics / projects

author: 741
author_rank: Frequent Contributor
timestamp: November 05, 2019, 04:34:15 pm
content: I've been measuring the resistance of an MQ2 sensor, trying to get something like the tempco results shown in the DS.This is a sensitive device, and something like a breeze (eg from a hairdryer) massively alters results. I can't be sure, but I think the net effect of the hairdryer (no cold setting!) is tolowerthe surface temperature of the ZnO sensor. However, also tried holding sensor over a convection heater, and similar results - hard to believe that is lowering surface temperature - though still, admittedly, there are convection currents.What I see is an initial slow, large rise in resistance, then (whilst trying to keep conditions steady) a slow fall. This is similar to the power-up results: A cold sensor has huge resistance (maybe the ZnO is not a semiconductor when cold?), then the resistance falls quite fast, then rises a bit: Hard to sepasrate from room conditions though.What are the top contributions to widely varying/wobbling sensor resistance in "stable/normal" room conditions: temperature, pollutants...moisture?Another thing is DS indicates a burn-in of up to 48 hours (for stock not used for a long time) - why so long? If I actually do this, will the thing stabilise, and what takes so long?

author: StuartA
author_rank: Regular Contributor
timestamp: November 05, 2019, 05:30:55 pm
content: I've never had a play with these types of sensors, but I have been involved with thermal conductivity detectors used in gas chromatography equipment. In those circuits, you tend to have one active sensor and one reference sensor and these are usually configured as a bridge circuit, seehttps://en.wikipedia.org/wiki/Thermal_conductivity_detector.With a brand new MQ-2, or one which has not been used for a long time, the oxide will probably have adsorbed some heavier organic compounds, typically evolved from plastic packaging etc and these may take quite a while to bake off. Be sure to store the sensors in something like aluminium foil; absolutely avoid plastic bags, which often have silicone release agents on them which can be the kiss of death for oxide sensors.What do you want the sensor to do for you?

author: DBecker
author_rank: Frequent Contributor
timestamp: November 05, 2019, 05:52:21 pm
content: These sensors do not have the selectivity and repeatability that people expect.The sensitivity to air flow is one of the issues. Take apart one and watch it operating. The heater coil is hot enough to glow when measuring. The slightest movement of air cools it off, but not evenly. Depending on the exact direction of air the resistance of the outer tube is going to vary quite a bit. This is a fundamental problem because you need to move air across the tube to get a valid sample.Another issue is the selectivity. The tube reacts to a wide range of molecules. You are only getting out a single resistance measurement that is the aggregate of the reactions. The coating can be tuned to react more strongly to a specific subset than others, but that only helps if you have an array of different types getting the same sample and are looking at the weighted differential response.These sensors are useful for turning on blower fans or changing vent positions if there might be a concentration of non-specific Bad Stuff. That's why the same model sensor is used on carbon monoxide engine room ventilation and "stink sensors" on luxury cars. It's irresponsible to claim that they are selective, accurate or repeatable enough for uses such as legally-valid breath alcohol detectors.

author: 741
author_rank: Frequent Contributor
timestamp: November 06, 2019, 09:36:05 am
content: Good information; I knew silicone poisoned some types of sensor - but I hadno ideaplastic can "release" silicone! Is this issue likely for a normal 'zip-seal' plastic bag, or only for plastic products intended for particular applications?I wonder how sensitive to supply voltage these are (P=V2/R). D.S. says 33 Ohms +/- 5% and run at 5V +/- 0.1V.

author: StuartA
author_rank: Regular Contributor
timestamp: November 06, 2019, 11:19:08 am
content: Silicone is not actually a constituent of the polythene, but silicone "releasing agents" (i.e. stops the plastic sticking to the machinery) are sometimes (often?) used in on the equipment used to make and handle polythene sheet and bags. It's hard to quantify the extent of the issue, but I remember being told about it as a new graduate back in the 70's. A colleague who worked for a company making sensors/alarms told me that they banned the use of silicone furniture polish in the admin block, such was their level of concern.