Sr. X - The Itching. Photograph by Kevin Walters.

Flammable liquids, vapours and gases can be dangerous especially odourless one. It is best to keep samples small and keep liquids away from ignition sources in sealed containers.

The general method was to introduce liquid samples on a soaked, solid-stem cotton bud about 5cm from the sensors for 90 seconds and then remove the sample for 90 seconds and then remove the bowl for 30 seconds to let air circulate and refresh. The breadboard was covered with an upturned glass bowl to reduce the effect of draughts but with a 3cm gap on one side to facilitate adding and removing the samples. Gases were injected from a plastic syringe at the edge of the bowl towards the sensor.

All liquids were at room temperature. The ambient temperature during measurement varied between 24.7 and 25.0 degrees celsius. Temperature and humidity do have a small effect on sensor output. Some more advanced sensors have built-in temperature compensation.

Some prior casual testing revealed the sensors are sensitive to airflow particularly cold draughts.

The graphs show the ratio of the resistance of the sensor with no sample (Ro) vs the the resistance of the sensor with sample (Rs). The data sheets show the reciprocal of this value. The y scale is logarithmic and kept constant across all graphs.

The power supply varied gradually from 5.10V to 4.86V as the batteries discharged. The value per test was used in the calculation of Ro/Rs.

Gas sensors require calibration against reference samples for accurate ppm measurement.