A general-purpose colourimetric air pressure indicator

•A general-purpose colourimetric air pressure indicator based on a very sensitive CO2 indicator.•Uses the pH indicator dye thymol blue and coated as an ink.•The TB indicator changes colour over a broad range of air pressures, P, (0.1–14 at m).•The colour change is monitored using a camera and colour analysis and related quantitatively to P. A general-purpose air pressure colourimetric indicator is described based on a sensitive CO2 indicator, which responds to the change in partial pressure of CO2 in air. The indicator uses the pH indicator dye thymol blue (TB) and is green in ambient air (at 1 atm), bright blue under high vacuum (10 atm). The TB indicator is cast from an ink and its change in colour monitored mainly using digital photography coupled with colour analysis and quantitatively related to air pressure. The response of the TB indicator is initially probed using different Ar/CO2 gas mixtures at 1 atm pressure and then used to monitor: (i) sub-ambient air pressures, P, over the range 0.1−1 atm and (ii) super ambient air pressures over the range 1−14 atm. Colour analysis of all the photographic images taken in the two studies reveals a consistent data set that is readily related quantitatively to P, with a typical 50 % colour change at 1.02 ± 0.03 atm of air, i.e. or (4.2 ± 0.1)×10−4 atm of CO2. The 90 % response and recovery times of the indicator to sudden changes in pressure are fast,