Humidity Indicator Cards

Moisture indicator strips are the simplest method of measuring relative humidity. The strips consist of different solutions of cobalt-II-chloride soaked pieces of paper. The paper changes depending on the RH changes in colour from pink to blue, with the current air humidity being indicated by the square that appears most likely to be grey (at 20 ° C). At 10 ° C, 5% RH would have to be added to the display value, at 30 ° C 5% RH would have to be subtracted.

Moisture indicator strips are available, individually or in combination, in increments of 5 - 90% RH. Strips with a colour change at 8% RH are used to control desiccants. The required measurement time is officially five hours, but a trend is often visible after a short time.

Indicator strips containing cobalt-II-chloride have been classified as toxic by the EU since 1988. Since then, cobalt-free indicator strips based on copper have also been available, which change from sky blue to brown. However, the colour change cannot be seen as well as with the cobalt-containing indicators. For this reason, "cobalt chloride-free" indicators are now available that also change from pink to blue. The dye here is cobalt bromide, which should not be less toxic than cobalt chloride.