[dzabakh]

Hello everyone!

Before asking the question, I have to warn: I am no chemist and English is not my native language so the text can mean not what I want it to mean:)

I'd like to design a small power-free device that could measure a time of its being in an environment with 100% humidity. Say, you take some sensor and put it into water. After some time you take it out, attach it to some microcontroller or a computer and it says for how long it has been under water. The device should have no in-built power source.

I think about it as some chemical device that irreversibly changes any of its electrical characteristics (resistance, capacitance, inductance) while being in high humidity. As we know the time-dependence of this characteristic in advance, we can find out the time.

What I can imagine it to be is for example two small metallic plates that become covered with oxide film and the resistance (capacitance) between them changes with time.

I know that probably it is a known concept in chemistry and everyone knows how to do that. Thanks in advance for any answer everyone!

[Borek]

Never seen anything like that and it definitely doesn't sound like a trivial concept (even if it is interesting).

But first things first, it is not clear what you are really interested in. Being in 100% humidity and being submerged are two different things. Humidity refers to the relative amount of water in a gas phase which - by definition - doesn't contain liquid water. 100% humidity means air is saturated with water vapor (it can't contain more, or the condensation will start).

[dzabakh]

Hello Borek!

Thanks for the reply.

doesn't sound like a trivial concept (even if it is interesting)

Thank you, I tried my best.

100% humidity means air is saturated with

I mean being inside an mammal's mouth. I think it refers more to being submerged.

[Enthalpy]

"In a mammal's mouth" changes everything. So it shall not be a poison, and I suppose you can't predict whether it will be in damp air or in saliva, and the proportion of CO₂ and HCl varies.

Oh good. I wouldn't look for a chemical change then - though other people here may. What about a temperature threshold instead?

Take a mechanical clock. Maybe you wind it up with a key. Something blocks its movement but melts below the mouth temperature: an alloy, a paraffin... To stop the movement, put the clock in free air if it's cold enough, or in the fridge.