Humidity in bathroom - measuring

No idea what the actual level of humidity you need is but a set time out is not really the answer as starting conditions are too variable. ISTM that you get far more moisture in the air from a shower than from a bath and the air you suck from elsewhere to replace the moisture laden air is similarly of variable humidity. You can however get humidistats that directly control the fan but be aware that if you go down that route you need to be careful at what level you set the humidity to avoid a drop in temperature triggering the fan at night when it isn't really necessary. And on the other hand if you are anxious to expel noxious smells a humidistat won't always oblige without an override button.

We had similar when we first moved in. We opened and locked a window 'on the latch' so there was always some ventilation and set the over-run on the fan to 20 minutes! Cleared the black mould in about three weeks and it's never returned.

A measurement taken in the rest of the house will show what is 'normal' for any particular day.

My bathrooms have humidistat-controlled fans. When there's an low-lying fog outside they just run and run which is annoying but otherwise they are great.

I have Greenwood Airvac ones, wired so that as well as a rise in humidity starting them, they can also be started manually. I didn't want the more conventional door-operated switch as I don't want to have to listen to a fan run for ages in the middle of the night or if someone has gone into the room just to change a loo-roll or whatever; instead there's a 'trigger' switch outside the bathroom next to the room light switch. I looked for ages for a push-to-make switch but eventually just ended up with a normal light switch which I usually just flip on & off - that's all the trigger the fan circuity needs. It can also be switched permanently on - useful if someone's got sickness and the trots and is going to be in the bathroom for much more than the normal run time.

So I can start the fan if I'm about to go for a shower (better/faster than waiting for it to detect high humidity and start automatically), or if the air's become smelly. The fan will run until the humidity is down. If a fan is running continuously on a foggy night I just turn it off at the safety isolator switch.

A while ago, but there was a device from Xpelair which was a humidistat and a timer combined. I had this installed when we redid our main bathroom, with a pull switch to work the timer. So you could turn it on when using the loo, it didn't automatically come on when you put the light on, and the humidistat coped with you forgetting to turn it on or there still being humidity around when the timer ran out if you were having a bath or shower. Coupled with two fans, one over the loo and the other over the bath, this worked really well.

Seem to recall having major problems explaining it all to the electrician who "didn't do things that way".

With regard to your question, we have digital clocks/weather stations which claim to measure internal humidity but they do not always agree with each other on temperature or humidity level - so I wouldn't expect too much accuracy.

Sounds like what is really needed is a dewpointistat

Yes, but electronic humidity meters are of very variable accuracy. Since the purchasers usually have nothing to check them against, the makers get away with selling fairly useless instruments.

When I was in uni, a long time ago, there was a project to build an Rh meter. Human hair was the most reliable measuring device. There are DIY plans on the web, some very simple, The only seller of human hair hygrometers I know of is Barometer World.

You could also fix up a sling hygrometer fairly easily; record wet and dry bulb temperatures, work out Rh from a psychrometric chart.

Dew point temperature below the wall's surface temperature. I'm not going to try to explain that here.

Given the cheapness of temp sensor chips and the availability of humidity sensors, it would be feasible. Given the price of existing humidistats ( I don;t know if most of those are mechanical or electronic now), it would be feasible to produce one with a profit margin. Pop a run on timer and a pull- cord trigger input in for free (virtually) too.

As if by magic, as I logged into Onecall just now, right on the front page:

Not having read the programmign datasheet, superficially it looks like, once set, the alarm outputs are automatic and simple...