Trusting a Humidity Sensor

[Musicmanager]

Hi Guys

I'm currently involved in building a large digital clock for my favourite Daughter in Law which apart from the proverbial DS3231 RTC includes a Sensor which provides a value for Temp, Baro pressure in hPa & a Humidity percentage.

My knowledge of weather and the like is almost as limited as that of electronics; I know that rain is wet and Martini's are Dry, but apart from that - Jack Diddley !

The Sensor is showing a value @ 61.8% Humidity, but my phone Weather App states 91% ;

Is that because the App is providing a generalisation and my Sensor is currently on a breadboard tucked under a shelf in my workshop, or do I need to think about making some adjustment ??

Any wisdom available please ?

Thanks

MM

 

[Nigel Goodwin]

Well the weather app is a generalisation over a large area, and outside as well - the sensor is measuring the humidity in your workshop, which you'd hope will be drier than outside?.

I'm also not very impressed with humidity sensors, put them side by side and they often read completely differently.

 

[Inquisitive]

Snip!

My knowledge of weather and the like is almost as limited as that of electronics; I know that rain is wet and Martini's are Dry, but apart from that - Jack Diddley !

Snip!

How about Bo Diddley?

 

[Ian Rogers]

How about Bo Diddley?

That would be worse... Bo didley in USA... Jack Didley here!

 

[Inquisitive]

Actually Bo Didley as in musician. Always heard it as Didley Squat here.

Now back to our regularly schedule thread content.

 

[JimB]

but my phone Weather App states 91%

A hot steamy electronics workshop in the leafy suburbs of Brum!
Who would have thought it?

Fit like loon?

JimB

 

[Ian Rogers]

Wow..MM Moderator magnet or what!!!!

I agree with Nigel... The room its in will be less humid than outside.. ( Unless your phone has a humidity sensor in it!!)

 

[Musicmanager]

Wow..MM Moderator magnet or what!!!!

.. .. .. and not ONE sense of humour between them !

The room its in will be less humid than outside..

Will It ?
So when I open the outside there is a rush of humidity in, or a lack of it out ?

Mmmm .. .. .

MM

 

[picbits]

91% is a pretty large value, we run our test cells between 45 and 60% RH which is pretty much dependent on the weather. Are you sure it isn't a 91% chance of rain ? RH sensors are notoriously inaccurate and we kill them regularly with contamination / fires / antifreeze etc.

 

[Musicmanager]

Are you sure it isn't a 91% chance of rain ?

Yes for sure, the same App stated chance of rain @ 51%

However, my son made a valid point this afternoon .. who knows ? who cares ? D in Law certainly won't !

MM

 

[unclejed613]

if you have air conditioning, you will have lower humidity than outside because that's one of the uses of air conditioning, to control humidity as well as temperature. that's why air conditioners drip water, they're removing moisture from the air.... unlike what we have here in Colorado USA, where the air is usually very dry, we use "swamp coolers" that increase the humidity and cool the air by evaporation of water. basically the water is sprayed on what's essentially a furnace filter, and air is drawn through the filter by a fan.

 

[Nigel Goodwin]

91% is a pretty large value,

It'd be higher than that if he lived in Manchester

 

[Nigel Goodwin]

if you have air conditioning.

Domestic air conditioning is pretty rare in the UK.

 

[Musicmanager]

Fit like loon?

Nae sae bad quinie, fit like yasel?


MM (A Wee Sassenach in Steamy Brum)

 

[Musicmanager]

Domestic air conditioning is pretty rare in the UK.

Air Conditioning in Brum ? You gotta be kidding !!

We're only just getting around to having air !!

 

[Nigel Goodwin]

Air Conditioning in Brum ? You gotta be kidding !!

We're only just getting around to having air !!

You've got air? - where did you steal it from

 

[JimB]

Nae sae bad quinie, fit like yasel?

Quinie!

Surely nobody could confuse me with a Quinie!

JimB

 

[Musicmanager]

Surely nobody could confuse me with a Quinie!

Maybe .. . ' .. .. at dusk, with the light behind you ! .. '

Sorry Jim .. .. the perils of a quick Google translate .. .. .

S

 

[Reloadron]

put them side by side and they often read completely differently.

That pretty well covers it. There are ways to calibrate them depending on the sensor but even then they are seldom accurate over their entire range. With central (whole house air) air-conditioning the inside humidity will be much lower normally than the outside air humidity. How do I calibrate the Relative Humidity Sensor? should get you started with a few methods. I used saturated salts quite a bit but we were doing quite a bit of humidity work so had access to quite a few salts. For a simple single point the wet bulb / dry bulb method works fine as long as you have two decent thermometers and a wick.

Generally "close" is good enough.

Ron

 

[schmitt trigger]

Also:
Some RH sensor devices may have significant hysteresis.