Weather Station Relative Humidity Measurements

[Hairy Celt]

Here's another query...

I've had an Oregon Scientific weather station running for about three weeks now. The outdoor temperature / humidity sensor is on a south-facing wall (temporarily, it'll be moved to a north-facing location very soon but it is currently shaded from direct sunlight all day) about 1.5m above the ground. About half the time it reports 98% relative humidity (RH). I've made the assumption that this should be 100% and have adjusted it in Weather Display accordingly. My query is this: does it seem realistic to have 100% RH so much of the time, or is it possible that I either have a faulty sensor or that the sensor's located inappropriately? I should add that the temperature measurements seem okay...

[Cloudburst]

Here's another query...

I've had an Oregon Scientific weather station running for about three weeks now. The outdoor temperature / humidity sensor is on a south-facing wall (temporarily, it'll be moved to a north-facing location very soon but it is currently shaded from direct sunlight all day) about 1.5m above the ground. About half the time it reports 98% relative humidity (RH). I've made the assumption that this should be 100% and have adjusted it in Weather Display accordingly. My query is this: does it seem realistic to have 100% RH so much of the time, or is it possible that I either have a faulty sensor or that the sensor's located inappropriately? I should add that the temperature measurements seem okay...

Well it all depends on the weather you have. Here in Worcester the max humidity i have recorded was 96% but only lasted for a few hours close to a weather front. What is the reading at present??

[Hairy Celt]

That's the nub of it: what to compare to... Currently 100% and has been since 22:00 last night apart from about 20 minutes this afternoon when it dropped to 97%.

[Nabraxas]

Mine nearly always reads the humidity as 98% (100%) at night, although if my weather knowledge was correct that would mean the air is saturated and the dewpoint would be equal to the temperature yet, according to East Midlands Airport, the temperature is 6c and the dewpoint is 4c.

[johnholmes]

hi

not sure why its so high for most of the time. For example, mine has varied between 70 and 95% over the past 24 hours. It should show a diurnal cycle(daily) higher at night and lower by day.

regards

John

[Cloudburst]

That's the nub of it: what to compare to... Currently 100% and has been since 22:00 last night apart from about 20 minutes this afternoon when it dropped to 97%.

It may be picking moisture up from the ground, try and get it away from the ground and any walls etc, as thye can give false readings. However if you have mist or dew and it reads quite high then i would say its working o.k.

[reef]

Certain stations (particularly older LaCrosse models) dont actually record humidities above 95%! I know this for certain on my LaCrosse WS2300 as I tested it out in a fogged up bathroom before I fitted it. The result was exactly the stated maximum of the model, which is 95%. Nevertheless, many nights the humidity hovers around 93-94%, I dont think Ive seen more than a handful of 95% readings in the time Ive had it running.

[frogesque]

If anyone wants to check an RH reading there's an online simplified psychrometric chart and how to use them. It's American so is in °F but it's easy enough to convert.

NebGuide: Air Properties

If you take wet and dry bulb thermometer readings you can determine the RH from the chart then compare it with your sensor readings. Wet bulb readings should be taken out of sunlight in moving air and the ususal method is to fix a linty rag or cotton wool round the bulb and part way up the stem of a good thermometer, reliable to 0.1C or better, saturate the wick then twirl it slowly round your head for a few minutes. This will always be lower than a standard dry bulb reading unless the RH is 100% when fog or other fine precipitation will usually be present. Under those cirmstances it will be the same (or virtually so).

Edit: Full Psycrometric Chart (Sorry - again it's in old units)

Edit 2: Finally! an SI chart with good detail. 1 page .pdf file, prints off nicely

Universal Industrial Gasses Inc.

Edit 3 Or if you're feeling lazy just plug in the numbers, even takes your altitude into consideration

Mumble-mutters - Thermodynamics was never that easy when I was a lad!

[Verglas]

I have a mechanical instrument on my desk with baro/temp/hygro and it seems to hover about 60% but havn't tried it outside or have anything to compare it with

[frogesque]

I have a mechanical instrument on my desk with baro/temp/hygro and it seems to hover about 60% but havn't tried it outside or have anything to compare it with

Just take a wet bulb reading as outlined above then plug the numbers with your altitude into the handy caculator I linked to. It will then spit out an RH you can compare to your indicator.

[frogesque]

Just checked my own RH in the house (Sad aren't I? )

Dry bulb reading 17.5C

Wet bulb reading 11.8C

Height asl 100m

Result RH = 50.85%

No special kit required, I used a home brew wine and beer thermomometer and tied a strip of rag around the bulb and stem, soaked it then waved it about for 5 mins until the reading settled for the wet bulb. The divisions aren't that accurate and the decimal places are estimates but using the same thermometer for both reading helps to eliminate errors.

Outside: Db = 9.3C Wb = 9.2, RH = 98.78% (as you would expect - it's blowing through a fine cold drizzle )

[Verglas]

Just checked my own RH in the house (Sad aren't I? )

Dry bulb reading 17.5C

Wet bulb reading 11.8C

Height asl 100m

Result RH = 50.85%

No special kit required, I used a home brew wine and beer thermomometer and tied a strip of rag around the bulb and stem, soaked it then waved it about for 5 mins until the reading settled for the wet bulb. The divisions aren't that accurate and the decimal places are estimates but using the same thermometer for both reading helps to eliminate errors

blimey, what are you brewing that's 50%

[frogesque]

blimey, what are you brewing that's 50%

Ah! That's another story I should perhaps add that the rag was soaked with plain tap water

[Verglas]

well now, done that,

inside

temp dry 20c

wet 12.5c

Psychrometric calculator says 41.81%, (my hygro says 62% !)

outside

temp dry 11.7c

wet 8.5c

Psychrometric calculator says 66.09%

Although it's a nice bit of german engineering I suppose the instrument is not classed as a scientific device, more a decorative home object, but maybe it just needs calibrating ? (can't see anything other than the baro' adjustments)

[frogesque]

Ah! Much as I suspected. 62% would be quite high for indoors unless you were in a bathroom or kitchen. To calibrate your little gismo you may have to remove the front bezel, pull the hand off then replace it so that it reads correctly as I doubt if there is an adjustment for it. Usually they are just decorative and will only provide an indication of changes, not absolute values - a bit better than seaweed perhaps but not a lot.

Just for the exercise (how much fun can you have on a wet aftrnoon ) I've put the heating on (Gas warm air) and I'm now getting

Db 23.4C

Wb 14.9C

giving an RH of 39.84% (40%)

[Verglas]

Ah! Much as I suspected. 62% would be quite high for indoors unless you were in a bathroom or kitchen. To calibrate your little gismo you may have to remove the front bezel, pull the hand off then replace it so that it reads correctly as I doubt if there is an adjustment for it. Usually they are just decorative and will only provide an indication of changes, not absolute values - a bit better than seaweed perhaps but not a lot.

Just for the exercise (how much fun can you have on a wet aftrnoon ) I've put the heating on (Gas warm air) and I'm now getting

Db 23.4C

Wb 14.9C

giving an RH of 39.84% (40%)

it's not built like that, it's the bottom of the triple cylindrical scales, baro on top, temp in the middle and hygro at the bottom, see the picture above in the previous post, there's nothing to get at !

[frogesque]

Just had a look at your photo ( ) - It's a nice looking little instrument and you'd think there aught to be a means of calibration. Does it have a manufacturer's name on it anywhere? It might be possible to find out a bit more about it. Certainly taking it apart looks like a job for a clock or instrument maker and you don't want to start poking about with it before you know what you are doing.

As a matter of interest though, do you take the dome off when taking readings as this could make a big difference in what it's sensing inside its own little greenhouse?

[Verglas]

Just had a look at your photo ( ) - It's a nice looking little instrument and you'd think there aught to be a means of calibration. Does it have a manufacturer's name on it anywhere? It might be possible to find out a bit more about it. Certainly taking it apart looks like a job for a clock or instrument maker and you don't want to start poking about with it before you know what you are doing.

As a matter of interest though, do you take the dome off when taking readings as this could make a big difference in what it's sensing inside its own little greenhouse?

Hi Froggy

It's one of Wm. Widdop's products, my xmas pressie in 1992 !. As you say it is a very pretty device, says made in Germany.

I'll have to see if I can find the instructions, probably put away safe somewhere !, or get Mrs V. to ask for a copy next time she is ordering from them (it came from her family business)

It has vents in the base for air, I don't have to take to cover off, doesn't seem to make any difference.

It has a beized metal foot under the wooden rim, which also gives access to the baro trimmer but nowhere is there any other adjustments apart from the numerous levers and arms on the baro section on top.

Another pic shows it better

[Hairy Celt]

I brought the device indoors yesterday and it quickly responded - RH down to about 55% within an hour. So it (probably) isn't faulty. All I can think is that it is pretty damp around here (close to the sea, sun very low, damp grass nearby), so maybe it really is right at 100% where it is, and a more representative reading would be made in free air up a tall pole - which probably how it's supposed to be done, not screwed to the garage wall at 2m agl!

[Cloudburst]

I brought the device indoors yesterday and it quickly responded - RH down to about 55% within an hour. So it (probably) isn't faulty. All I can think is that it is pretty damp around here (close to the sea, sun very low, damp grass nearby), so maybe it really is right at 100% where it is, and a more representative reading would be made in free air up a tall pole - which probably how it's supposed to be done, not screwed to the garage wall at 2m agl!

Sounds like a good idea to put it up higher :lol: , seen as your not far from the sea i would say that your RH is correct, i have my Temp sensor on a 1 metre pole attached to my bathroom roof, and the data seems to be not affected in any way.