Humidity Settings

[geoffp]

Could anyone advise me how I could set up my Humidity settings, I compare my readings with two other sites in the near vicinity to me and to be somewhere near I have had to put a +7 offset into Cumulus, but as the day goes on I find that I may end up being 7or more % higher than these other stations.

Am I doing right by putting in an offset or should I be using the multiplier? ant advice will be appreciated.

[nking]

Hi Geoff,

I have noticed recently that my humidity seems out, higher than that reported by the other close sites. It’s also been slow to react to changes. From what you've described this is the same for you.

I've thought that either I'm in a micro climate that really is more humid, the position of the sensor is causing erroneous readings or the sensor is just faulty..... the Jury is still out!!

[geoffp]

Hi Neil,
In the past I have had my Sensor in various places to try and cure the problem but to no avail, I have now got it mounted 4ft 6 inches above a lawn and not in direct sunlight but it is still the same, maybe Steve will come to our rescue, he normally has a good solution to these problems

[steve]

The humidity sensor on Fine Offset stations seems to be quite poor. I occasionally compare my FO with my VP2, and sometimes they are quite close, and sometimes there's a large difference. Right now, VP2 says 83% and FO says 79%, which isn't too bad.

I don't think there's a simple solution, perhaps a combination of offset and multiplier is required (multipliers are applied first, then offsets).

[geoffp]

Many thanks Steve, I have just been putting on a straight offset, what would you suggest as a start for the multiplier?

[steve]

What you need is a set of FO readings and a set of 'correct' readings, and then we might be able to work out a mapping. I could do it with my data, but the result probably wouldn't be the same for you.

[steve]

It looks like it's hopeless. Here's a few comparisons from my data, sorted in ascending order of FO reading. The VP2 order is pretty much the opposite!

Code: Select all

FO VP2
66 94
77 92
79 83
82 86

[serowe]

The problem is, as has been hinted in other replies, the issue of 'micro climates' is coming into play.

One possible method is to make comparisons with an 'official' met reporting station that is close by. But, in order to help you, it would need to be very close (well under 1000 metres!)

The other method (expensive but the only real way to calibrate the sensor) is to have it laboratory calibrated. But, as mentioned, expensive and you have to ask yourself 'is this cost justifiable'?

Personally I would be using the readings it give and, if anyone asks, the only answer you can honestly give them is 'this is the reading where the sensor is placed'. (The same really goes for all home weather station sensors - we get what we pay for and, whether we like it or not, only paying a few hundred dollars/pounds/francs/rubles, really doesn't cut the mustard if oyu want super accurate readings 100% of the time.

One final thought - if you are close to a met office, you *might* be able to get them to calibrate it for you - but this would depend on the person and day of the week when you asked! (some of the observers at Australian met offices outside the capital cities have been kown to do this sort of thing for the locals, but this is usually in the smaller, outback obs offices (East Sale here in Victoria used to have one observer that would do this but, unfortunaely he left the Bureau about 10 years ago and his replacement was a right PITA).

[geoffp]

Thanks Steve and Serowe, Looking at your figures Steve I think it may be best to leave what will be will be.

[beteljuice]

Another thing to consider with the comparisons is the 'time to respond' of the sensors.

They can takes hours to cope with a large change (As I discovered when I dried out an over-damp hygro a couple of times )

[sooty]

The other method (expensive but the only real way to calibrate the sensor) is to have it laboratory calibrated.

You can actually calibrate the sensor inexpensively at home if you are up for a little science experiment. The humidity of the air in a closed container holding a saturated salt solution will be predictable and only dependent on the salt and the temperature. Google will find many how-to's on the web, like this bare bones one http://www.smartec.nl/pdf/apphs1002.pdf, and even kits (for illustration only, I wouldn't call this one inexpensive) http://www.nkhome.com/support/instructi ... it_07.html

[nking]

As I had a spare kit (purchased from Maplin on a special low price some months ago) I decided to switch over the sensor as I was convinced that my Humidity reading was high.
I made the mistake of not leaving the sensor outside long enough to acclimatise so at the moment the Humidity is very low reading 47% whereas the old one was reading 81%. Will have to wait a few hours to see what happens.
I’m reminded of a saying which goes along the lines of “Give a man a thermometer and he knows the temperature. Give him two thermometers and he is not sure what the temperature is!” Same goes for humidity sensors