On Sep 23, 7:19*pm, Steve J wrote:
On 23 Sep, 17:35, Paul Hyett wrote:

How come, when temperature can be measured to an accuracy of 0.1C on
instruments costing under £100, almost all RH instruments quote an
accuracy of +/- 5% at best?

Hi Paul

At Bablake, we have a brand new Met Office AWS, we have a Campbell
Scientific AWS still maintained to Met Office standards and a
conventional Met Office screen with calibrated, Met Office checked
sheathed wet & dry bulbs from which we work out RH each day.

The AWSs have RH electronic sensors, but they don't always agree with
each other, and they can both be several % points at variance with the
wet'dry bulb thermometers. So there you have it - 3 readings every day
and all can be different even in their screens!

However, I don't use a whirling psychrometer very often, but if I did
I thinkk that might be a fourth different reading!

Does that help?

Thought not:-)

Steve Jackson FRMetS
Bablake Weather Station
Coventry UKwww.bablakeweather.co.uk

The humidity measured from wet- and dry-bulbs in a screen is
unreliable because of the unknown speed of the draught over the
bulbs. The formula used is something of a compromise, a guess even,
at the speed of the air movement over the bulbs and at low wind speeds
the wet-bulb depression is strongly dependent on the wind speed. A
whirling psychrometer is much more reliable (if whirled quick enough)
and is the method I use though I don't do it on a regular basis. I
don't know what type of sensor is used in AWS's - it probably doesn't
involve the evaporation of water but depends on the variation of
properties of some substance with humidity and is therefore likely to
be no better than a hair hygrometer and may have a time lag and also
respond differently at different temperatures. If I were still at
work I'd suck 100 litres of air through a cold trap and weigh the
water - not feasible, obviously. It's quite a tricky problem, it
seems, but ought not to be.

Tudor Hughes, Warlingham, Surrey