Keith (Southend) wrote:

Ah, I think I found the problem.

Sometime ago I used a wet/dry and the hourly metar readings from
Southend Airport and compared each hour with what my Davis was
reading. I calculated the average deviation and made the necessary
adjustment on the console. Subsequently, I 'tweaked' the reading up by
7%. Unfortunately I seem to remember I was told that the accuracy
above 90% RH is not good with these sensors and upto now I have not
had a problem. However, I obviously need to tweak things back a tadge.

I have tweaked it back 3%...

All the same I will have another monitoring excersise to do.

You might also need to monitor at the same location -- relative
humidity can vary considerably over relatively short distances
depending on the immediate environment as well as temperatures.

Nor are you likely to find an accurate linear relationship between the
Davis sensor and values derived from a Mason hygrometer (wet and dry
bulb). Mine varies by at least 5%, and possibly much more, across the
normal range of observations -- which, IMHO, makes it doubtful if
there's ever any point in tweaking.

--
John H