Thanks for the ideas. I think a capacitative sensor is the most accurate. I need the 0-1 volt output to use for a chart recorder. The realistic humidity range in the UK is about 40% - 100%. I have read that some cap. sensors don't / can't read above 90%, as the dielectric then becomes saturated and loses accuracy. I have also seen one which is affected by light!
The ones I have found are all quite expensive, specialist , and are not easy to find. Has anyone made up a circuit using HIH 3610 (Honeywell), H1 (Philips) or SMTHS 10 (selectronic) ? These are the commonest ones. I need to trim the output so that 100% gives 1Volt. Any help please. Thanks.