From: Neville Michie
Sender: time-nuts-bounces@febo.com
To: richard@karlquist.com
To: Time-Nuts
Subject: Re: [time-nuts] HP 10811 Response I Replies
Sent: Sep 22, 2011 8:10 PM

Humidity is a confusing subject to many engineers and scientists.

Unlike most parameters it is a quantity with two input variables, concentration and temperature. There are many ways to combine these to give different units.

As a research scientist I spent most of my career working with “composite materials” which exhibit great sensitivity to humidity.

Most composite materials respond to the Relative Humidity with only a small temperature dependance. So 80% RH has the same effect at any temperature. Note that the Absolute Humidity varies exponentially with temperature. For a fixed Absolute Humidity (say 10gms /m3) and at 70% RH the relative humidity changes about 10% per degree celsius. So if you have a sealed container with some water vapour in it the RH will vary about 10% per C*.

If you have a fixed ambient humidity, a heated enclosure will have a humidity that falls 10%/K as the temperature rises.

Now there are many grandiose “environmental chambers” sold to scientists and engineers that perform poorly. They have internal temperature gradients, so even if the concentration of water vapour is uniform the distribution of relative humidity is not.

If a chamber set to 80% RH has a 2 degree gradient it could have internal condensation.

The problems are made worse by the plethora of nearly useless humidity probes made by manufacturers who are having a bidding war based on claimed specs.

Since there are very few facilities to calibrate humidity sensors, and no company can make a dollar by having their humidity measured more accurately, there is no pressure to improve instrument quality and the situation remains that there is a lot of misunderstanding about humidity.

The instruments I have built have an inaccuracy of less than 0.1% RH, and I have built isothermal chambers that can be programmed to 0.1% RH. They are based on calculable processes for calibration, and so have absolute calibration.

In the case of quartz crystals in ovens, when the oven is 30K above ambient the relative humidity is very low, so you would expect there to be very little absorbed or adsorbed water to interfere with stability. The main effects are surface leakage on hydrophilic surfaces and dielectric absorption in composite material insulators. There is a second order effect that the dielectric constant of air changes with absolute humidity. Humidity sensitivity would seem to me to be a problem of the measurement system rather than the item being tested.

Cheers, Neville Michie