Stan Schultz wrote: Anything that produces heat can b

It's now mid or late spring in South Africa. You don't have to worry about excessively low temperatures for another 6 or 8 months. That should be plenty of time for you to "think outside the box," acquire the necessary hardware from scrap heaps and second hand stores, and finally take a weekend to throw together a respectable little case to hold your collection safe from the cold.

It is actually summer at present, but our weather seems to be a bit topsy turvy at the moment.
Normally by this time of the year we hit 30 degrees (86F) + and for weeks on end, and hardly have rain.
Over the past couple of weeks we have had rain almost every week
The hottest day was about 3 weeks ago when we had a heat wave. other wise the temps only get to 28 degrees (82.4F) a little hotter during the day and colder at night.

It is now 21:31 at night and the temps outside are sitting at 18 degrees (64.4F), still does not warrant the use of heating.pad though.

To come back to your Question.

I only use Hygrometers for my snakes, and when a female is sitting on an egg sack other wise they are packed away in cupboard.

Electronic meters seem to be the most accurate. give or take the price and brand.
But if you want to make sure if it is working properly get your self a mechanical meter as well.
And put both into the the area you want to check and you will work out more or less.

But really if you are just buying to check your temps and humidity in your spider room or cage it not really important if it is a couple of degrees off,, same goes for an egg sack a small percentage of lose want be to much of a problem.
That why they say 80% to 90% you given a wide target to be able to hit.

So it comes down to you and what you are willing to spend of a device.
Repti-zoo seems to be a good product and there prices are descent.

Perhaps this will answer your question..

people.ucalgary.ca/~schultz/Humidity.html