Old Measurements Never Die

Maastricht is the second oldest city in the Netherlands. With its strategic location on the Maas River, it has experienced its fair share of wars and battles. Many millions of years ago, this region lay at the bottom of a tropical ocean; the fossils that can be found in many marl caves in the vicinity bear tangible witness to a distant past rich in water.

Maastricht nestles close to St Pietersberg. Although St Pietersberg is more of a hill than a real mountain, with a bit of imagination you can treat yourself to the thrills of acrophobia. At the top of the mountain, volunteers have set up a small exhibition on the mountain, marl quarrying, the equipment used for this purpose, and so on (bear with me while I get started with my story…) – and that’s where I saw this instrument.

It’s a hygrometer. Nothing special, you might say, but the measuring method is pleasantly interesting. The instrument has two thermometers: one dry and the other wet. The difference in their temperatures is a measure of the relative humidity, which you can read from a table. If only everything were this simple.

I was reminded of this instrument by our preparations for a weather station project, which have been underway for a while now, and our work on a new design for the CO2 meter published in the January 2008 issue. We plan to upgrade the original design with a temperature sensor and a humidity sensor. Naturally, we intend to use a ready-made humidity sensor for this, but the old instrument still has a certain charm, perhaps due to its simplicity or the self-evident operating principle. It seems like you learn something from this instrument. The operating principle is somewhat similar to that of a differential amplifier.

Who knows, maybe one of you will put together an electronic version? Or am I simply making things unnecessarily complicated again?

Wisse Hettinga