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Hygrometers

by Chris Woodford. Last updated: April 18, 2014.

If you've ever been in the dry of a desert or the sopping steamy heat of a rainforest, you'll certainly remember it. What makes these extreme environments so different from one another is their humidity: the amount of water vapor in the atmosphere. Deserts, obviously, contain little or no water, while trekking through a rainforest can feel just like walking through a shower. Measuring humidity is an essential part of weather forecasting and it's also very helpful for gardeners with glasshouses and people who run saunas. We can do it simply and efficiently with cunning instruments called hygrometers. Let's take a look at how they work!

Photo: This Stevenson screen contains a hygrometer and other weather forecasting equipment. The white louvered box protects the instruments from the direct heat of the Sun but allows air to circulate inside, so giving more reliable measurements.

How can we measure changes in humidity?

Top: A pine cone closed up in wet weather. Bottom: A pine cone opened up in dry weather.

Lots of plants react to changes in humidity. Pine cones open their spines when it's dry (to release seeds) and close them tight when it's wet. That's why (as most children know) you can use a fallen pine cone to figure out how humid it is outside. Pine-cones aren't the most accurate hygrometers, however, not least because it takes quite a while for them to open and close—but you can still make fun and interesting home hygrometers with them and they make good science fair experiments (see in the links down below).

Photo: A pine cone is a simple hygrometer. It closes up tightly when it's wet (top) and opens when it's dry (bottom). Although you can build a decent little home hygrometer with a pine cone, it will take some time to respond to changes in humidity.

How a typical twisted fiber hygrometer works. This one was invented by Louis Ullman and shown here in his patent application drawing, US patent #25,457, from September 13, 1859.

Twisted-fiber hygrometers

Some humidity measuring devices aren't much more sophisticated than pine cones. In a weather "house," a little man and a little woman stand in two doorways of a closed wooden box. When it's going to rain, the man comes out of his door with an umbrella; when it's dry, the man goes inside and woman pops out of her door instead. Inside the weather house, the two figures are mounted on a turntable and suspended from a piece of tightly twisted hair (or plant fiber). When it's dry, the hair tightens up and twists the turntable one way. In wet conditions, the hair loosens and the turntable rotates the other way instead. Just as you can make a home hygrometer from a pine cone, so you can do the same thing with a chunk of your own hair—or an obliging friend's! (Again, you'll find some links below.)

Artwork: Left: A typical twisted-fiber hygrometer. Before electronic hygrometers became popular in the 20th century, most inexpensive hygrometers worked like this one, patented by Louis Ullman of Nashville, Tennessee in 1859. It has a box (open to the air—so moisture can get in and out) with a piece of twisted plant fiber (orange) inside. The fiber is connected to a pointer (red) that turns around a dial and, as the humidity changes, the fiber either tightens or loosens, moving the pointer up or down the dial. As Ullman explains in his patent, various different plant fibers can be used, including ones from Geranium erodium. Artwork from US Patent #25,457: Hygrometer courtesy of US Patent and Trademark Office.

Psychrometers

Line drawing of a wet-dry bulb thermometer type of hygrometer, also known as a psychrometer.

Pine cones and weather houses give a fairly vague indication of humidity, at best. How can we put some numbers to humidity and measure it more accurately? One way is to use an instrument called a psychrometer (also known as a wet- and dry-bulb thermometer). It uses a pair of thermometers standing side by side. One has a bulb open to the air; the other has a bulb covered in a wet cloth. The water on the cloth causes evaporation and loss of heat from the bulb, making its reading lower than that on the dry-bulb thermometer. The amount of evaporation (and the lowering of the temperature) depends on how much water vapor there is in the atmosphere already. Measuring the temperature difference between the two thermometers lets you measure what's called the relative humidity: how much water vapor there is in the air compared to how much there would be if the air were totally saturated, written as a percentage.

Artwork: Right: A typical psychrometer (wet-dry bulb thermometer) has two thermometers side by side. One of them (left) is a dry bulb thermometer and simply measures the temperature of the surrounding air, like any conventional thermometer. The other thermometer (right) is the wet bulb: its bulb is immersed in a bottle or reservoir of liquid (green) at the base. You measure the humidity by comparing the readings from the two thermometers. Using the sliding pointer (blue), you can then read the humidity off the rotating chart (yellow) in the center, which is essentially a look-up table that converts temperature differences into humidity measurements. This particular version of the wet-dry bulb hygrometer was invented in the 1930s by John Leonard Schwartz of Philadelphia and the drawing comes from his US Patent #1,933,283: Hygrometer, courtesy of US Patent and Trademark Office.

Electronic hygrometers

A dial hygrometer made by Holmes

Photo (left): This Holmes electronic hygrometer has an easy-to-read dial. There are many other brands available, including Honeywell and GE Panametrics. Photo by courtesy of Ben Winslow, published on Flickr in 2008 under a Creative Commons Licence.

Sensing membrane from an electronic hygrometer

In an age where virtually everything is measured for us, instantly and electronically, the last thing many of us want to do is fiddle about with thermometers and wet cloths. Thank heavens, then, for electronic hygrometers. Typically, they measure the capacitance or resistance of a sample of air and calculate the humidity from that. In a capacitive hygrometer, there are two metal plates with air in between them. The more water there is in the air, the more it affects the plates' capacitance (ability to store a static electric charge). By measuring how much charge can be stored, it's possible to measure the humidity quickly and accurately. In a resistive sensor, electricity flows through a piece of ceramic material exposed to the air. The higher the humidity, the more water vapor condenses inside the ceramic, changing its resistance. Measuring how much current flows through the ceramic gives an accurate measurement of the humidity.

These days, you can even get hygrometer apps for cellphones, and you might wonder how a little electronic gadget with no obvious humidity sensor can make those kinds of measurement. It's a cheat! The app figures out your location from its GPS satellite receiver and sends a query to the local weather station server, which sends back the humidity measurement for your phone to display. Now that's a rough measurement of the humidity at the weather station, which may be many kilometers or miles from your home. It'll give you a rough idea of the overall humidity (if it's a dry or a wet day), but not of the exact local humidity right where you are.

Photo (right): The ceramic sensing membrane from an electronic hygrometer. Photo by courtesy of NASA Langley Research Center (NASA-LaRC).

Hygrometers in weather stations

Photo showing US Air Force officer reading the instruments on a weather station with a remote-control panel.

A typical electronic weather station contains a thermometer (measures temperature), a barometer (measures maximum and minimum air pressure), a pecipitation (rainfall) gauge, an anemometer, and our old friend, the hygrometer!

Modern electronic weather stations usually have an LCD display that shows all the measurements automatically and updates them instantaneously, saving the need to read the instruments one by one. Using the measurements, a microchip inside will also figure out and display something called the tendency (a rough forecast for the next day's weather, summarized with a simple picture such as a sun (fine day), partly obscured sun (dull day), or raincloud (wet day). Electronic stations also generally have a memory so they can record hundreds of separate measurements covering the last few months. Some stations can be connected to a computer with a USB cable so you can upload your data and draw proper weather and climate charts.

Photo: The main parts of a portable, military weather station. This one can send its readings automatically using a solar-powered transmitter. Photo by Maynelinne De La Cruz courtesy of US Air Force.

Inventing your own weather station

Weather station design from a 1942 patent by Harry Diamond and Wilbur Hinman, US NBS, assigned to the US government, US patent 2287786.

It's easy enough to collect together a few basic weather-measuring instruments—a thermometer, hygrometer, rainfall gauge, and so on—to make your own local records and forecasts, but what about doing it automatically? Is there any alternative to buying a readymade electronic weather station? Of course! Thanks to the widespread availability of electronic microcontrollers such as the Arduino and Raspberry Pi, it's relatively easy to turn your PC into a DIY weather station that can receive data from electronic sensors and compile weather forecasts and climate charts. I've added a few links to Arduino-type weather-station projects at the bottom of the further reading below.

Now it's pretty cool and cutting-edge to build your own weather station, but how about if you'd been trying to do that a half century ago before computers and microelectronics revolutionized the world. Impossible? Not a bit of it! Flick back through the patents on record at the US Patent and Trademark Office and you'll find quite a few people have attempted to make mechanical, electrical, and electronic instruments that can automatically record data from weather stations. In 1942, Harry Diamond and Wilbur Hinman, Jr. of the United States National Bureau of Standards (NBS) built the wonderful automatic weather recording equipment you can see here. Using a cunning mixture of mechanical devices (levers, gears, and clockwork), simple electrical circuits, and a radio transmitter, it collected data about pressure, temperature, humidity, wind direction, wind velocity, and rainfall and automatically beamed it to a receiving office using coded radio signals. Eat your heart out, Arduino!

Artwork: How Diamond and Hinman's automatic weather station worked: First, they developed a generic mechanism (1, green) that could convert movements caused by various kinds of mechanical sensors into movements of a variable resistor—in other words, turning mechanical movements into measurable electric currents. Next, they produced simple mechanical weather sensors that would drive this mechanism in different ways. Three of them are shown here. 2 is an aneroid barometer in which an expanding and contracting aneroid cell moves a lever up and down with changes in pressure; 3 is a humidity sensor, which measures humidity using the tension of those wires, which presumably tighten or loosen according to how wet or dry the air is; 4 is a rainfall gauge in which a bucket moves downward and turns wheels as it fills with rain. Finally, they devised a way of turning the resistance measurements into codes that could be transmitted by radio signals. Read more about it in US Patent 2,287,786: Automatic Weather Station by Harry Diamond and Wilbur Hinman, Jr, assignees to the US government, patented June 30, 1942. Artwork by courtesy of US Patent and Trademark Office with a bit of minor editing and coloring of the original to improve clarity.

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How to make a hygrometer

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Text copyright © Chris Woodford 2009, 2012. All rights reserved. Full copyright notice and terms of use.

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Woodford, Chris. (2009) Hygrometers. Retrieved from http://www.explainthatstuff.com/hygrometers.html. [Accessed (Insert date here)]

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