People who regularly watch the weather news on television are familiar with the term relative humidity. They understand that higher humidity numbers mean less comfort in the warm months. Scientists have many more reasons to track humidity beyond simple comfort. For engineers, scientists and weather reporters, humidity sensors provide the information. This short piece will introduce the types of measurement and sensors used.

The starting point of any discussion of humidity is a definition. Air or any gas can hold an amount of water vapor. This vapor content in a gas is humidity and is not visible. Visible forms of water (steam, fog, snowflakes) are condensing from vapor to liquid. There are three basic measurements of humidity. Each different type uses different measurement techniques and is reported in specific units of measurement. The ranges of these measuring systems overlap and allow mathematical conversion between units of measure.

The most common humidity measurements are dew point, relative humidity, and relative humidity. Each of these is best suited for specific applications where the range and accuracy of the measured values is used in research or process control. Humidity measurement has advanced along with other technologies and is a common part daily life.

The “bad hair day” is caused by the way human hair responds to humidity. Early scientists took note of this effect and built the first relative humidity instruments around it. Horse hair was also used in some indicators and chart recorders. With the invention of synthetic materials like nylon, hair-based instruments became obsolete. Synthetic fibers have more predictable responses and are more easily calibrated. The invention of transistors and solid-state electronics led to the modern humidity sensor that is tiny, relatively inexpensive and much more robust.

Dew point is the combination of temperature and gas pressure at which water vapor begins to condense into liquid. Most people understand this in weather report terms but it is an important factor in many scientific calculations and processes outside of weather. Early dew point measurements used chilled mirror technology which was both expensive and temperature limited. Modern electronic dew point sensors are accurate down to -40degrees F.

The most precise humidity measurement is absolute humidity. This is the measurement of the weight or mass of water vapor in a specified volume of gas. The unit of measurement is grams per cubic meter or grains per cubic foot. Absolute humidity is useful in most engineering or scientific applications where the values are used in calculations. Today’s sensors can measure absolute humidity in polluted environments up to 200degrees C.

Engineers sometimes encounter problems with manufacturing processes that are related to humidity. With modern sensors the environment can be monitored and the process adjusted for better results. Painting processes are particularly sensitive to humidity changes.

Some internet research can find humidity sensors to meet almost any requirement. Today’s sensors are accurate, tiny, relatively inexpensive and much more durable than the early horse-hair based indicators. And you will not need to rely on a weatherman.

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