| Most people are familiar with relative humidity as | | | | The temperature at which water vapor begins to |
| discussed by the local weather reporter. Humidity has | | | | condense out of a gas at a specified pressure is |
| a significant effect on comfort as most of us know. | | | | called the dew point. It is a common weather report |
| Very few people realize that humidity is important to | | | | term. It is also important in scientific calculations and is |
| scientists and engineers in many different ways. | | | | used in manufacturing situations or processes. The |
| Weather reporters, scientists and engineers all need | | | | first dew point instruments used a chilled mirror to |
| some way to measure humidity. This short article will | | | | determine the temperature of condensation. These |
| describe some types of humidity sensors commonly | | | | sensors were expensive and difficult to keep |
| used to get the measurements. | | | | calibrated. Electronic technology used in modern dew |
| Humidity is the measure of water vapor suspended is | | | | point sensors can make measurements at |
| air or other gas. Water vapor is invisible when | | | | temperatures as cold as -40degrees F. |
| suspended in a gas. Fog, steam, rain, clouds or other | | | | The third common humidity measurement is absolute |
| precipitation are water condensing out of suspension | | | | humidity. This is the calculation or measurement of |
| and becoming visible (and wet). Scientists have | | | | the weight (actually mass) of water vapor in a |
| different methods to measure humidity in terms | | | | volume of gas. This measurement is expressed as |
| directly related to the way the measurements are | | | | grains per cubic foot or grams per cubic meter. |
| used. Some humidity measurements can be | | | | Absolute humidity is the most useful measurement |
| mathematically converted to other measurement | | | | for calculations and is found in many applications. |
| scales (within certain limits). | | | | Electronic sensors can measure absolute humidity in |
| The most often used humidity scales are absolute | | | | gases as hot as 200 degrees C., even in polluted |
| humidity, relative humidity and dew point. These | | | | environments. |
| types of measurements, although measuring the | | | | Engineers sometimes encounter problems with |
| same thing, have different units of measure. The | | | | manufacturing processes that are related to humidity. |
| way the measurements are used determines the | | | | With modern sensors the environment can be |
| best method and scale for a particular application. | | | | monitored and the process adjusted for better |
| Most people have experienced the "bad hair day" | | | | results. Painting processes are particularly sensitive to |
| that is caused by high relative humidity. Scientists | | | | humidity changes. |
| used the changes in human hair to detect and | | | | The demand for humidity sensors has prodded the |
| indicate relative humidity in early sensors. Horse hair | | | | development of a wide range of sensors from |
| was also used in mechanical devices to indicate | | | | several manufacturers. Engineers can choose |
| humidity. When material science began producing | | | | combinations of cost, robustness, measurement |
| artificial materials such as nylon, researchers soon | | | | range and accuracy to meet any sensing requirement. |
| realized these synthetic materials were more | | | | The internet is the best starting point for sensor |
| predictable and more accurately calibrated. The | | | | research. The researcher can quickly see the range |
| evolution of humidity measurement has led to the | | | | of available products and which best meet the |
| solid-state humidity sensor that is relatively cheap, | | | | requirements. |
| extremely accurate and damage resistant. | | | | |