In 1450, Nicolas Crifts invented the first hygrometer by weighing a tuft of wool. In 1480, Leonardo DaVinci re-invented the scale type hygrometer and recorded it in the Codex Atlanticus. In 1783, Horace Benedict de Saussere invented the first hair hygrometer.
For 562 years, people have been inventing different technologies designed to measure the water vapor content of air (humidity). As we all know, humidity is a tough measurement and choosing the most appropriate technology for your application helps to make the measurement more accurate, repeatable and reliable. You may find a hygrometer at eBay or at your local Radio Shack store with a price as low as $35 and you may find a high end chilled mirror hygrometer costing as much as $50,000. Price, accuracy, repeatability, and calibration requirements each determine the level of measurement reliability or risk of failure. The type of technology you choose matters. It’s important to match the technology to the application and related measurement environment. For this discussion, we’ll focus on humidity measurement technologies commonly used in controlled environments in the modern world.
Mechanical Hygrometers. A mechanical hygrometer exploits the expansion and contraction of certain organic materials like hair, cat gut, textile or plastic. As the material changes along with changes in relative humidity a series of levers connected to the material will move a needle and indicate the level of relative humidity. Pros: inexpensive and no power required. Cons: doesn’t work at high or low RH; drifts excessively and high uncertainty. This type of hygrometer is rarely used in industrial or controlled environments.
Psychrometer. A psychrometer uses the physical principle of wet bulb temperature as compared to the dry bulb temperature to calculate relative humidity and other humidity measurement parameters. By placing a wet sock or cloth around a temperature measurement device and passing a light breeze across the bulb, the water evaporates at a rate determined by the ambient relative humidity. The evaporation consumes energy and causes a lower temperature around the evaporating water. There are many different types of psychrometers and it is still a fairly common means to measure humidity. Pros: simple and fundamental measurement; low price; and tolerates condensation. Cons: high uncertainty; difficult to maintain consistency of operation; requires some way to calculate the output; and many variables affect the reliability of the result.
Condensation Hygrometer or Chilled Mirror. This technology measures the dew point temperature by measuring the diffraction of a light source reflected off of a mirror. The mirror is gradually cooled until condensation occurs. As condensation occurs, the light defraction is picked up by a sensor and the measured temperature of the mirror at the time is an accurate measure of the dew point. Pros: very low uncertainty; wide measurement range; excellent long term stability; and a fundamental measurement. Cons: expensive; requires training and skill to operate; mirror may not be able to differentiate between water or ice; high amount of cleaning maintenance; and very complex to construct.
Electronic Sensors. This technology includes two types of sensors; resistive and capacitive. Resistive sensors measure the electrical resistance across a small sensor that is coated with some type of hygroscopic material. As the relative humidity increases, the resistance decreases proportionately allowing a correlation between the resistance and the RH. Pros: small footprint; inexpensive; easy to mass produce; and interchangeable. Cons: limited range of measurement; high temperature dependency; and sensitive to contamination.
Capacitive sensors rely on the capacitance of an electrode. The dielectric is typically some type or polymer that reacts to changes in relative humidity. As the dielectric properties change, the capacitance changes and can be correlated to the level of RH in the surrounding air. Pros: wide measurement range; fast response; resistant to contamination; and excellent long term stability. Cons: requires electrical power and electronics; requires regular calibration; and low accuracy in very dry conditions (<5% RH).
Choosing the most appropriate sensor technology depends on many factors in addition to the pros and cons listed here. The measurement environment, required accuracy, and level of acceptable risk are all important considerations. For example, measuring RH in an office environment may not be as critical as measuring RH in a drug or medical device storage facility. Measuring the dew point temperature in a class 1 dry compressed air system requires a very different technology as compared to measuring relative humidity in a concrete curing room.