Hygrometers and humidity meters are instruments used to measure the amount of water vapor in gases, most often air. The amount of water vapor in air, the humidity, is an important consideration in a wide range of commercial and industrial processes. High humidity can cause corrosion and other moisture-related problems. High humidity also makes temperatures feel warmer, affecting comfort levels and straining HVAC systems. Low humidity, on the other hand, will dry things out and can lead to a build-up of static electricity. While humidity meters do not in any way add or remove water vapor from a sample, they are important instruments for knowing how much water vapor is present allowing measures to control it or to mitigate potentially harmful effects.

Why Measure Humidity?

Nature controls humidity via the evaporation and condensation of water. Air is typically about 1% water vapor by mass though it can vary quite a bit depending upon environmental factors including temperature and atmospheric pressure. For example, the water content of air at sea level can get as high as 3% by mass at 30°C (86°F) compared to no more than about 0.5% by mass at 0°C (32°F). Since the amount of water vapor the air is capable of holding depends on multiple factors, there is more than one way to quantify humidity.

Humidity is generally measured in one of three ways:

Absolute humidity refers to the amount of water vapor present in a unit volume of air, usually expressed in grams per cubic meter. At 30°C (86°F), absolute humidity can range from near zero to about 30 grams per meter.

Specific humidity is the ratio of water vapor to dry air. It is equal to the mass of water vapor in the air divided by the total mass of the air parcel, including the water vapor. The value is expressed as a percentage.

Relative humidity is the ratio of the amount of water vapor in the air to the amount required for saturation at a given temperature and pressure. The value is expressed as a percentage.

Depending upon the application it may be preferable to measure absolute, specific or relative humidity.

The Effects of Humidity

Humidity affects many properties of air, and of materials in contact with air. Since humidity is simply water vapor, any material of process that would be affected by the presence of water would also be affected by humidity. Measuring and controlling humidity is necessary wherever there is a need to prevent condensation, corrosion, mold, warping or other spoilage of products. This is highly relevant for foods, pharmaceuticals, chemicals, fuels, wood, paper, and many other products.

Humidity also has a strong influence on human perception of comfort. We are sensitive to humidity since the human body uses the evaporation of sweat as the primary means of regulating body temperature. When humidity is high, the air is more saturated with water vapor reducing the rate at which perspiration can evaporate. Without evaporation carrying away body heat, we feel hotter in humid climates versus drier climates, even if the temperature is the same.

Humidity and its effect on comfort has huge implications when designing and operating HVAC systems. Air-conditioning systems in buildings often control humidity, and significant energy may go into cooling the air to remove water vapor. Hygrometers are often installed as parts of a building management system.

Electronic devices may also be susceptible to the effects of humidity. Many electronics devices include humidity specifications, such as 5 to 95% RH. Either end of the scale could potentially cause problems with the device.

High humidity can cause condensation on electrical devices. This could lead to a short circuit inside the device which could cause substantial damage and potential for injury. At the other end, very low humidity could cause materials to become brittle or favor the build-up of static electricity, which may result in electronics behaving erratically should the static discharge. Electrostatic discharges can also cause dielectric breakdown in solid state devices, resulting in irreversible damage. Data centers often monitor relative humidity levels for these reasons.

Applications involving compressed gases are also very susceptible to humidity. The act of pressurizing a gas concentrates any water vapor within that gas thus raising the humidity level. Compressing the gas warms it, as the gas cools, condensation occurs. The liquid water caused by condensation can corrode the inside of pipelines, pumps and holding tanks as well as clog gas jets.

Humidity meters / hygrometers are used in a host of other applications as well. They are used in food service applications where humidity may lead to spoilage and building applications where high humidity levels can lead to mold or warping of materials or low humidity levels can lead to shrinkage. Museums use humidity meters to protect valuable works of art from decay caused by moisture. Wooden musical instruments such a guitars, violins and pianos can also be damaged by improper humidity levels. Hygrometers are used in the coating industry because the application of paint and other coatings may be very sensitive to humidity.

Types of Humidity Meters / Hygrometers

Humidity meters / hygrometers use various technologies to measure the content of water vapor in a gas. Since environmental conditions have such an meaningful impact on humidity, most meters incorporate other measurements as well such as temperature, pressure, mass or a mechanical or electrical change in a substance as moisture is absorbed.

Humidity is also something of a catch-all term that, to some, includes moisture and dew point. Though these are not the same things, many meters are able to calculate other parameters based upon readings. Wet bulb, dry bulb, dew point, concentration, saturation, and partial pressure are some of the calculated measurements available with humidity meters / hygrometers.

Chilled mirrors

Chilled mirror hygrometers are amongst the most accurate humidity meters available. They work by passing a sample of air or gas, kept at a constant pressure, across a temperature controlled mirror. The temperature of the mirror is reduced from high to low, and the exact temperature at which the sample begins to condense on the mirror surface is the dew point temperature meaning the sample is fully saturated with water vapor and is at 100% RH. By knowing the dew point temperature of the sample, its moisture content can be calculated.

Condensation on the mirror surface is hard to detect by the naked eye so an optoelectronic mechanism is used to determine the exact point condensation begins. Electronic feedback from the mirror surface provides precise temperature control allowing the unit to maintain a dynamic equilibrium between condensation and evaporation on the mirror surface thus increasing the accuracy of measurement.

Chilled mirror hygrometers are capable of a dew point accuracy of ±0.2°C which translates to about a ±1.2% accuracy for relative humidity in typical environments. To reach this level of accuracy, a skilled operator and frequent cleaning are needed as the meter can drift in smoky or impure air or be misread in environments containing other materials which are condensable such as alcohol, glycol or hydrocarbons.

Capacitive polymer humidity sensors

Capacitive polymer humidity sensors operate on the electrical characteristics of a capacitor. A capacitor is made of two conductive plates isolated from one another by a dielectric. Capable of storing an electrical charge, that charge varies depending upon the conductivity of the dielectric.

Capacitive polymer humidity sensors consist of a lower electrode deposited on a ceramic substrate. A thin polymer hygroscopic layer acts as the dielectric, and on top of this is the upper plate, which acts as the second electrode. The upper electrode allows water vapor to pass through it which is absorbed by the polymer until it reaches equilibrium with the ambient air or gas. The dielectric constant changes incrementally as water vapor is absorbed and is nearly directly proportional to the relative humidity of the surrounding environment. Thus, by monitoring the change in capacitance, relative humidity can be derived.

Capacitive polymer humidity sensors are subject to contamination, drift and aging effects, but are suitable for many applications especially those where cost, space, or fragility are relevant. When properly calibrated, capacitive humidity sensors are capable of being accurate to ±2% RH over the range of 5 to 95% RH. Without calibration, the accuracy is rarely better than ±4 to 6% RH.

Resistive humidity sensors

Resistive humidity sensors measure the change in electrical impedance or resistance of a hygroscopic medium—such as conductive polymer, salt, or a treated substrate—in the presence of water vapor.

Resistive humidity sensors generally are made of electrodes either laid on a substrate or wire wound on a cylinder. The substrate is coated with a salt or conductive polymer. In the presence of humidity, the sensor absorbs water vapor, which lowers the resistance between the electrodes directly proportional to the humidity level.

Resistive humidity sensors are small and inexpensive. They offer an excellent response time and are very reliable. Resistive sensors have a more limited measuring range—generally 15 to 95% RH—than other measurement types and offer an accuracy in the range of ±3% RH. Resistive sensors are less sensitive than capacitive sensors so they require more sophisticated circuitry. Resistive sensor can be affects by contamination and condensation as well as all substances that affect resistance including salts, hydrogen, oxidizing agents, and other chemicals

Resistive humidity sensors are also very temperature dependent so that the sensor is usually combined with a temperature sensor in order to compensate readings.

Thin film aluminum/silicon oxide sensors

Thin film aluminum or silicon oxide moisture sensors are quite similar in design to capacitive moisture sensor but maintain distinct differences. Oxide sensors are made of an inert substrate material—usually aluminum or silicon—which is oxidized to form a very thin layer of aluminum oxide or silicone oxide. To this a very thin layer of gold is applied via chemical vapor deposition. The gold and aluminum form the electrodes of a capacitor. As water vapor passes through the gold layer it is adsorbed onto the pore walls of the oxide layer until equilibrium with the ambient humidity is reached.

As with capacitive sensors, the adsorbed water vapor (it is absorbed in capacitive sensors) changes the dielectric constant of the sensor in proportion to the relative humidity. The capacitance of the sensor is measured which is then converted to the humidity value.

Aluminum or silicon oxide sensors offer an inexpensive, accurate humidity measuring option. Adsorbed water vapor take time to enter and exit the oxide pore so response time is slow compared to other technologies. Contaminants and corrosives may damage and clog the pores causing drift in the calibration.

Things to Consider When Selecting a Humidity Meter / Hygrometer:

  • What parameter are you measuring—relative humidity, dew point, or something else?
  • What are the ranges, both for humidity and temperature?
  • What level of instrument performance is needed (resolution, short-term stability, long-term drift, speed of response, non-linearity, hysteresis, temperature coefficient, uncertainty of calibration)?
  • How will the measurement system be configured? Will it be mounted to a wall for measuring ambient conditions or in a pipe for monitoring the moisture in compressed air/gas? Are there any space constraints?
  • How will the instrument be powered, battery or mains?
  • What data output format —display, analog voltage or current, digitally-read output, data logging—is needed?
  • Is an alarm needed if humidity passes any limit?
  • Is sensor output to be used to automatically control something?
  • Is there any need for compatibility with any unusual gases? What about contamination protect from dust or chemicals?
  • Is it for use hazardous areas? If so, is certification required?
  • What are the upfront costs as well as cost for upkeep?
  • How often does the instrument need calibration? What doers calibration entail?
  • How easy is it to use the instrument?

If you have any questions regarding humidity meters / hygrometers please don't hesitate to speak with one of our engineers by e-mailing us at sales@instrumart.com or calling 1-800-884-4967.