Some experts argue that every conceivable product is either directly affected by humidity or works differently at certain relative humidity levels, so using hygrometers to measure ambient air moisture could be critical for protecting homes, businesses and products. Humidity affects performance of materials in the environment, industrial processes and physical well-being, but for hygrometers to measure humidity effectively, they must contact the environment that affects each type of process. Contamination, degradation and calibration issues arise, so effective measurement requires many types of hygrometers and calibration processes to get reliable results.
- In complicated industrial processes, small changes in humidity levels can generate significant variances.
- Sensors must control or contain variables such as pressure, temperature and the environment or materials being measured to get accurate readings.
- Humidity plays a role in every manufacturing process, so monitoring moisture and sometimes correcting humidity becomes critical.
- One of the simplest examples of the effects of humidity becomes evident when baking bread and cakes where moisture affects how these foods rise.
Types of hygrometers perform an astonishing range of work in homes, offices, manufacturing plants, hospital environments, research labs, drying processes, food production facilities, incubators, greenhouses, museums and textile and paper industries.
- Hair-tension hygrometers
Displacement sensors use hair, cellulose or nylon to measure expansion and contraction due to humidity. Inexpensive and relatively immune to contamination, these hygrometers have fibers that connect with dials and cause displacements of needles or gauges as they expand or contract. These meters are useful for measuring general humidity in homes, instrument cases, terrariums, basements and attics. Use a simple hygrometer to find out if excess moisture pockets could be causing bacteria and mold growth or exacerbating allergies.
- Metal-paper coil hygrometers
These devices are often used inexpensively to monitor moisture where absolute accuracy is not essential. A salt-impregnated paper is attached to a metal coil, and it changes shape due to moisture absorption or loss, which causes the coil to move a needle or dial. Only accurate to about 10 percent, these hygrometers can easily indicate changes in humidity without being 100-percent accurate.
- Humidity cards
Humidity cards are among the simplest ways to measure moisture. Designed for specific humidity ranges, spots on the cards change colors to indicate whether humidity is below the range, in the middle of it or at the top limit of the range or above. Spots have cobalt chlorine solutions and various additives to cover a range of moisture conditions. Some cards have multiple spots to cover larger ranges of humidity. These cards are useful for measuring humidity in containers and packaging when observed through clear windows. Cards can measure from 5 percent up to 95 percent relative humidity.
Psychrometers consist of matched pairs of thermometers, one of which has a dry bulb while the other is wet. Evaporation cools the wet version and lowers temperature, and a chart determines relative humidity by the differences in temperature between the two measurements. You can’t use this model in temperatures below freezing because the moisture freezes, and this specialized hygrometer doesn’t measure accurately below 20 percent relative humidity. Typically used in larger areas, the sling psychrometer doesn’t work in small, enclosed spaces because the device introduces moisture into these environments. Sling psychrometers whirl about large areas to sample ambient air while aspirating psychrometers are compact, mechanized units that can be transported to different areas. Psychrometers are great for measuring outdoor humidity and moisture in museums where dry storage conditions are essential to preserve exhibits.
- Mechanical hygrometers
Mechanical meters can be simple wall devices, clocks, watchlike instruments or elegant meters made of brass that measure humidity with high degrees of precision and cost upwards of $1,500. These analog instruments have provided reliable readings for hundreds of years, but testing and calibrating them becomes necessary periodically to ensure that readings remain accurate. You can test a mechanical hydrometer by comparing an outdoor reading with local weather forecasts, wrapping a damp rag around the instrument and placing it into a sealed container and making sure that the reading is 95 percent or greater after a few hours or use a sling psychrometer to compare humidity readings. Most mechanical hygrometers can be calibrated by turning a screw. Home environments that are considered the healthiest are those that fall between 40 and 60 percent relative humidity. Precision mechanical hygrometers are used for indoor and outdoor measurements or to ensure humidity in controlled environments. High-precision hygrometers measure humidity with a 1 to 3 percent margin of error but are expensive for the most accurate models.
- Gravimetric hygrometers
Gravimetric hygrometers measure a given sample of air as compared to an equal volume of controlled, dry air. Highly accurate and expensive, gravimetric hygrometers are used to set standards throughout the United States, United Kingdom, European Union and Japan. Using these devices is difficult and challenging, so they are usually reserved for calibrating less accurate hygrometers.
- Electrical hygrometers
Electrical hygrometers base their measurements on changes in electrical resistance or capacitance. Usually, a probe houses a sensor that measures changes in a layer of lithium chloride or other type of semiconductor. The most accurate type of electrical hygrometer passes a quantity of gas over a moisture-absorbing material and determines how much water is absorbed by weighing the gas before and after the process. Electrical hygrometers operate by absorbing or adsorbing water. Capacitive hygrometers measure condensation to determine how it affects how much current a material can hold; resistive hygrometers absorb water and measure electrical resistance. Capacitive sensors can withstand condensation, but resistive sensors can’t, and both types of hygrometers are vulnerable to contaminants.
- Manual hygrometers
Manual hygrometers work for hundreds of personal, industrial and scientific applications and non-precise field work. Portable instruments are affordable, user-friendly and reliable when calibrated regularly. Other types of manual hygrometers include wall hygrometers, models for instrument cases, moisture analyzers for humidors and home-use models to prevent mold, chapped skin, breathing difficulties and problems with electrical equipment.
Hygrothermographs measure and record humidity and temperature data for defined periods, and the National Park Service uses these devices to monitor conditions in museum exhibit and storage areas. The hygrothermograph measures humidity and temperatures with bimetal temperature sensors and hygroscopic humidity sensors. These devices require frequent calibration.
- Chilled mirror hygrometers
Chilled mirror hygrometers are often used in delicate scientific research where the smallest traces of moisture must be identified. Precise and capable of measuring trace moisture as little as one part per billion, chilled mirror hygrometers include dew-point and frost-point hygrometers. The dew-point hygrometer determines the saturation point for water vapor in a gas. The mirror’s surface is chilled until dew droplets form causing reflected light to scatter, which halts the cooling process. At temperatures below 0-degrees C, water manifests as frost. Frost-point hygrometers are used to track climate changes and measure distribution of water vapor throughout the world. Chilled mirror hygrometers use various cooling methods including dry ice, compressed carbon dioxide, mechanical refrigeration and liquid air to chill mirror surfaces until the gas, temperature and pressure reach maximum saturation, which causes droplets to condense on the mirror. These mirrors never need calibration and work in situations where routine calibration is impractical.
- Research hygrometers
Research hygrometers include handheld portable models, chilled mirror hygrometers, and advanced humidity control systems utilizing frost-point moisture analyzers. You can choose from mechanical, chemical, electronic and digital equipment and models with built-in vacuum pumps to take samples from low-pressure sources. Various types of specialty hygrometers allow you to achieve speed, stability and repeatability when measuring, which are key qualities of the scientific process. Whether analyzing air for military, firefighting or medical applications, hygrometers analyze samples accurately, increase safety and ensure uninterrupted supplies of air in aircraft, submersibles, simulation equipment and pressure suits. Even trace amounts of moisture can compromise processing equipment in certain industrial and scientific processes, but advanced hygrometers and complementary processing equipment can reduce risk, ensure effective performance and analyze critical intelligence about the environment.