Moisture refers to the presence of a liquid, generally water, in minute amounts. Moisture is found nearly everywhere, including materials we consider to be “dry”. Moisture meters
are the class of instruments that are able to measure the trace amounts of moisture in solids, gases and hydrocarbon liquids. Controlling moisture is vital as water vapor above
certain levels can lead to condensation, corrosion, mold or other issues.
The Importance of Controlling Moisture
Each material and application is affected differently by moisture and each has a range of moisture most suitable for it. Depending upon the application or material, too little
moisture can be as bad as too much moisture. To demonstrate the importance of moisture, let’s take a look at how it affects some common applications:
Commercial Food Production: Controlling moisture is very important in commercial food production applications. Too much moisture in foods intended to be crunchy,
like potato chips, will make them mushy and stale. Too little moisture in foods intended to be moist, like a cupcake, will make the item dry and unpalatable. High moisture levels
also contribute to mold and bacteria growth which will spoil the food. This is also very important in the bulk storage of grains and other food commodities. Moisture levels can
also be manipulated to add water weight to foods sold in bulk or to reduce the caloric content of foods
Building Trades: Building trades require moisture control on a number of levels. Individual building materials, like concrete and wood, have ideal moisture levels.
Too much moisture can cause these materials to swell while too little causes shrinkage and cracking. Either of these conditions can affect the quality of the building. Additionally,
uncontrolled moisture within the building envelope can rapidly degrade building materials and insulation or lead to mold and mildew issues that can be dangerous to occupants.
Energy: From the homeowner who burns wood for heat realizing a lower moisture content allows wood to burn hotter and cleaner, to industrial scale natural gas plants
that must remove excess moisture before the gas is marketable, energy is another industry with far reaching moisture control needs. Moisture can also adversely affect the insulation
or motor windings of power generation equipment as well as internal combustion engines that burn gasoline tainted with water.
Manufacturing: A huge amount of manufactured items rely on adequate control of moisture. Nearly all plastics, for example, are sensitive to moisture and levels too
high or too low can affect curing. Pharmaceuticals, as well, need to control moisture to prevent clumping of ingredients or spoilage due to mold. Excessive moisture can also corrode
sensitive electronics.
There are many, many other applications in which controlling moisture is a paramount concern. As the above examples demonstrate, excess moisture can lead to mold, mildew, odors,
sogginess, swelling, inefficient combustion, clumping and a host of other bad effects. Too little moisture can cause shrinkage, cracking, a build-up of static electricity, dust, etc.
Only a moisture meter can let you know when moisture levels are ideal.
Moisture vs Humidity
There are many ways to describe the amount of water in solids, gases and hydrocarbon liquids. It can be stated as a percentage of the total volume, as mass-volume ratio, by parts per
million or billion, or as the mole fraction of water vapor in the air. The type of measurement you need is determined by the application.
Though the terms moisture and humidity (and their variants relative humidity, absolute humidity and dew point) are all used to describe the presence of water in some form and are used,
to some degree, interchangeably, there are differences.
Humidity is the amount of water vapor in air or another gas. It can be shown as either absolute humidity, the total amount of water vapor in a specified
volume or air, or as relative humidity, the ratio of water vapor in the air to the maximum amount of water vapor the air can hold which is dependent upon temperature and
pressure. Absolute humidity is calculated as mass per volume while relative humidity is calculated as a percentage.
Dew point is the temperature at which water vapor condenses. Expressed in degrees Fahrenheit or Celsius, dew point is an indicator of how much water vapor is in air or
gases. If the air is fully saturated with water vapor at a constant barometric pressure, the dew point will be equal to the ambient temperature.
Unlike humidity or dew point, which refer only to water vapor in air or gases, moisture can refer to either water vapor in air/gases or trace water in solids or hydrocarbon
liquids. Moisture is generally calculated as a percentage or as parts per million/billion depending upon the instrument used.
Since trace water measurements are somewhat similar, we find that many meters used to measure it are capable of measuring some or all of the parameters. The primary difference is in
measuring moisture in solids which uses more specialized equipment.
Types of Moisture Meters
As you would expect with any class of instrument tasked to measure such a wide range of materials, moisture meters are available in a wide range of styles and employ a number of measurement
technologies. The styles, of course, reflect the purpose of the instrument. The solids most typically tested for moisture content are building materials or grains/crops. The nature of these
tests suggests they would be conducted in the field so handheld meters with simple, portable technology dominate this style of meter.
Testing liquids and gases, on the other hand, is often part of a process in a closed system. It may also require a greater level of precision. In these applications we find fixed mount
systems with highly accurate sensing technology.
Given the relationship between meter style, measuring technology, and application; the best way to examine the type of meters is group them into handheld or fixed mount units.
Handheld Moisture Meters
Handheld moisture meters are designed for high portability and ease of use. They are most often used to measure the moisture content of building materials (such as wood products,
concrete, drywall, etc.), soils and aggregates, or agricultural products (like hay bales or bulk grain). There are four types of handheld moisture meters.
Pin-Type: Pin-type meters are an invasive way to measure moisture. They consist of a pair of pins attached to the meter which are pushed or driven into the material
being tested. Since the pins penetrate the material, they do cause damage to the surface so testing should be done to a scrap piece or the back of the material.
Pin-type meters work on the principle of electrical resistance. With the pins inserted into the material, a small electrical current is passed between them. The amount of resistance
correlates to the amount of moisture in the material. Because moisture is a good conductor, higher moisture levels result in lower resistance values. In this way the moisture level
can be quantified.
Pin-less meters usually measure moisture as a percentage or apply a relative scale for comparison. They are generally accurate to 1 or 2%. Pin-type measurements are affected by variances
in the naturally-occurring chemical composition of wood species, so the meters are often calibrated for specific woods. Since pin-type meters measure only a very small area, they can be
subject to moisture fluxuations found within a sample.
Pinless: Pinless moisture meters are non-invasive instruments that use a variety of measuring technologies without damaging the surface of the material being tested.
Pinless meters emit electromagnetic waves, usually either radio waves or an electric current, to measure moisture. These waves are affected by the presence of moisture in the material
being measured. By detecting how the returning wave is affected, moisture can be calculated. If the meter uses electrical current, this calculation is based on the fact that that a
material's resistance will vary inversely from its moisture content. If the meters uses radio waves, the characteristics of those waves can determine moisture content which is normally
presented on a relative scale rather than as an absolute percentage.
Pinless meters, unlike pin-type meters, are capable of quickly measuring a large surface area and can “scan” the wood for a more complete moisture content picture. They do have limitations,
though. Reading depth, for example, is limited to about 1/4 - 3/4 inches maximum, depending on the meter. Also, pinless meters are very susceptible to surface moisture. Just wiping the
surface with a damp cloth can make the readings skyrocket.
Reagents: Though not common, there are some meters that use reagents to measure moisture. These meters usually involve grinding up a test sample and mixing it with a
reagent that reacts with moisture. That reaction produces gas which increases the pressure inside the testing chamber in proportion to the amount of moisture in the sample. By simply
measuring that increase in pressure, moisture content can be calculated.
Probes: Another type of handheld moisture meter, also found on some pin-type and pinless models, includes an outlet with which to attach a moisture measurement probe.
Probes provide a very versatile method for getting to specific areas of solids, for example inside a bale of hay, rather than only being able to test near the surface.
Moisture probes can use conductivity or capacitive polymer sensing. Conductivity probes work just like pin-type meters. The probe emits a small electrical charge from one point to
another. The amount of resistance correlates to the amount of moisture in the material. Because moisture is a good conductor, higher moisture levels result in lower resistance (or higher
conductivity) values. In this way the moisture level can be quantified.
Capacitive polymers 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. As moisture is absorbed by a thin polymer hygroscopic layer that acts as the dielectric,
the conductivity of the dielectric changes incrementally and proportionally to the amount of moisture present.
Fixed Mount Moisture Meters
Fixed mount moisture meters are intended to remain in place to measure moisture levels in gas or hydrocarbon liquids. Measuring moisture levels in these substances required much different
technology than for solids.
Chilled mirrors: These work on the principle that when gas flows over a chilled surface, in this case a mirror, the moisture will condense on it. The exact temperature at
which this condensation begins is the dew point. The temperature of this mirror is reduced from high to low, and the temperature is read exactly when the dew is observed. By obtaining the
dew point temperature, one can calculate moisture content in the gas.
The temperature of a chilled mirror is controlled by the flow of a refrigerant over the mirror or by using a thermoelectric cooler. The condensation is detected by visual or optical means.
For example, a light source can be reflected off the mirror into a detector and condensation detected by changes in light reflected. The observation can also be done visually; however the
exact point at which condensation begins is not discernible to the eye.
Aluminum/silicone oxide sensors: Oxide sensors are quite similar in design to capacitive moisture sensors but maintain distinct differences. They 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 moisture (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 moisture value.
Aluminum or silicon oxide sensors offer an inexpensive, accurate moisture measuring option. Adsorbed water vapor takes 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.
Laser: High definition laser absorption spectroscopy (HDLAS) provides high accuracy, high sensitivity measurement of moisture content in gas over a range of 0 to 10 parts
per million. Moisture analysis with a laser involves scanning a sample across a narrow frequency band. Moisture in the sample will absorb the photonic energy of the laser resulting in loss
of intensity of the laser at certain frequencies. This loss of intensity is proportional to the water concentration in the sample.
Things to Consider When Selecting a Moisture Meter:
- What type of material will be measured? Solid, liquid, or gas?
- What parameter will we require to quantify water vapor?
- What is the expected range of measurement?
- What level of performance will we require? Uncertainty? Long term stability? Response time?
- Resolution of output?
- Is any output required?
- What are the installation requirements?
If you have any questions regarding moisture meters 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.
