How do portable grain moisture meters measure grain moisture?

What factors affect their accuracy and can they be calibrated?

Len Caddick, CSIRO Stored Grain Research Laboratory

The moisture content of grain has an important influence on quality changes that occur during storage. Bio-deterioration due to insect and mould activity and the rate that chemical and enzymatic processes take place within the grain are strongly related to moisture content. Other factors that influence grain quality during storage are age or condition of grain at harvest, temperature at which grain is stored, and storage time. All these factors interact to determine the rate that grain quality is lost during storage. Grain characteristics, such as hardness, specific weight and electrical properties, are also influenced by moisture content.

Portable grain moisture meters

A range of portable moisture meters is commercially available. Hand-held meters most commonly used to measure grain moisture content in the paddock are conductivity or capacitance type instruments. Influence of moisture on the electrical properties of grain is the basis for moisture measurement using these instruments.

Electrical conductivity or resistance type instruments measure the ability of a compressed grain sample to conduct a current relative to its moisture content. A direct current circuit, often based on a Wheatstone bridge, is established between two electrodes. Whole seed or ground meal is placed between the electrodes, and the flow of current measured. Conductivity of the substrate increases as the moisture content increases.

Capacitance type instruments measure the ability of a set volume of air and grain to store an electric charge relative to its moisture content. The dielectric constant or electrical permittivity is determined using a known mass of grain packed using a standard procedure. Grain is placed between two conducting plates forming a capacitor or condenser and is exposed to high frequency voltage. The amount of electrical energy absorbed by hydrogen atoms of water molecules present in the sample is converted to moisture content.

It is not easy to recommend the best meter for all types of grain. There are many factors that affect the electrical properties of grain. Temperature is a good example, where increases in temperature of the test sample, increase the rate of conductivity and capacity of a sample to store an electric charge. Portable moisture meters need to be calibrated and used in a consistent manner to enable comparable results. A properly calibrated portable meter, operated under controlled conditions, can be expected to provide accurate moisture readings. Grain samples also need to be prepared and presented in a consistent way.

Factors that can influence the performance of portable meters, include:
Temperature differences between the test sample and measurement cell
Temperature differences between ambient conditions and meter
Moisture differences between individual kernels
Moisture gradients within kernels
Grain type, kernel size and varietal differences
Stage of maturity
Weathering, surface texture and moulding
Sample size
Loading technique and changes packing or bulk density - capacitance type meters
Variation in compression force - conductance type meters
Variation due to grinding (when required)

Contemporary moisture meters are designed to compensate (to a degree) temperature influences, and provide facility to obtain consistent sample presentation. Conductivity type meters are designed to apply a uniform compression force. Capacitance meters rely on the operator to pour grain into a test cell in a controlled manner to achieve a consistent packing density. This process has been automated in some models where the grain is dropped into the test cell using a spring-loaded valve. Capacitance meters are generally calibrated to a packing density of 70% grain to 30% air. Variation from this ratio will vary the result obtained.

Conductance meters measure the path of least resistance of an electric charge through a "wheat-stone" bridge and any differences in moisture between individual kernels can cause incorrect readings. Immature green kernels for example can cause wet spots in the sample and should be removed prior to testing. The accuracy of capacitance meters is affected by the presence of moisture gradients within individual kernels due to re-wetting or drying. Capacitance meters are designed to measure moisture present on the grain surface and in interstitial air between kernels. The moisture content of grain taken from rapid dryers will be well below the actual level. In contrast, wetted grain will produce a high reading. The influence of other factors on the accuracy of portable grain moisture meters is difficult to quantify and an extensive field study is required. An individual factor may not have a substantial affect on the accuracy of meters. However, the simultaneous occurrence of a number of factors can be significant.

Sampling crops

Variability of grain moisture content within and between plants in a ripe crop makes accurate determination difficult. Such variability is likely to be greatest early in the harvest season and especially for large-seeded crops such chickpeas, field peas, lupins and maize. Differences in moisture between the outer and inner portions of the grain due to re-wetting caused by heavy dew or rainfall will also affect the accuracy of portable meters.

Increasing the number and representative nature of grain samples taken for moisture assessment will provide a better profile of the overall moisture content of a crop. Accuracy of portable meters can be improved by placing grain into sealed bags and standing them together with the portable meter in a shaded location for 30 minutes prior to testing.

Calibration of meters

Calibration of a portable meter is not difficult but can be time consuming. To achieve an accurate calibration curve a series of grain samples covering a range of different moisture levels should be selected and a recognised standard method used to accurately determine moisture content. Well-equipped grain quality testing laboratories should have the necessary equipment such as a fan forced oven, suitable sample dishes, desiccators and accurate electronic balances to conduct standard tests. Alternatively, a calibration curve can be produced against the instrument used by the local receival agent. When delivering grain to an established industry receival limit, samples may be selected at moisture levels concentrated around this limit.

A properly calibrated portable meter, operated under controlled conditions, can be expected to provide accurate moisture readings. Growers need to take advantage of testing facilities provided by local receival agents to check the accuracy of their portable meter before harvest. Near infrared transmittance (NIT) or reflectance (NIR) systems are now used extensively by commercial grain storers to measure grain moisture content. Results from NIT/NIR systems can be compared to values obtained on a portable meter to determine the difference in moisture content readings.

Collect 4 to 5 composite samples from different parts of a paddock, set them aside in a shaded locality for about 30 minutes, then test them through the NIT/NIR system used by your local agent. Check these at the same time using your own portable meter at the receival site, preferably under similar conditions. There is likely to be variation in measurement between the portable meter and NIT/NIR instrument and the magnitude of the difference may also vary across a given moisture range. A simple calibration curve can be established by plotting values obtained from each instrument on a two-dimensional chart. Using grain samples from your own paddock to calibrate a portable meter against an NIT/NIR system used by your local agent is a better option than relying on calibrations offered by distributors of portable devices.