September 2006

What do Grain Moisture Meters Measure and 

How are They Calibrated? 

By G. Diane Lee 

Did you know that grain moisture meters do not measure moisture? Most grain moisture 

meters measure an electrical property, such as capacitance, related to the dielectric 

constant of a test cell filled with grain. Although there are other types of grain moisture 

meters using different technologies to predict moisture, this article addresses grain 

moisture meters that make use of the dielectric constant of grain to predict moisture. 

What is capacitance, what is a dielectric constant, and how do these relate to percent 

moisture? This article answers these questions, providing information on capacitance and 

dielectric constants, and how these measurements are used to determine the percent 

moisture of a grain sample and how they relate to the calibration of moisture meters. 

What Grain Moisture Meters Measure 

Water is a good insulator. Insulators have tightly bound electrons and can store electrical 

charges. A dielectric or electrical insulator is a material that is highly resistant to the 

flow of electrical current. Capacitance is a measure of the amount of electric charge 

stored (or separated) for a given electric potential. Capacitance exists between two 

conductors insulated from one another. A dielectric constant can be determined by 

measuring the capacitance of a capacitor (two conductors or plates) with air between the 

plates, then measuring the capacitance with a dielectric material between the plates. The 

ratio of these measurements is used to determine the dielectric constant. 

 

The dielectric (capacitance) technology used in many grain moisture meters is based on 

the principle that a functional relationship exists between the moisture content of grain 

and its dielectric constant. As grain increases in moisture content (water), its dielectric 

constant increases. The rate at which the dielectric constant increases as grain moisture 

increases is not the same for all grain types; therefore, a unique calibration equation must 

be developed for each grain type to be measured. Moisture meters based on the dielectric 

principle typically incorporate a test cell in the form of an electrical capacitor, that is, two 

conductors separated by an insulator. When the cell is empty, only air separates the two 

conductors, and the insulator is air. 

When a grain sample to be measured is placed between the conducting surfaces of the 

test cell, the grain displaces most of the air. By sensing the change in the electrical 

characteristics of the capacitor due to the dielectric properties of the grain sample, the 

meter can predict the moisture content of the sample. Because the bulk density and the 

temperature of the grain sample also affect the electrical characteristics of the grain-filled 

test cell, the meter must measure these parameters and apply the necessary corrections. 

Some instruments using this method also have the capability to correct for surface 

moisture (conductance) effects. Year-to-year and regional changes in growing conditions 

and genotype variations within a single grain type can also result in changes in the 
relationship between dielectric constant and moisture. These changes are effectively 

'averaged out' by developing calibrations using three or more years of data representing 

all geographic areas. 

Grain Moisture Meter Calibrations 

As noted above, dielectric grain meter technology requires the use of unique calibration 

equations for different grains. Because meters measure the effect of moisture on certain 

electrical properties of grain, and because a functional relationship exists between 

moisture and these measured properties, a calibration equation or table can be developed 

that assigns a unique moisture value (air-oven reference method* moisture value) for 

each set of measured parameters. 

To develop meter calibrations, grain samples are collected from a wide variety of 

moisture ranges, varieties, geographical diversities, and several crop years. Data are 

collected for oven moisture values, capacitance (or a related parameter), sample weight 

(per test-cell volume), and sample temperature. The data are evaluated and adjusted to 

remove outliers and to make baseline and other corrections; the data are then fitted to a 

calibration curve. (See Figure 1, Typical Calibration Data). Thus, measurement results 

obtained with a grain meter are only as good as the meter calibration. Since the 

calibrations are based on the data collected from a sample set of grain, if the sample grain 

used to test the meter is atypical (an outlier) or if a grain sample used to test the meter is 

not represented in the calibration sample set, the instrument may not provide accurate 

results on these samples when in use. 

 *The air-oven reference method is used to determine the reference moisture of grains. 

The air-oven reference method involves weighing a grain sample prior to and after a 

designated amount of heat is applied to the sample of grain over a specified amount of 

time. The grain sample is prepared (ground or air dried) prior to the oven test. Percent 

moisture is calculated base on the amount of weight loss in the grain sample. 

Figure 1: Typical Calibration Data 9diagram courtesy of John W. Barber, 

 Grain Moisture Meters from Theory to Practice) 
Every year States are requested to participate in providing samples to the USDA, GIPSA 

for the national sample set that is used to develop calibrations for NTEP grain moisture 

meters. The more representative the sample set used to establish the calibrations the 

more representative the calibrations will be. The NIST Weights and Measures Division 

continues to encourage States to annually submit grain samples for the national sample 

set. 

Future articles will follow that discuss other technologies used in determining grain 

moisture. Contact Diane Lee of the NIST Weights and Measures Division by phone at 

301-975-4405 or by e-mail at diane.lee@nist.gov for additional information concerning 

grain moisture measurements.