May 2004 

Testing Grain Moisture Meters (GMMs) 

By G. Diane Lee 

Spring is here and many States choose this time of year to inspect and test grain moisture 

meters. However, inspection and testing is conducted at the discretion of the State and is 

not, in many cases, confined to the spring. In many States, winter wheat is the first grain 

to be harvested (typically from May to July). Inspecting and testing meters in early 

spring provides the device owners with verification of meter accuracy and if the device is 

rejected, allows the device owner time to repair the device before the harvest season 

begins. This inspection time frame also allows the weights and measures inspector time 

for callbacks or follow-ups on any rejected devices before the harvest season begins. 

 

 Usual Start of Winter Wheat Harvest

Two methods for testing GMMs are recognized in NIST Handbook 44 (HB 44). GMMs 

may be tested using the air oven reference test method (grain samples are used to test the 

meters) or by like-type meter test method (use of standardized reference like-type meters, 

in a side–by-side comparison of a grain sample). This article provides some helpful hints 

on proper sample prepara-tion and testing of grain moisture meters using the air oven 

reference test method (grain samples are used to test the device). 

Using grain samples to test moisture meters requires that grain be collected, cleaned, 

tested, stored in proper containers, retested, and then distributed to weights and measures 

inspectors for use in the inspection and test of grain moisture meters. 

Collection, Cleaning and Storing 

Several different lots of various types of grain (wheat, soybeans, corn, etc.) should be 

collected from different locations within the State and at different moisture levels (high 

and low moistures). These grains are typically collected from the prior year grain crop. 

The grain is cleaned using a mechanical cleaning device (typically a Carter Day Dockage 

machine) and further hand-sieved and cleaned as necessary. The grain sample is mixed 

using a Boerner Divider (a device used to mix and separate grain samples) to ensure 
homogeneity, and then the grain is evaluated. Evaluation of the sample to ensure that it is 

adequate for use as a field sample involves a comparison of a grain sample moisture, 

which is determined using the air oven method, to a grain sample moisture, which is 

determined using the laboratory standard grain moisture meters. (The laboratory must 

have one laboratory standard moisture meter for each type of moisture meter used in the 

State.) The grain sample air oven moistures compared to the laboratory standard meter 

moistures must not exceed a difference of 0.5 % moisture to be approved for use as field 

samples. Samples that pass this evalu-ation are stored in the refrigerator for future use as 

field-testing samples. 

Tips 

• Ensure that enough samples are collected for each type of grain. Some grains 

may not pass the evaluation criteria for comparison of air oven moisture to 

laboratory standard meter moisture. 

• Grain samples should be cleaned thoroughly. Bugs and other foreign material 

may affect the moisture stability of the field grain sample and test weight results. 

• Once grain samples have been tested to determine they are appropriate for use as 

field samples, they should be stored in appropriately sealed glass gallon jars. 

• Grain samples must be stored in the refrigerator between 1.5 °C and 7 °C (35 °F 

and 45 °F). Grain must not be frozen. 

Retesting Grain Samples for Use as Field Grain Samples 

When ready to use, the grain samples that are appropriate for field-testing are removed 

from the refrigerator and allowed to equilibrate to room temperature before sample 

containers are opened. This prevents condensation on the grain. The grain samples are 

mixed in a Boerner Divider three times. The sample is then retested for moisture using 

the air oven and retested for moisture on the laboratory standard meters to ensure that the 

sample still meets the criteria. (The difference between the air oven moisture and meter 

cannot exceed 0.5 %.) The samples are then divided into field samples typically 2 to 3 (1 

pint or 1 quart) grain samples of the same lot with a high moisture content and 2 to 3 (1 

pint or 1 quart) grain samples of the same lot with a low moisture content for each grain 

type (wheat, soybeans, corn, etc.). A set of these grains is issued to the inspector for 

testing. 

Tip

• Consider using three (1 pint or 1 quart) samples per moisture level (high and low), 

one sample of which will be used as a back up to verify other moisture reading 

and will not be opened unless it is necessary. The back-up sample may be used if 

there are concerns with the validity of the moisture content of the other two 

samples. 

 

Field Inspection and Testing Grain Moisture Meters 

Meters must be inspected to ensure they meet the requirements of HB 44. A minimum 

test of a grain moisture meter consist of tests with samples (typically 1 pint or 1 quart) 

which need not exceed three for each grain or seed for which the device is used and with 

samples having at least two different moisture content values (high and low moisture 
content) within the operating range of the device. If applicable, test the test weight per 

bushel indications with at least the lowest moisture samples. Three samples of the same 

moisture level from the same lot of grain should be issued for each grain type and 

moisture level. The samples are typically stored in a portable refrigerator until needed. 

About two hours before use, the samples are removed from the refrigerator and placed in 

coolers to allow the grain to stabilize to ambient temperature. During this time the grain 

is shaken several times for homogeneity. One high and low sample of each grain type are 

run through the meters 3 times each, and an average of each high and low sample 

moisture is calculated and compared to the air oven moisture values of the grain to 

determine if it meets the tolerances of HB 44. 

Tips 

• Each sample must not be used for more than 24 drops (18 drops for 

• high moisture corn and soybeans). Samples may begin to deteriorate after this point. 

• Test your high moisture samples first so that they can be returned to containers and 

placed in storage to prevent loss of moisture.