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Weights and Measures Quarterly, September 2008 Article Ref: C-008

September 2008 

Grain Preparation, Maintenance and Storage of Grain Transfer 

Standards, Equipment and Apparatus, and Field Test Procedures 

Grain Moisture Meter (GMM) Series Part 4 

By G. Diane Lee 

The most widely used test method for the inspection and testing of GMMs in the United States is the use of grain as 

a transfer standard. Unlike other commercial devices, these transfer standards are biological samples that require 

preparation and care to maintain. There are various types of equipment and apparatus needed to test GMMs, 

including storage and transport containers, temperature measuring devices, and refrigeration equipment. This 

article, which is the fourth in a series of six articles on Grain Moisture Meters, addresses grain preparation prior to

field testing; maintenance and storage of grain transfer standards for field-testing; the equipment and apparatus 

needed to test grain moisture meters and field test procedures. 

The reader is encouraged to review previous articles in this series, including Part 1 “Overview of GMM Series 

Topics,” Part 2 “Economic Impact of Grain Moisture Meters,” and Part 3 “Grain Moisture Meter Measurement 

Technology”. The reader is also encouraged to review the May 2004 Weights and Measures Newsletter article for 

an overview of the process for testing GMMs. These articles can be found on the NIST WMD’s web site at 

www.nist.gov/owm by selecting “Weights and Measures Quarterly Newsletter Articles” which is located 

under “Publications,” and then selecting “Grain Moisture Meter/NIR.” 

Grain Preparation for Field Testing 

The transfer standards used to test GMMs are grain samples. Samples of grain, which are typically purchased or 

sold within the State’s Weights and Measures jurisdiction, are collected from various farms and elevators. “low” 

and “high” moisture samples of each grain type are collected. The laboratory staff cleans, labels, and stores the 

grain for laboratory testing. Laboratory testing is performed to determine which grain samples will be appropriate 

for use as a transfer standard. GIPSA air-oven test procedures are used to determine the official moisture value of 

the grain sample. After the official moisture of the grain is determined using the air-oven test method, the moisture 

of the grain sample is also determined on grain moisture meters in the laboratory. These laboratory meters, which 

include the same make and models as are used in commercial applications, are maintained in good operating 

condition, serviced as needed, and contain the most current meter calibrations. The laboratory maintains a meter 

type of each commercial grain moisture meter within the weights and measures jurisdiction. The difference 

between the air-oven moisture value and meter moisture value of the grain sample is calculated. If this difference 

exceeds 0.5% for a particular sample, that sample is considered an “outlier” for that specific meter, and that grain 

sample should not be used as a transfer standard for testing that specific moisture meter. Such samples are 

considered “outliers” for these meters because they may not provide an accurate moisture reading when used to test 

these meters. Outliers may exist because a particular variety of and/or growing seasonal change in a grain sample 

may not be represented in the pool of grain samples that were used to develop the calibration for the moisture meter. 

Non-NTEP meters may be more readily affected by outliers because the calibrations for these devices may not be 

based on a pool of samples that represent grains from across the United States. The calibration of NTEP meters are 

based on a national sample set of grains from the United States. Grain samples in the national sample set are 

annually collected using samples that are submitted on a voluntary basis from various States. The national sample 

set also includes grains that are collected by the NTEP laboratory from various locations within the United States. 

Along with the current-crop-year’s grain moisture data, the national sample set data also includes the previous two 

years of grain data, for a total of 3 years of grain moisture data. Although less-frequently occurring in NTEP meters 

than in non-NTEP meters, outliers may still exist. This is because grains in the national sample set, which are used 

to develop the meter calibrations, may not include every type and variety of grain available across the country. As 

mentioned in previous articles, it is for this reason that NIST Weights and Measures Division encourages States to 

participate in the annual collection of grain that will be used in the pool of samples for the national grain sample set. 
 

 

 

 

 

 

1-quart and 1-pint Transfer standard storage containers with label. 

Maintenance and Storage of Field Grain Transfer Standards 

A set of transfer standards may contain three or more 1-quart or 1-pint jars of each grain type that is representative 

of the grains grown and sold in a weights and measures jurisdiction at a “high” and “low” moisture for each grain 

type. Some NTEP meters will need samples of about one quart and Non-NTEP meters will need samples of about 

one pint for testing. Since both NTEP and non-NTEP meters are typically present in a weights and measures 

jurisdiction, many laboratories will prepare a transfer standard set with one-quart sample jars so that both NTEP and 

non-NTEP meters can be tested with the same transfer standard set. 

One of the transfer standards for each grain type and moisture level will serve as a back-up standard. The back-up 

standard is not opened or dropped as often as the other grain transfer standards. Since the back-up standard is use 

less frequently, it will serve as verification for the grain transfer standard in use. If it is suspected that a particular 

transfer standard may not be accurate or if a device fails the initial test, the back-up standard can be used to verify 

the test results. 

The grain should be stored in glass containers (see picture of transfer standard storage containers below). The use of 

tinted glass containers can reduce the effects of environmental conditions on the grain sample. Each one-quart or 

one-pint sample is labeled with the official air-oven moisture percentage and weight per bushel, grain type, sample 

identification, and space to record the number of times the sample was opened, warmed and dropped. Drops are 

when the sample is placed in and run through the meter for testing. Because grain transfer standards are biological 

samples that will change over time, the use of this transfer standard is limited in the number of times it can be used. 

Some state studies show that the moisture level of grain samples may begin to change after 18 drops for high 

moisture (over 18 %) corn and soybeans or 24 drops for other grain types and moistures. Therefore, it is 

recommended that grain transfer standards should not be used for more than the number of drops noted above, 

unless your jurisdiction has data to show that the number of drops may be increased without affecting the integrity 

of the sample. Once an inspector begins using a sample to test meters, time may also become a factor in monitoring 

the integrity of the grain sample. 

The grain transfer standards are to be stored in a refrigerator and maintained at 2 oC (35 °F) to 4 °C (40 °F) until 

needed (see “Equipment and Testing Apparatus” section in this article). Approximately two hours before use 

(consider travel time in this two-hour period), the grain transfer standards that will be used for testing that day 

should be removed from the refrigerator and gently shaken several times (for homogeneity) and placed in a transport 

cooler (see “Equipment and Testing Apparatus” section in this article) WITHOUT ICE. This will allow them to 

stabilize to ambient temperature without undergoing temperature extremes. The samples should be spread out to 

allow the air to move freely about them. Upon arrival at the test site, take the cooler containing the grain transfer 

standards into the room where the moisture meter is located, and remove the grain transfer standards from the 

transport cooler. Then, place them in racks near the moisture meter to allow them to equilibrate to room temperature 

(to within -12 oC (10 oF) of the room temperature) before the jar is opened. To verify that the sample has reached 

the appropriate temperature, the temperature of the grain sample should be taken with the least amount of exposure 

to the environment. One method is to replace the lid on the sample with a holed rubber stopper or with a holed lid 

so that a thermometer can be inserted into the sample. Another method is to use a separate container with a holed lid 

or holed rubber stopper. 

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Equipment, Apparatus and Documentation 

From Top Right – Portable refrigerators; Digital thermometer; Coolers, Documentation and Grain transfer standards; Rack; and Liquid-in-glass 

thermometers. 

Appropriate test equipment, apparatus, and documentation is needed to test grain moisture meters. A general 

description of the equipment used to test these devices are as follows: 

Certified digital thermometer or certified liquid-in-glass thermometer - A certified digital thermometer or certified 

liquid-in-glass thermometer that meet NIST HB 105-6 requirements is used to monitor the refrigerator and grain 

sample temperatures. 

Cooler – A cooler of adequate size is used in tempering the official grain samples (Note: Grain samples must not be 

stored on ice). 

Portable refrigerator – A portable refrigerator is used for transporting and maintaining the condition of the samples; 

a connection to an energy source is needed to maintain refrigerant conditions when transporting the samples from 

location to location (e.g. some portable refrigerators may be attached to the cigarette lighter in a motor vehicle). 

Racks – Racks are used to carry standard grain samples from meter to meter during testing. 

Jar lid with a hole or rubber stopper with a hole – A Jar lid with a hole or rubber stopper with a hole is needed to 

measure the temperature of the grain to determine if the grain sample temperature is appropriate before using the 

sample to test the meter. 

Additional containers may be needed to safely carry any hand tools and mercury-in-glass thermometers. 

Documentation - The documentation needed during meter testing includes weights and measures jurisdictional 

policy and inspection procedures, moisture meter operating instructions, current Certificates of Conformance, and a 

list of moisture meters within the jurisdiction and the locations, type of meter(s), and type(s) of grain purchased by 

establishments in the jurisdictions. 

Field Test Procedures 

The following is an overview of the field test procedures for inspecting and testing a grain moisture meter. For 

additional information please review NIST HB 44 Section 5.56b. 

Prior to testing a grain moisture meter, verify that the meter is functioning properly, check to make sure all moving 

parts are moving smoothly and correctly, ensure that all displays are indicating properly, and ensure that there are no 

broken parts on the meter. Proceed with inspecting the meter in accordance with NIST HB 44, Sections 5.56(a) for 

NTEP meters and 5.56(b) for non-NTEP meters prior to and during testing as appropriate. The following are stepby-step procedures for testing a grain moisture meter for accuracy. 

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1. Follow the meter operating instructions and select the grain type to be tested on the GMM. 

2. Fill the grain hopper with the grain transfer standard which represents the grain type that was selected on the 

GMM, starting with the high moisture sample first. 

3. Drop the grain through the meter and record the results. Repeat the test for a total of three moisture readings. 

4. Repeat step 3 with the other meters at the location in assembly-line fashion as quickly as possible with the 

same grain transfer standard. 

5. Return the sample to its original clean jar, seal the jar with the lid, mark the jar with the number of drops and 

return the jar to the cooler. 

6. Analyze the test results only after the selected grain type and selected corresponding grain transfer standard at 

the selected moisture level are tested in all the meters at the device location and the sample has been resealed 

and returned to the cooler. 

7. Average the three moisture readings for each meter and compare each of the results with the official transfer 

standard moisture and determine the error, then compare the results with the applicable tolerance. 

8. If the meter is in tolerance, proceed to the lower moisture transfer standard of that grain and repeat steps 1 

through 7. 

9. If the meter is not in tolerance repeat steps 1 through 7 with the back-up sample for that meter. If the results 

from the back-up sample agree with the results from the primary sample, record the results and return the 

back-up sample to the cooler and continue to use the primary sample as the standard. 

10. If the results from the back-up sample are different, return the primary sample to the portable refrigerator for 

return to the laboratory. Use the back-up sample as the primary from then on. 

11. Repeat steps 1 through 10 for each grain and each moisture level. 

12. If appropriate, test the test weight per volume (e.g., test weight per bushel) indications with at least the lowest 

moisture sample. To do this, drop the lowest moisture sample through the meter 3 times and record the 

results. Note: Evaluation of the test weight per bushel indications can be performed while testing the 

moisture indications or weights and measures jurisdictions may choose to use a separate low moisture sample 

to test the test weight accuracy. 

13. Compare the meter results with the official weight per volume of the transfer standard then compare the 

results with the applicable tolerance for meter test weight per bushel. 

14. As appropriate, follow the NTEP certificate “Field Inspection Notes” and review the audit trail of the GMM 

device for compliance with NIST HB 44. 

15. Complete the report form and explain the results to the device owner. 

16. As you are conducting the tests, mark each sample label with the number of times each sample is dropped and 

the number of times the sample was warmed and verify that the seals of the jars are tight before returning the 

samples to storage. 

17. Return the samples to the portable refrigerator as soon as testing is completed for the day. 

18. When a sample reaches 18 drops for corn and soybeans above 18% or 24 drops for all other grain types or 

moistures, or a greater number of drops, as specified by the jurisdiction, (specified by the jurisdictions based 

on data that shows that the number of drops can be increased without affecting the integrity of the grain 

sample), the sample must be returned to the laboratory and retested. 

Look for Part 5 in this series of articles on grain moisture meters, which will address the evaluation of grain 

moisture meters including a review of the evaluation procedure outline contained in the Grain Moisture Meter field 

manual. 

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