Article Ref: C009 

Weights and Measures Connection, Vol. 2 , Issue 2 Page 1

May 1, 2011

Grain Moisture Meter Test Methods and Tolerances

Byline: G. Diane Lee

Commercial grain moisture meters are responsible for making measurements that affect large volumes 

of product per device and have a large economic impact. Many of the commercial grain moisture 

meters in service today are meters that must meet the requirements of NIST Handbook (HB) 44, Section 

5.56(a) Grain Moisture Meters. This code applies to all grain moisture meters manufactured or placed 

into commercial service after January 1, 1998. This code identifies two test methods, the Air Oven 

Reference Method and the Meter to Like-Type Meter Method; separate tolerances are applied to each 

method. This article has been written to describe how to use the two test methods and apply the 

associated tolerances specified in NIST HB 44 Section 5.56(a) in field testing grain moisture meters. 

Background

To better understand the system for field testing grain moisture meters, it is necessary to provide some 

background on the development of calibrations for grain moisture meters. The National Type 

Evaluation Program (NTEP) for grain moisture meters differs from the type evaluation program for other 

commercial devices in that there are two phases to the program. In addition to a device evaluation, 

Phase I, in which a limited number of grains are tested to evaluate the design and calibration of the 

device, grain moisture meters are also subjected to an annual calibration testing program; referred to as 

Phase II. The Phase II program verifies the calibration performance with a large number of samples, 

including difficult to store, high moisture samples, which would be hard to use in Phase I tests. The 

Phase II program also facilitates meter calibration and standardization to a national, common sample set 

and a single air oven laboratory. In the NTEP Phase II program, raw data needed for calibration updates 

to grain moisture meters is collected. The collection of this data is necessary for grain moisture meters 

because although the device’s standard features may remain unchanged each year, individual grain 

calibrations may change to reflect long-term changes in grain characteristics. 

Moisture meters predict moisture content based on an optical (near infrared meter) or dielectric 

(dielectric meter) measurement of a meter to a grain sample. The current technology for grain moisture 

meters is grain type/class (e.g., wheat is a grain type, durum such as durum wheat is a class of wheat) 

specific. As such, a calibration is developed for each grain type/class based on the characteristics of that 

grain type/class. Because the current technology is grain type/class specific, the relationship between 

moisture and a grain moisture meter’s optical or dielectric response to the biological characteristics of 

the grain sample (variety, size, weight, etc.) may change with each crop season, thereby necessitating a 

change to the meter’s calibration for that grain type/class. In addition, there is variation in the 

calibrations for each type of grain moisture meter due to the manufacturer’s prediction equations which 

are used to develop the calibrations for their meter and due to the specific sensing technologies used in 

different meter types. Simply put, calibrations must be reviewed and adjusted to account for 

differences in grain crops from year to year.
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Weights and Measures Connection, Vol. 2 , Issue 2 Page 2

To determine whether or not a calibration change is needed, as noted above, raw data is collected in the 

Phase II program on the NTEP laboratory meters using the national grain sample set. The national grain 

sample set consists of hundreds of current crop year grain samples that are representative of grains 

from across the United States for the grain types/classes in the NTEP program. The raw data from the 

NTEP laboratory meters is provided to the manufacturers and the manufacturers use this data to make 

calibration updates to the grain moisture meters as needed. An NTEP Certificate of Conformance is 

available to state weights and measures officials, which includes the updated calibrations needed for 

field inspection of devices. The manufacturers provide the updated calibrations to the device owners to 

ensure that all commercial meters have the latest calibrations installed in the meters. 

AIR OVEN REFERENCE METHOD - FIELD TESTS

When using the Air Oven Reference Method, a grain sample with a reference moisture value is used to 

test field meters. The reference moisture value of the grain sample is determined using a GIPSA Air 

Oven test procedure. The field test is performed by measuring the moisture of reference grain samples 

in the field meter. The field meter moisture measurement is compared to the reference moisture values 

of the grains. This method is a “mini” test or verification of the meter’s calibration; it determines how 

well the meter’s calibration responds to the two to three grain samples (typically of high and low 

moisture content for each grain type) that are used for testing. Due to the current technology for 

developing meter calibrations, there is a probability that the samples selected may not perform well on 

all NTEP meter types. Why would this happen? As discussed in the background section of this article, 

the current technology for grain moisture meters relies on grain type/class specific calibrations that will 

vary per meter type. There are a number of samples represented in the national grain sample set, but 

due to the secondary measurement methods (measuring an optical or dielectric response of a grain 

sample to predict moisture) and the differences in measurement technologies (prediction equations and 

sensing technologies), individual samples are predicted with differing levels of agreement to the 

reference method. Also there is the possibility that if a grain sample that is chosen to test the meter is 

not represented (due to its variety or other biological characteristic) in the national sample set used to 

develop the calibration for some measurement technologies, the grain sample may not perform well on 

the meter. To prevent this occurrence, it is necessary to screen the sample by running the samples 

through laboratory meters that are maintained in good operating condition, have the latest calibrations 

installed, and are of the same type as the jurisdiction’s field meters that will be tested. If the grain 

sample moisture tested on the laboratory meter types are within 0.5 % of the reference moisture, the 

sample can be used to test field meters. Careful sample selection is needed for this test method to 

avoid failing meters that are operating correctly. 

AIR OVEN REFERENCE METHOD – TOLERANCES FOR FIELD TEST

The tolerances that apply to the Air Oven Reference Method are found in NIST HB 44 Section 5.56.(a) 

Paragraph T.2.1, Table T.2.1. The tolerances are the same for both acceptance and maintenance 

tolerance. See tolerances below:
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Weights and Measures Connection, Vol. 2 , Issue 2 Page 3

Table T.2.1.

Acceptance and Maintenance Tolerances

Air Oven Reference Method

Type of Grain, Class,

or Seed Tolerance MinimumTolerance

Corn, oats, rice, sorghum, 

sunflower

0.05 of the perdent moisture 

content

0.8 % 

in moisture content

All other cereal grains and oil 

seeds

0.04 of the percent 

moisture content

0.7 %

in moisture content

(Amended 2001)

The minimum tolerance referenced in the table is the minimum tolerance to be applied to a test; if the 

calculated tolerance falls below the minimum tolerance level, the minimum tolerance is applied. To 

calculate the tolerance, the percent reference moisture of the grain sample that is used for testing is 

multiplied by the tolerance in Table T.2.1. 

Example 1: 

Corn Sample with a reference moisture of 14.5 %

14.5 % x 0.05 = 0.72 %

Since 0.72 % is less than the minimum tolerance of 0.8 %, the applicable tolerance for the test is 0.8 %. 

Example 2:

Corn Sample with a reference moisture of 16.0 %

16.0 % x 0.05 = 0.8 %

Since 0.8 % is equal to the minimum tolerance of 0.8 %, the applicable tolerance for the test is 0.8 %.

Example 3:

Corn Sample with a reference moisture of 18.0 %

18.0 % x 0.05 = 0.9 %

Since 0.9 % is greater than the minimum tolerance of 0.8 %, the applicable tolerance for the test is 

0.9 %.

METER TO LIKE-TYPE METER REFERENCE METHOD - FIELD TEST

When using the “meter to like-type meter” test method, a grain sample is used as a comparison medium 

for a side by side comparison test of a “reference meter” to a field meter of like-type. A reference 

meter is a meter that is maintained by the laboratory in good working condition, has the latest 

calibrations installed, and is of the same meter type as the jurisdiction’s field meters. The reference 

meters are used in side-by-side comparisons in the field to test commercial meters of like type. The 

grain sample is measured in the reference meter and then measured in the field meter under test, and 

the results are compared. The tolerance in NIST HB 44, Section 5.56(a), Table T.2.1. (see below) is 

applied to the difference between the two results. It is important to note that the meter to like-type 
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Weights and Measures Connection, Vol. 2 , Issue 2 Page 4

meter reference method can only be used with NTEP meters because the Phase II calibration testing 

performed on NTEP meters is a check of the calibration accuracy. This test method verifies that the field 

meter is functioning similarly to the reference meter and determines if a meter is malfunctioning. It is 

important that the side-by-side comparison of meters be conducted with a like-type meter, because 

although the meter calibrations are based on the same sample set, the prediction equations used to 

develop the calibrations vary per meter type; the sensing technologies may also vary, which causes 

various meter types to measure differently. If a meter of one type is used to test a meter of another 

type, errors will be introduced in the test results because of these differences.

Reviews and reports of the variation between meter types in the NTEP Phase II program are annually 

reviewed at the Grain Analyzer Sector meetings and are included in the reports of the Sector. It should 

be noted that these variations are much less than in past years because of the NTEP Phase II program. 

The data below include the grain moisture meter Phase II comparisons for Corn, 2007 - 2009 crops 

years, which were reported in August 2010. These data illustrate the value of using like-type meters 

when using a meter to meter comparison method. In the first graph, the official meter has a bias with 

respect to the air oven of approximately 0.5 % in the 10 % to 12 % range, while another meter type in 

the same range has a bias of approximately 0.19 %. The official meter has a bias to air oven of 0.31 % 

higher than this NTEP meter type. If a meter of like-type is not used in the meter to like-type meter test 

method, any error created by a difference in meter types will be added to the test results.
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Weights and Measures Connection, Vol. 2 , Issue 2 Page 5

METER TO LIKE-TYPE METER REFERENCE METHOD – TOLERANCES FOR FIELD TESTS

The tolerances that apply to the meter to like-type meter test method are found in NIST HB 44 Section 

5.56(a) Paragraph T.2.2. The tolerances are the same for both acceptance and maintenance tolerance 

as shown.

Table T.2.2. 

Acceptance and Maintenance Tolerances Meter to Like-Type Meter Method

Sample Reference Moisture Tolerance

Up to 22 % 0.5 %

in moisture content

(Added 2001)
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Unlike the air oven reference method for field testing, no calculation is needed to apply the tolerance 

for meter to like-type meter. The meter to like-type meter tolerance is expressed as a percent value 

within the table and is applied directly to the difference in the measurement results of the reference 

meter of like-type to the measurement results of the field meter.

Example 1

A corn sample reads 16.0 % on the reference meter of like -type and the same sample reads 14.5 % on a 

field meter under test.

14.5 % − 16.0 % = − 1.5 %

Since the difference is greater than ± 0.5 %, the field meter under test does not meet the tolerance 

specified in Table T.2.2.

Example 2

A corn sample reads 18.0 % on the reference meter of like-type and the same sample reads 18.5 % on a 

field meter under test.

18.5 % − 18.0 % = 0.5 %

Since the difference is equal to ±0.5 %, the field meter under test meets the tolerance specified in 

Table T.2.2.

Example 3

A corn sample reads 14.5% on a reference meter of like-type and the same sample reads 14.7 % on the 

field meter under test.

14.7 % – 14.5% = 0.2% 

Since the difference is less than ± 0.5 %, the field meter under test meets the tolerance specified in 

Table T.2.2.

It is very important that correct test procedures and tolerances be applied during testing. Grain 

moisture meters, although few compared to other commercial devices, affect high dollar values of 

product per device. 

FUTURE DIRECTION FOR GRAIN MOISTURE METERS

In spite of the limitations of the current technology, there has been progress. Prior to the NTEP program 

there was a lack of calibration uniformity due in part to the use of different grain sample sets. 

Calibrations for different meter types were developed with grain samples from different areas of the 

United States. Also many of the meters were designed such that the user was required to make 

judgments concerning certain user-controlled factors that would affect the accuracy of the moisture 

measurement. When the NTEP program was developed, the same meter technology was in use, but the 

design criteria for the devices changed to remove many of these user-controlled judgments, including 

sample weighing and moisture look-up charts. NTEP meters directly indicate the moisture on the device 

and also do not require the user to measure the weight of the grain sample prior to measuring the 

moisture content of the grain. In addition, instead of calibrations being developed based on varying 

grain sample sets, calibrations for all meter types are based on the same grain sample set, made up of 

grains from across the United States. As the calibrations for NTEP meters become more robust, that is 

more and more grain varieties are included in the grain sample set, over time there may be reduced 
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Weights and Measures Connection, Vol. 2 , Issue 2 Page 7

issues with sample outliers (grain samples that do not perform well on a meter) in field evaluations of 

NTEP meters. 

Currently, grain moisture meters are being developed that use the same dielectric technology but 

measurements are made at a higher radio frequency and a uniform equation has been developed. It is 

anticipated that these new grain moisture meters will not require separate calibrations for each grain 

type; will be more stable over time; and with the uniform calibration equation, will allow the use of the 

same calibration equation for all grain moisture meter types. Look for more details on these new grain 

moisture meters in a future article.

A special thank you goes to Mr. Jack Barber, JB Associates; Ms. Cathy Brenner, USDA, GIPSA; Dr. Richard 

Pierce, USDA, GIPSA; and Mrs. Tina Butcher, NIST, WMD for their reviews and comments to this article.