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Weights and Measures Connection, Vol. 3, Issue 6

November 19, 2012

Grain Moisture Air-Oven Reference Methods 

in the United States

Byline: G. Diane Lee

Grain moisture air-oven reference methods are used by many stakeholders in the United States and 

these reference methods play an important role in measurement accuracy and uniformity of commercial 

grain moisture meter measurements in the country. This article will provide a basic description of the 

air-oven reference methods and their use; discuss the importance of grain moisture measurements in 

legal-for-trade applications; identify the key users of the air-oven reference methods for commercial 

use; review the origins of these air-oven reference methods; and outline the specific air-oven reference 

methods used for the major U.S. grains.

What are the grain moisture air-oven reference methods in the United States and how are they used?

The grain moisture air-oven reference methods are procedures used to determine the percent moisture 

content in grain. Basically, a small sample of grain (ground or unground) that represents a larger sample 

set of that grain is placed in a small metal dish and the weight of the sample is recorded. The sample is 

then placed in an oven at a specific temperature and heated for a specific amount of time. The sample 

is removed from the oven and cooled in a container that prevents loss or gain of moisture during the 

cooling process. Then the sample is reweighed. The loss in the sample mass is calculated as the percent 

moisture content of the grain. 

Commercial grain moisture meters are devices that provide a rapid prediction of moisture in grain and 

are used to test the moisture of grains that are bought and sold in the United States. Calibrations for 

commercial grain moisture meters are developed by comparing the grain moisture meter reading to 

grain moisture air-oven reference values. Also, air-oven reference methods determine the reference 

moisture values of grain samples that verify the accuracy and operation of commercial grain moisture 

meters. 

Why are grain moisture measurements important in commercial (legal-for-trade) applications?

Moisture is a critical commercial measurement in grain because “discounts” to the price per bushel of 

grain are made at the time of sale, based on the moisture content of the grain. “Discounts” are 

reductions to the price per bushel of grain. Two examples of discounts are drying and shrinkage 

discounts. A grain buyer must dry grain that is too high in moisture to prevent molding during storage. 

The cost associated with drying the grain or a “drying discount” is determined based on the grain 

moisture measurement at the time of sale. This discount reduces the amount of money the seller 

receives for the grain. For example, a buyer may be purchasing corn from a seller at $4.00 per bushel of 

corn at a target or desired 15 % moisture content. If the seller’s corn is higher in moisture content at 
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the time of sale than is targeted or desired, the buyer will reduce the unit price per bushel he will give 

the seller for the grain. Grains that are too high in moisture may be placed into storage with aeration 

causing shrinkage. The cost associated with shrinkage or a “shrinkage discount” is also based on the 

moisture of the grain at the time of sale. Errors in the moisture measurements may result in large losses 

to either the seller or buyer of grain. See article number C-006, March 2007 “The Economic Impact of 

Errors in Moisture Measurements, Part 2, Grain Moisture Meter Series,” located at: 

www.nist.gov/pml/wmd/pubs/archives-grain-moisture.cfm, for additional information on discounts. 

Who uses air-oven reference methods?

Stakeholders in the United States that use air-oven reference methods as the basis for commercial 

measurements of moisture in grain include:

• The U.S. Department of Agriculture, Grain Inspection, Packers, and Stockyards 

Administration (USDA, GIPSA);

• The National Type Evaluation Program (NTEP) Laboratory for Grain Moisture Meters;

• Grain Moisture Meter (GMM) Manufacturers; and

• State Weights and Measures and other state officials. 

These organizations work together to ensure equity in the commercial grain market as illustrated in 

Figure 1, the system of commercial grain moisture measurements in the United States and Figure 2, the 

movement of grain in the United States and inspection points.

The USDA, GIPSA uses air-oven reference methods to develop the calibrations for the official grain 

moisture meter(s). These official grain moisture meters are used to measure the moisture of grains at 

export locations and upon request at interior grain facilities in the United States. 

The National Type Evaluation Program (NTEP) Laboratory at the USDA, GIPSA uses these air-oven 

reference methods when analyzing a national grain sample set and providing manufacturers with data 

to develop the calibrations for all grain moisture meters used in commercial service in the United States. 

These reference methods are also used to evaluate commercial (i.e., legal for trade) grain moisture 

meters in the NTEP laboratory to determine if prototypes of the device meet the requirements of the 

National Institute of Standards and Technology (NIST) Handbook (HB) 44 “Specifications, Tolerances, and 

Other Technical Requirements for Weighing and Measuring Devices,” prior to being used commercially 

in the United States. See Figure 1, the system of commercial grain moisture measurements in the 

United States. 

State Weights and Measures Officials and other State Officials use these reference methods to 

determine the moisture in grain samples that are used to test commercial grain moisture meters. 

Typically, the grain moisture meters tested by State Weights and Measures or other state officials are at 

local elevators that purchase grain from farmers in their jurisdictions. See Figure 1, the system of 

commercial grain moisture measurements in the United States and Figure 2, the movement of grain in 

the United States and inspection points. 
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Manufacturers of grain moisture meters use these reference air-oven moisture methods when 

developing the calibrations for grain moisture meters that they sell for use in legal-for-trade 

applications. (See Figure 1, The system of commercial grain moisture measurements in the United 

States.)

USDA 

GIPSA

Air-Oven 

Moistures

Manufacturers 

Air-Oven 

Moistures

States AirOven 

Moistures

NTEP Lab

 Meters

GIPSA 

Master 

Meter 

Manufacturer's 

Laboratory 

Standard Meters

Manufacturers 

Production 

Master Meters

Meters to be 

Shipped

(These Become 

Field Meters)

GIPSA Field 

Meter

GIPSA Field 

Meter

GIPSA Field 

Meter

Field Meter

Field Meter

Field Meter

Industry Expectation

Official System State Weights and 

Measures System

NTEP and Manufacturing 

System

Figure 1.

The system of commercial grain moisture measurements in the United States.
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Farm

Local Elevator

Terminal

Elevator

(Grain May be 

exported from this 

location)

Export 

Elevators

Via Trucks

Usually Via Railway, 

 Barges or Ships) 

State Weights and Measures Field Inspection 

Test of commercial meters, typically at local elevators.

Usually Via Railway, 

 Barges or Ships)

GIPSA Official Inspections 

The official meter is used to test grain 

moisture at Interior facilities and at export 

locations. 

Figure 2.

The movement of grain in the United States and inspection points.

History of Air-oven Reference Methods in the United States

The most widely recognized reference methods for determining moisture content in grain are based on 

drying known weights of grain in various types of ovens and calculating moisture content from the 

weight lost in the drying operation.

i The USDA, GIPSA, identifies the “air-oven methods” as its official 

reference methods for determining moisture in grain and oil seeds. According to the history of changes 

to USDA, GIPSA grain standards, these air-oven reference methods have been in use since July 1, 1935. 

The USDA, GIPSA air-oven reference methods were accepted by the American Association of Cereal 

Chemists (AACC, currently known as AACC International), Method 44-15A. Although oven methods are 

subject to error due to the loss of substances other than water during the drying process, Hart and 

Neustadt adapted the use of Karl Fisher (KF) Titration in 1957 to test the accuracy of the USDA air-oven 

moisture methods.

ii

 The USDA air-oven methods were designed to give results which agree with those 

obtained by KF Titration. KF Titration provides moisture values by extracting water from grain with the 

use of a solvent, such as methanol, and is more specific for grain moisture. Historical records have 

noted that KF Titration required more time for testing than was practical and, due to the test process at 

that time, there were repeatability issues from operator to operator due to portions of the test that 

were not automated. Frank E. Jones and Carroll S. Brickenkamp, National Bureau of Standards (NBS, 

now known as the National Institute of Standards and Technology), National Engineering Laboratory, 

noted that techniques were developed to automate KF Titration to address some of the earlier 

disadvantages of this method. Jones and Brickenkamp also noted that, although the KF Titration is more 

specific for moisture, additional tests are needed to determine if other substances react with the solvent 

used to extract water.iii The air-oven reference methods remains the reference methods for moisture in 
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the United States; this is likely because these methods have been compared to KF Titration and/or other 

more definitive tests and have proven to provide very repeatable measurements.

Air-oven Reference Methods for Major U.S. Grains

There are currently eight air-oven moisture reference methods used to determine moisture in the major 

U.S. grains and these methods are recognized internationally. These reference methods vary per grain 

type, moisture, and commodity and are maintained at the USDA, GIPSA in the Technology and Science 

Division’s Laboratory Work Instructions. These work instructions include specific procedures, guidance 

on test sample size, equipment and material, and acceptance criteria needed for each test. Because 

these air-oven reference methods vary per grain type, the following tables are provided to summarize 

the different air-oven reference methods used in the United States. 

AO_1 

Single-Stage Air-Oven Reference Method for Moisture in Grains, Oilseeds, and Commodities: is the 

difference in weight after drying a ground sample of grain for one hour at 130 oC ± 1 oC (drying 

time is two hours for soybean meal). The following grains and commodities are tested using this 

method:

Grains and Oilseeds Commodities

Description

Moisture

Restriction Description

Barley ≤ 16 % Barley - submitted

Lentils ≤ 16 % Bulgur

Oats ≤ 16 % Corn Soy Blend

Pea ≤ 16 % Corn Soya Flour

Rice, brown ≤ 16 % Corn Soy Milk

Rice, milled ≤ 16 % Corn - submitted

Rice, rough ≤ 13 % Corn Meal

Rye ≤ 16 % Dessert Powder

Sorghum ≤ 16 % Egg Noodles

Soybeans ≤ 10 % Hominy Grits

Sunflower ground 50/50 with celite

Non (for 

moisture 

correction 

of crude oil 

results

Icing Mix Powder

Triticale ≤ 16 % Lasagna

Wheat ≤ 16 % Macaroni

Macaroni and Cheese

Rotini

Rolled Wheat

Sorghum Grits

Soybean Meal

Spaghetti
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AO_1 

Single-Stage Air-Oven Reference Method for Moisture in Grains, Oilseeds, and Commodities: is the 

difference in weight after drying a ground sample of grain for one hour at 130 oC ± 1 oC (drying 

time is two hours for soybean meal). The following grains and commodities are tested using this 

method:

Grains and Oilseeds Commodities

Description

Moisture

Restriction Description

Wheat Flour

Wheat Soy Blend

Wheat Soy Milk

Wheat – submitted

Cited References: AOCS Method Ba 2a-38; AACC Method 44-15A; AOAC Method 925.10; ASTM; NIST; Kenyon, 

A.S., Black, J.C., & Layloff, T.P. (1995); J. Assoc. Off. Anal. Chem. 78, 1109-1111

AO_2 

Two-Stage Air-Oven Reference Method for Moisture in Grains and Oilseeds is the 

difference in weight after air-drying a unground sample of grain until its moisture 

content is < 16 % (< 10 % for soybeans and < 13 % for rough rice), then drying 

the ground, air-dried sample for one hour at 130 oC ± 1 oC (two hours for soybean 

meal). The following grains are tested using this method:

Grains and Oilseeds

Description Moisture

Restriction

Barley > 16 %

Lentils > 16 %

Oats > 16 %

Peas > 16 %

Rice, brown > 16 %

Rice, milled > 16 %

Rice, Rough > 13 %

Rye > 16 %

Sorghum > 16 %

Soybeans > 10 %

Triticale > 16 %

wheat > 16 %

Cited References: AACC Method 44-15A;, ASTM; NIST; Kenyon, A.S., Black, J.C., & Layloff, 

T.P. (1995); J. Assoc. Off. Anal. Chem. 78, 1109-1111.
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AO_3 

Air-Oven Reference Method for Moisture in Corn and Beans is the difference in 

weight after drying an unground sample of grain for 72 hours at 103 oC ±1 oC. 

The following grains are tested using this method:

Grains

Description Moisture

Corn Any

Beans Any

Cited References: AACC Method 44-15A; ASTM; NIST; Kenyon, A.S., Black, J.C., & Layloff, T.P. 

(1995); J. Assoc. Off. Anal. Chem. 78, 1109-1111.

AO_4 

Air-Oven Reference Method for Moisture in Mustard, Canola and Rapeseed is the 

difference in weight after drying an unground sample of grain for one hour at 

103 oC ±1 oC (two hours for soybean meal), repeat the drying and if the difference 

between the 1st and 2nd weight after each drying is greater than 0.01 g the sample is 

dried for an additional hour. The following grains are tested using this method:

Grains

Description Moisture

Mustard Any

Canola Any

Rapeseed Any

Cited References: ISO 665; ASTM; NIST; Kenyon, A.S., Black, J.C., & Layloff, T.P. (1995); J. 

Assoc. Off. Anal. Chem. 78, 1109-1111.

AO_5 

Air-Oven Reference Method for Moisture in Flaxseed is the difference in weight

after drying an unground sample of seed for four hours at 103 oC ±1 oC. The

following grains are tested using this method:

Grain

Description Moisture

Flaxseed Any

Cited References: AACC Method 44-15A; ASAE Method S352.2; ASTM; NIST; Kenyon, A.S., 

Black, J.C., & Layloff, T.P. (1995); J. Assoc. Off. Anal. Chem. 78, 1109-1111.

AO_6 

Air-Oven Reference Method for Moisture in Safflower seeds is the difference in 

weight after drying an unground sample of seed for one hour at 130 oC ±1 oC. The 

following grain is tested using this method:

Grain

Description Moisture

Safflower seeds Any

Cited References: ASAE Method S352.2; ASTM; NIST; Kenyon, A.S., Black, J.C., & Layloff, T.P. 

(1995); J. Assoc. Off. Anal. Chem. 78, 1109-1111.
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AO_7 

Air-Oven Reference Method for Moisture in Sunflower seeds is the difference in 

weight after drying an unground sample of seed for three hours at 130 oC ±1 oC. 

The following grain is tested using this method:

Grain

Description Moisture

Sunflower seeds Any

Cited References: AOAC Method Ac 2-41; AOAC Method Ai 2-75; ASTM; NIST; Kenyon, A.S., 

Black, J.C., & Layloff, T.P. (1995); J. Assoc. Off. Anal. Chem. 78, 1109-1111.

AO_8 

Air-Oven Reference Method for Moisture in Corn Gluten Feed is the difference in 

weight after drying the ground sample for four hours at 103 oC ±1 oC. The

following sample is tested using this method:

Grain

Description Moisture

Corn Gluten Feed Any

Cited References: Method of the Corn Refiners Association; ASTM; NIST; Kenyon, A.S., Black, 

J.C., & Layloff, T.P. (1995); J. Assoc. Off. Anal. Chem. 78, 1109-1111.

Many studies of air-oven reference methods have been conducted over the years since the first use of 

these reference methods. These studies resulted in changes to the air-oven reference methods that 

improved the accuracy of grain moisture meter measurements. The air-oven reference methods are 

well established in the United States. Stakeholders use these reference methods to develop the 

calibrations for all commercial grain moisture meters and to test these devices to provide uniformity in 

the market place. 

NIST, OWM greatly appreciates the review and comments of Dr. Richard Pierce, USDA, GIPSA during the 

development of this article. 

AO_10 through AO_15 are reference methods for other commodities and processes 

for laboratory check samples and equipment maintenance:

AO_10 100 o C Vacuum Oven Reference Method for moisture in Commodity 

Samples

AO_11 Processing the Air-Oven Laboratory Wheat Check Sample

AO_12 Processing the Air-Oven Laboratory Corn Check Sample

AO_13 Air-Oven Uniformity Check

AO_14 Calibrating the Thermometers Used in the Air-Oven Laboratory 

AO_15 Moisture Dish Maintenance and Tare Weight Check Schedule
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For additional information concerning this article, please contact Diane Lee by e-mail at 

diane.lee@nist.gov or the Office of Weights and Measures at (301) 975-4004, e-mail at owm@nist.gov.

 i i Storage of Cereal Grains and Their Products, Chapter 1: Moisture–Its Significance, Beahavior, and Measurement, 

W. Harward Hung and S.W. Pixton.

iii Journal of AOAC, Volume 64, Number 6, 1981, Frank E. Jones and Carroll S. Brickenkamp.

Editor’s Note: July 18, 2016, content updates were made to the moisture allowance in several tables.