Using Moisture Meters Properly
January 01, 1997
by Teresa Acklin
Three key guidelines can prevent costly mistakes in assessing grain moisture.
By Dirk E Maier
The proper use of a moisture meter is critical in making sound grain drying, storage and marketing decisions. Inaccurate moisture readings can lead to serious consequences, including higher-than-necessary drying costs, excessive shrink or outright spoilage.
To obtain accurate moisture readings, a representative sample must be obtained, the tester must be used properly, and the tester must be calibrated periodically.
1. Grain Sampling
When sampling a load of grain, scooping a can-full off the top is not adequate. Either probe the load in at least two locations (avoid the center and the corners) or preferably sample the flowing grain during unloading.
To sample from a grain flow, pass a can across the grain stream about every tonne and collect the grain into a bucket. After unloading is completed, mix the grain in the bucket — for example, by emptying it back and forth several times into a second bucket — then draw out a sample for a moisture test. To avoid changes in the moisture content, cover the bucket if the grain will not be tested immediately.
When checking moisture of binned grain, a probe to collect samples from various depths will provide the most representative samples. Going as deep as possible at the bin center and several other locations will help determine the progress of drying or reveal potential trouble spots in storage bins.
Do not mix the samples, and be sure to test them separately. Knowing the moisture at different locations will help in making better management decisions. Routine sampling of binned grain periodically during the storage season helps to avoid hot spots and spoilage problems.
2. Tester Use
For portable moisture meters, make sure the battery is good because a low battery causes inaccurate readings. Replace the battery at least once a year. Also, removing the battery during long, idle periods prevents acid leakage, which could damage the meter.
Carefully read and follow the meter manufacturer's instructions. All moisture testers show some variability when the same sample is tested repeatedly, and an average of three successive readings should be used.
Attention must be paid particularly to the tester's temperature compensation because grain temperature has a major effect on moisture readings. Some testers have automatic compensation, and some compensate when a button is pushed. Others require separate measurement of grain temperature with a thermometer and the addition or subtraction of a correction factor to the moisture reading.
Even with temperature compensation, moisture meters are not capable of testing grain above 32°C with any accuracy.
When cold grain is removed from storage on a warm day, or when cold samples are brought inside during harvest, moisture will condense on the kernel surfaces. Condensed surface moisture produces erroneously high readings in electronic testers.
Avoid this problem by placing the grain into a sealed container and allowing it to equilibrate to the ambient temperature before testing. When selling grain in warm weather, warm the grain by aeration to avoid erroneous high moisture readings.
Electronic testers will underestimate the moisture content of hot grain that has not reached equilibrium by at least one to two percentage points. To get an accurate reading of hot grain, let it cool slowly in a sealed container such as freezer bags before testing.
An alternative for estimating the moisture content of hot grain is to cool the sample rapidly in an open container, or cooling box. Although definitely not as accurate, it reduces the temperature compensation problem.
A cooling box is equipped with a fan that cools a half-kilogram sample within three to five minutes. It consists of a metal mesh sieve, a meat thermometer, a 5-minute timer, paper air filter, small squirrel-cage blower and plywood. The total cost of constructing such a box is about U.S.$70, with the blower being about 50% of the cost.
3. Calibrating for Accuracy
Elevator moisture testers should be inspected and certified regularly. Most equipment dealers that sell and service moisture testers have an inspected and certified unit available for calibration purposes.
Readings from several samples at different moisture contents on a moisture tester should be compared against a certified unit. At least three samples should be tested at both the 15% and 25% moisture levels.
Testing each sample three times in the meter and three times in a calibrated unit allows for the calculation of an average moisture content difference. The difference in readings at the higher and lower moisture contents is generally not the same. However, if the difference for any single sample is greater than 1.0 percentage points or if the average difference for all samples at one moisture level is greater than 0.5 point, the tester should be serviced by the manufacturer.
Dirk E. Maier is assistant professor and extension agricultural engineer at Purdue University, West Lafayette, Indiana, U.S. He specializes in post harvest engineering, including grain and feed handling, drying, storage and processing.