Research to develop practical user guidelines to maximise the accuracy of moisture meters

Abstract

Accurate and reliable moisture measurement is essential for drying calculations, as well as for safe storage and marketing of grain and oilseeds. Most farmers rely on capacitance or resistance moisture meters for this task. These have limitations of accuracy: usually in the range ± 0.5%. This has implications for quality loss during storage as well as sale to end-users with added costs of rejections or claims.

The aim of this project was to assess problems that might occur under practical conditions and develop end-user recommendations to improve the accuracy of moisture measurement on farms. A review of factors most likely to cause variation in use was undertaken, in co-operation with meter manufacturers and selected farmers. This was followed by a survey of farmers and meter use on a number of farms to assess the variations under practical conditions.

Laboratory experiments were devised to compare readings from meters with the oven method (ISO 712:1998). No difference between the performance of capacitance and resistance meters was shown. Moisture content readings were repeatable for homogenous samples but readings from variable, but well-mixed, samples gave variable results even with the meter that used the largest sample.

The effect of variety on the results given by several different meters was assessed during both laboratory and field testing. A number of hard and soft endosperm varieties of wheat were compared in laboratory tests but no consistent effect on meter reading was found. No differences were detected in response of meters when testing different varieties of barley.

Resistance meters were inaccurate when the sample was under-compressed and the need for regular servicing of grinders was identified. Capacitance meters should only be used on a level surface.

Samples were taken from the output of a high temperature dryer. Readings taken using a capacitance meter were higher on average by 0.4% after six days when compared with readings taken immediately ex drier. No such difference was observed using a resistance meter.

An assessment of moisture probes showed their value in obtaining in-situ data from grain bulks. However, results were more variable than those of conventional meters and there was often a significant difference between moisture contents determined by probes and oven tests on samples removed close to the probe sensor.

On-farm assessments indicated that some farm meters gave markedly different results from manufacturer supplied test meters used in the work or oven tests. In several cases poorly maintained grinders were the source of error. Meters tended to under-read moisture at values above about 17%, often by more than 1%. This error was not seen during laboratory testing of samples.

A survey showed that most farmers had a realistic view of the accuracy of their meter, but were often more concerned with their meters agreeing with those of the merchant or end-user than with accuracy. This could have serious implications for quality and food safety, as accurate moisture content measurements are important when deciding on the need for drying to the correct moisture content and preventing ochratoxin A formation during storage.