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The effect of wetting or drying wheat and barley on the calibration of two moisture meters
J. stored Prod. Res. Vol. 24, No. 3, pp. 151-155,1988 Printed in Great Britain. Ail rights reserved
Copyright 0
0022-474X/88 $3.00 + 0.00 1988 Pergamon Press plc
THE EFFECT OF WETTING OR DRYING WHEAT AND BARLEY ON THE CALIBRATION OF TWO MOISTURE METERS
Slough Laboratory,
SYJ_.WA HENDERSON Ministry of Agriculture, Fisheries and Food, London Road, Slough, Berks., England (Received 7 February 1988)
Abstract-Samples of wheat and barley pre-dried or pre-wetted in the laboratory were conditioned to the same range of moisture contents as each other so that they were adsorbing or desorbing moisture respectively. Their moisture contents were determined with a capacitance and a resistance moisture meter and compared with results obtained with an oven-drying standard method. Abnormally high meter results were obtained for adsorbing samples one day after adding water, but results were not af%cted one day after drying. Otherwise, moisture content results changed little with time during the IO-week experiment.
INTRODUCTION
Electrical moisture meters measure electrical characteristics of the grain that are related to its moisture content (m.c.) rather than making a direct reading, thus they must be calibrated against some form of standard, usually an oven-drying method. Manufacturers usually calibrate their meters using grain obtained from farm stores and use it without further wetting or drying. Scientists sometimes need to measure the m.c. of grain samples that have been artificially wetted or dried in the laboratory. Past experience at this laboratory (unpublished results) has shown that recent wetting or drying may affect m.c. results obtained with meters, and the object of the experiment described here was to find out if this were so. The finding could also be relevant to field situations where grain has been wetted by rain or dried on farm driers. MATERIALS
AND METHODS
A capacitance and a resistance moisture meter were chosen for this study because they were meters used in the laboratory at the time. The results obtained in this investigation may not apply to other makes or models of moisture meter. The capacitance meter was a Sinar Agritec Moisture Computer and the resistance meter was a Protimeter Mini Mark IV. No discrimination is intended in favour of or against the makes of moisture meter used in this investigation. Grain
Tests were carried out using Flanders, a soft variety of wheat received at 14.9% m.c. and a sample of barley (unknown mixture of varieties) received at 14.8% m.c. Both lots of grain were about 1 year old when this experiment began. Conditioning
the grain
The m.c. of 5 kg of wheat and 5 kg of barley was increased to about 18% by the addition of distilied water followed by thorough mixing. Another 5 kg of each was air-dried at 30-35°C to about 9% m.c. Six hundred grams of each, neither wetted nor dried, were retained as control samples. The samples were sealed in glass jars and left to equilibrate at 10°C for 3 weeks with occasional mixing by manual shaking. Seven 600 g lots of the pre-wetted grain were then dried to approximately 11, 12, 13, 14, 15, 16 and 17% m.c. to give desorption samples and seven lots of pre-dried grain were wetted to similar m.c.‘s to give adsorption samples. This range of m.c. was chosen because most of the grain samples handled at this laboratory come within this range. After preparation, all samples were stored in airtight glass jars at room temperature. 151
SYLVIA HENDERSON
152
Moisture content determination The m.c. of most samples was measured 1 day after preparation but it took 2 days to dry the samples to 11% m.c., so these were not measured until the third day. Moisture contents were then measured 7, 9, 14, 17, 21, 28 days and 10 weeks after preparation. Samples at 11% m.c. were not measured with the Protimeter because it was not calibrated at this level. The m.c. of the control samples was measured on each occasion. Moisture content was measured with both meters in triplicate at room temperature, which averaged 24°C and ranged between 21 and 30°C. About 200 g of grain was removed from the jar for use with the Sinar and, as it was not ground, it was returned to the jar and the contents were mixed before removing the next 200 g replicate. For measurement with the Protimeter, three 20 g samples were ground using the manufacturers hand grinder. The m.c. of all samples was also measured in duplicate by an oven-drying standard, BS 4317, Part 3, 1987 (IS0 712, 1985). Both meters had been calibrated by their manufacturers using this method. RESULTS
AND
DISCUSSION
Both meters gave consistent results and triplicate determinations came within 0.6% m.c. of each other on all except two occasions when results differed by 0.7 and 0.8% m.c. Triplicate meter results were usually closer together in the middle of the range of m.c. than at the extremes. It is likely that some variation was due to the grain, since each determination was made on a separate sample from the jar. Results obtained with the meters were compared with the BS oven-drying method (Figs l-4). The m.c. measured by the BS oven method varied little with time, the means of duplicates obtained during the 10 weeks coming within 0.4% m.c. of each other. This variation was probably due partly to changes in the m.c. of the grain with time and partly to errors in determination. The
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Fig. 1. The differences in moisture content between the BS oven-drying method and meter results for wheat, measured at intervals of up to 70 days after adding water. Moisture contents in the column on the right are the means of the oven-drying results.
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Fig. 2. The differences in moisture content between the BS oven-drying method and meter results for wheat, measured at intervals of up to 70 days after drying. Moisture contents in the column on the right are the means of the oven-drying results. SINAR tI -I
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Fig. 3. The differences in moisture content between the BS oven-drying method and meter results for barley, measured at intervals of up to 70 days after adding water. Moisture contents in the column on the right are the means of the oven-drying results.
154
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Fig. 4. The differences in moisture content between the BS oven-drying method and meter results for barley, measured at intervals of up to 70 days after drying. Moisture contents in the column on the right are the means of the oven-drying results.
mean of all oven-drying results for each sample was calculated and is given in Figs 14. It is obvious that the meter readings were consistently high for some samples and low for others. The main point of interest, however, is that the mean m.c. measured by the Sinar exceeded the oven-drying results by a greater amount one day after wetting the grain than during the rest of the experiment for 13 of the 14 samples (Figs 1 and 3). Similarly with the Protimeter, 7 out of 12 means were higher one day after wetting. After the first day, the differences between meter and oven-drying m.c’s remained almost constant with time, although the actual m.c.‘s changed slightly towards the end of the experiment, probably due to continually removing grain from the jars. Recent drying did not appear to affect the results (Figs 2 and 4). The mean m.c.‘s for the control samples determined by oven-drying method were 14.9% for wheat and 14.8% for barley. Moisture contents measured by the Sinar were between 0.3 and 0.7% m.c. high for the wheat control sample and were within f0.2% m.c. of the oven-drying results for the barley control. The Protimeter over-estimated the m.c. of the wheat control sample by between 0.0 and 0.7% m.c. and under-estimated by 0.2 to 1.3% m.c. for the barley control sample. These are similar to the results for the adsorption and desorption samples at about 15% m.c. Consistent differences between meter and oven-drying results are less important than trends with time, because the calibration of the meters can be adjusted. It was noticed that meter results were nearly always higher for samples adsorbing moisture than for those desorbing moisture, but differences were less than 0.5% m.c. This is generally of little practical importance, except in precise laboratory tests when, at this laboratory, m.c.‘s are always measured by an oven-drying method. The methods used in this work to add or remove water may have influenced the results obtained by either or both the meters and oven-drying. These methods are used as a routine at this laboratory because they are rapid and repeatable and in any case are representative of some of
Calibration of moisture meters
IS5
the ways on which grain may be wetted or dried in practice. For example, grain may be wetted by rain and it is often dried by flows of ambient air. This work was carried out on individual examples of a capacitance and a resistance meter. It is possible that different models and makes of meter may give different results. CONCLUSIONS The accuracy of both meters was reduced when measuring the m.c. of grain recently wetted in the laboratory. This must be taken into account when measuring the m.c. of grain recently wetted by rain because results are likely to be erroneously high. The Protimeter used in this investigation has an adjustable scale which lowers the reading by 0.5-l .O% m.c., when the meter is used for whole grains after rain. It is possible that such an adjustment should be made for ground grain, since this would generally improve the accuracy of the results reported here for newly wetted samples. Recent drying did not appear to affect the results. This investigation highlights the potential errors when measuring the m.c. of grain using electrical meters. It is advisable to check frequently the calibration of any moisture meter using an oven-drying standard and perferably using the type of sample and conditions most often encountered. Electrical moisture meters are invaluable for rapid checks on the m.c. of grain during drying or storage. However, it is unrealistic to expect them to provide the same accuracy as an oven-drying method of measuring m.c. although it should be noted that errors can occur during oven dete~inations (Henderson and Wilkin, 1985). During commercial transactions an appropriate margin should be allowed to accommodate measurement discrepancies and the potentially large errors associated with sampling. Acknowledgement-The
the manufacturers
author would like to thank Miss M. Gibbs, who determined most of the moisture contents and of the meters for their co-operation.
REFERENCE Henderson S. and Wilkin D. R. (1985) Measuring moisture content of wheat. Agritrade Ott 1985, pp. 1l-14.
S.P.R. 24,3-c
Copyright 0
0022-474X/88 $3.00 + 0.00 1988 Pergamon Press plc
THE EFFECT OF WETTING OR DRYING WHEAT AND BARLEY ON THE CALIBRATION OF TWO MOISTURE METERS
Slough Laboratory,
SYJ_.WA HENDERSON Ministry of Agriculture, Fisheries and Food, London Road, Slough, Berks., England (Received 7 February 1988)
Abstract-Samples of wheat and barley pre-dried or pre-wetted in the laboratory were conditioned to the same range of moisture contents as each other so that they were adsorbing or desorbing moisture respectively. Their moisture contents were determined with a capacitance and a resistance moisture meter and compared with results obtained with an oven-drying standard method. Abnormally high meter results were obtained for adsorbing samples one day after adding water, but results were not af%cted one day after drying. Otherwise, moisture content results changed little with time during the IO-week experiment.
INTRODUCTION
Electrical moisture meters measure electrical characteristics of the grain that are related to its moisture content (m.c.) rather than making a direct reading, thus they must be calibrated against some form of standard, usually an oven-drying method. Manufacturers usually calibrate their meters using grain obtained from farm stores and use it without further wetting or drying. Scientists sometimes need to measure the m.c. of grain samples that have been artificially wetted or dried in the laboratory. Past experience at this laboratory (unpublished results) has shown that recent wetting or drying may affect m.c. results obtained with meters, and the object of the experiment described here was to find out if this were so. The finding could also be relevant to field situations where grain has been wetted by rain or dried on farm driers. MATERIALS
AND METHODS
A capacitance and a resistance moisture meter were chosen for this study because they were meters used in the laboratory at the time. The results obtained in this investigation may not apply to other makes or models of moisture meter. The capacitance meter was a Sinar Agritec Moisture Computer and the resistance meter was a Protimeter Mini Mark IV. No discrimination is intended in favour of or against the makes of moisture meter used in this investigation. Grain
Tests were carried out using Flanders, a soft variety of wheat received at 14.9% m.c. and a sample of barley (unknown mixture of varieties) received at 14.8% m.c. Both lots of grain were about 1 year old when this experiment began. Conditioning
the grain
The m.c. of 5 kg of wheat and 5 kg of barley was increased to about 18% by the addition of distilied water followed by thorough mixing. Another 5 kg of each was air-dried at 30-35°C to about 9% m.c. Six hundred grams of each, neither wetted nor dried, were retained as control samples. The samples were sealed in glass jars and left to equilibrate at 10°C for 3 weeks with occasional mixing by manual shaking. Seven 600 g lots of the pre-wetted grain were then dried to approximately 11, 12, 13, 14, 15, 16 and 17% m.c. to give desorption samples and seven lots of pre-dried grain were wetted to similar m.c.‘s to give adsorption samples. This range of m.c. was chosen because most of the grain samples handled at this laboratory come within this range. After preparation, all samples were stored in airtight glass jars at room temperature. 151
SYLVIA HENDERSON
152
Moisture content determination The m.c. of most samples was measured 1 day after preparation but it took 2 days to dry the samples to 11% m.c., so these were not measured until the third day. Moisture contents were then measured 7, 9, 14, 17, 21, 28 days and 10 weeks after preparation. Samples at 11% m.c. were not measured with the Protimeter because it was not calibrated at this level. The m.c. of the control samples was measured on each occasion. Moisture content was measured with both meters in triplicate at room temperature, which averaged 24°C and ranged between 21 and 30°C. About 200 g of grain was removed from the jar for use with the Sinar and, as it was not ground, it was returned to the jar and the contents were mixed before removing the next 200 g replicate. For measurement with the Protimeter, three 20 g samples were ground using the manufacturers hand grinder. The m.c. of all samples was also measured in duplicate by an oven-drying standard, BS 4317, Part 3, 1987 (IS0 712, 1985). Both meters had been calibrated by their manufacturers using this method. RESULTS
AND
DISCUSSION
Both meters gave consistent results and triplicate determinations came within 0.6% m.c. of each other on all except two occasions when results differed by 0.7 and 0.8% m.c. Triplicate meter results were usually closer together in the middle of the range of m.c. than at the extremes. It is likely that some variation was due to the grain, since each determination was made on a separate sample from the jar. Results obtained with the meters were compared with the BS oven-drying method (Figs l-4). The m.c. measured by the BS oven method varied little with time, the means of duplicates obtained during the 10 weeks coming within 0.4% m.c. of each other. This variation was probably due partly to changes in the m.c. of the grain with time and partly to errors in determination. The
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l,,.0 ----70
DAYS
Fig. 1. The differences in moisture content between the BS oven-drying method and meter results for wheat, measured at intervals of up to 70 days after adding water. Moisture contents in the column on the right are the means of the oven-drying results.
Calibration of moisture meters SINAR +I** 0
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DAYS
Fig. 2. The differences in moisture content between the BS oven-drying method and meter results for wheat, measured at intervals of up to 70 days after drying. Moisture contents in the column on the right are the means of the oven-drying results. SINAR tI -I
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Fig. 3. The differences in moisture content between the BS oven-drying method and meter results for barley, measured at intervals of up to 70 days after adding water. Moisture contents in the column on the right are the means of the oven-drying results.
154
SYLVIA
SINAR
.
HENDERSON PROTIMETER
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DAYS
Fig. 4. The differences in moisture content between the BS oven-drying method and meter results for barley, measured at intervals of up to 70 days after drying. Moisture contents in the column on the right are the means of the oven-drying results.
mean of all oven-drying results for each sample was calculated and is given in Figs 14. It is obvious that the meter readings were consistently high for some samples and low for others. The main point of interest, however, is that the mean m.c. measured by the Sinar exceeded the oven-drying results by a greater amount one day after wetting the grain than during the rest of the experiment for 13 of the 14 samples (Figs 1 and 3). Similarly with the Protimeter, 7 out of 12 means were higher one day after wetting. After the first day, the differences between meter and oven-drying m.c’s remained almost constant with time, although the actual m.c.‘s changed slightly towards the end of the experiment, probably due to continually removing grain from the jars. Recent drying did not appear to affect the results (Figs 2 and 4). The mean m.c.‘s for the control samples determined by oven-drying method were 14.9% for wheat and 14.8% for barley. Moisture contents measured by the Sinar were between 0.3 and 0.7% m.c. high for the wheat control sample and were within f0.2% m.c. of the oven-drying results for the barley control. The Protimeter over-estimated the m.c. of the wheat control sample by between 0.0 and 0.7% m.c. and under-estimated by 0.2 to 1.3% m.c. for the barley control sample. These are similar to the results for the adsorption and desorption samples at about 15% m.c. Consistent differences between meter and oven-drying results are less important than trends with time, because the calibration of the meters can be adjusted. It was noticed that meter results were nearly always higher for samples adsorbing moisture than for those desorbing moisture, but differences were less than 0.5% m.c. This is generally of little practical importance, except in precise laboratory tests when, at this laboratory, m.c.‘s are always measured by an oven-drying method. The methods used in this work to add or remove water may have influenced the results obtained by either or both the meters and oven-drying. These methods are used as a routine at this laboratory because they are rapid and repeatable and in any case are representative of some of
Calibration of moisture meters
IS5
the ways on which grain may be wetted or dried in practice. For example, grain may be wetted by rain and it is often dried by flows of ambient air. This work was carried out on individual examples of a capacitance and a resistance meter. It is possible that different models and makes of meter may give different results. CONCLUSIONS The accuracy of both meters was reduced when measuring the m.c. of grain recently wetted in the laboratory. This must be taken into account when measuring the m.c. of grain recently wetted by rain because results are likely to be erroneously high. The Protimeter used in this investigation has an adjustable scale which lowers the reading by 0.5-l .O% m.c., when the meter is used for whole grains after rain. It is possible that such an adjustment should be made for ground grain, since this would generally improve the accuracy of the results reported here for newly wetted samples. Recent drying did not appear to affect the results. This investigation highlights the potential errors when measuring the m.c. of grain using electrical meters. It is advisable to check frequently the calibration of any moisture meter using an oven-drying standard and perferably using the type of sample and conditions most often encountered. Electrical moisture meters are invaluable for rapid checks on the m.c. of grain during drying or storage. However, it is unrealistic to expect them to provide the same accuracy as an oven-drying method of measuring m.c. although it should be noted that errors can occur during oven dete~inations (Henderson and Wilkin, 1985). During commercial transactions an appropriate margin should be allowed to accommodate measurement discrepancies and the potentially large errors associated with sampling. Acknowledgement-The
the manufacturers
author would like to thank Miss M. Gibbs, who determined most of the moisture contents and of the meters for their co-operation.
REFERENCE Henderson S. and Wilkin D. R. (1985) Measuring moisture content of wheat. Agritrade Ott 1985, pp. 1l-14.
S.P.R. 24,3-c