With increasing interest I have read your abstracts of the application of methyl bromide (see World Grain, April 1999, September 1999 and February 2001).
Referring to your list of possible alternatives to the use of methyl bromide, you have also mentioned the use of chilled air. According to your report, the expenses are 100 to 270 times higher than applying methyl bromide. Could you explain how these extremely high expenses in cooling arise?
The power consumption depends, among others, of the ambient conditions (°C / % r.h.). In moderate conditions it takes about 100 kWh to cool 100 cubic feed grain; in tropical conditions up to 200 kWh are necessary. Assuming energy costs of U.S.$10 per kWh, the result is a factor of only .8 up to 5.0 compared with the application of methyl bromide.
Worldwide, for over 30 years, chilling is being applied as an economical and natural way of conservation during grain storage with reduction of insect development and without environmental damage. The application of methyl bromide as well as heat can only be realized in empty buildings. In many countries, there are some mills or bakeries that carry out heat treatment with great success.
Sulzer-Escher Wyss GmbH, Lindau, Germany
Editor’s reply: An article in the February 2001 issue of World Grain that compared the cost of cold treatment with methyl bromide, stated, "This method (cold) takes longer and is more expensive than methyl bromide because temperatures must be held cool for a month." The cost difference between the two methods was stated to be "Cold: $1,200/1000 cubic feet per month; methyl bromide: $4.34-$13/1000 cubic feet."
The information was taken from the Fall 2000 Fumigants and Pheromones newsletter, published by Fumigation Service and Supply, Westfield, Indiana, U.S. David Mueller, FSS president, said he based the figures for the cold treatment cost comparison on a study conducted by Purdue University that examined the use of cold air to treat silos.
I was quite interested in the article, "Level systems: How accurate are they?" (see World Grain, July 2001). It is very timely for us, given our current effort to develop an automated, "real time" bin volume measurement process. We have tested ultrasonic and plumb-bob type bin level measurement devices.
As your article effectively points out, the "angle of repose" for grain incorporates an automatic margin of error to grain volume measurement. We have determined that, given the angle of repose for grain, locating the measuring device approximately 79% from the center of a round bin will minimize the margin of error. In other words, the measurement location will result in approximately the same volume determination whether the grain is coned up (from filling) or coned down (from emptying).
However, the most important point to have an accurate measurement of the level of grain in a bin is when the bin is near empty. For example, the lower hopper of our 25-foot diameter steel bins contains approximately 1,000 bushels of grain. This "margin of error" is too large for a 15,000 bushel bin, which means that the best place to locate a measuring device is as close to the center as possible.
It appears that the only solution to maintaining some degree of accuracy with bin level measurement devices is to install two of them in each bin — an expensive proposition. The article suggested that the best method for determining weight is to measure weight. Can the authors recommend a cost effective weigh scale device that can be installed permanently under a 15,000 bushel bin? We will need 30 of them.
L.M. Prefontaine, CMA
Prairie Pulse Inc., Vanscoy, Saskatchewan, Canada