The tribunal has been told it would be "practically impossible" to grind large quantities of fertiliser using coffee-grinders to prepare home-made explosives.
Supt Kevin Lennon told the tribunal that the Garda Technical Bureau had conducted experiments to grind fertiliser using coffee-grinders.
The tribunal is examining allegations by Ms Adrienne McGlinchey that, together with suspended Det Garda Noel McMahon and Supt Kevin Lennon, she mixed explosives that were later used in bogus Garda finds of terrorist arms.Both men have denied those claims, and Ms McGlinchey denies she was an informer.
The Technical Bureau office produced 2.45 kg or 5.5 lb of ground fertiliser in 20 minutes before the coffee-grinder broke, Supt Lennon said. It would therefore take 22 grinders to produce 1 cwt of ground fertiliser, he calculated. The Technical Bureau also found that by allowing the grinder to cool for 15 minutes after 10 minutes of grinding, after four grinds the grinder was still functional, producing an average of 0.92 kg, or half a pound.
Supt Lennon calculated that based on this figure, it would take 93.5 hours to produce 1 cwt of ground fertiliser. He asked would it be "practically impossible to do with coffee-grinders"
"I would agree with that, my lord," said Mr Aidan Murray, a retired detective sergeant who was giving evidence about explosives finds in south Donegal in 1993 and 1994.
Mr Murray served in Ballyshannon, and was the scene of crimes officer involved in a find of "a quarter ton" of explosive mix (ground fertiliser/icing sugar) near Donegal town, and a find including 800 lb of fertiliser mix near Rossnowlagh.
The Rossnowlagh find was discovered on July 18th, 1994, close to where a large Orange parade takes place every July. The Donegal town find, in the townland of Ardchicken, took place eight months earlier, on November, 18th 1993.
The retired garda said his information from his usual sources in south Donegal (in Ballyshannon and Bundoran) was that this find was not part of a south Donegal IRA cache.
The finds were unusual, in that he would expect to find Armalites and machine-gun ammunition, not .22 bullets. This was the only occasion on which .22 bullets had been part of a find in south Donegal, he said.