A hand-held 'sniffer' that was supposed to beep when it detected explosives wasn't reliable and should have only been regarded as a deterrent, a British explosives expert testified at the Air India trial on Monday.

The so-called sniffer was used at Toronto's Lester B. Pearson International Airport on June 22, 1985, after an X-ray machine broke down.

Timothy Sheldon, manager of explosives at the Police Scientific Development Branch in England, said, that test trials showed that a range of substances tend to cause this type of instrument to alarm, or beep, such as medicines, antiseptics and even spices such as curry powder.

Any vapours would be difficult for the sniffer to detect through layers of material in a suitcase, Sheldon told Justice Ian Josephson, who is hearing the case without a jury.

"The (sniffer) was the least important of the instruments that we were testing because it was not expected to perform," Sheldon said. "We were more interested in the newer, more sophisticated instruments."

Sheldon is one of hundreds of witnesses to testify in the trial of Ripudaman Singh Malik, 56, and 53-year-old Ajaib Singh Bagri. They face eight charges, including first-degree murder, attempted murder and conspiracy in the bombing of Flight 182, which killed 329 people aboard.

Malik, a millionaire Vancouver businessman, and Bagri, a Kamloops sawmill worker, are also charged in the deaths of two baggage handlers at Tokyo's Narita airport after a suitcase bomb exploded 54 minutes earlier. Air India Flight 301 was the intended target.

A third man, Inderjit Singh Reyat, pleaded guilty earlier this year to manslaughter. He was sentenced to five years in prison on top of 15 years he had already spent in custody for his part in building the Narita bomb.