The ultimate quest for any man - to measure his sexual potency - may soon be satisfied thanks to a technological breakthrough by aerospace engineers.

The researchers, based at Glasgow and Sheffield universities, have filed a patent for a machine based on wind tunnel research that can automatically quantify sperm's effectiveness.

'We can produce a reading from a sample in a few minutes, one that reveals just how potent the donor is likely to be,' said the project leader, Dr Richard Green of Glasgow University's department of aerospace engineering. 'Essentially, we have developed a new science - spermodynamics.'

The sperm counter has also been hailed by andrologists - doctors who study male fertility problems - because current techniques for assessing infertility are highly subjective and prone to considerable error rates.

'If you send samples of a man's sperm to seven different laboratories and ask them to test it, you are likely to get seven different sets of results,' said fertility expert Dr Allan Pacey of Sheffield University. 'That is what is so exciting about this device. It is completely consistent as well as being very quick.'

A man produces several hundred million sperm each time he has sex and can make more than 2,000 billion in his lifetime. However, on average, only two or three sperm result in pregnancies and, in some cases, there is no result at all. The tail, which should thrash around to drive an individual sperm, sometimes fails to function, for example.

One in seven couples suffers from fertility difficulties and in about 30 per cent of these cases the problem can be traced to the man. But establishing that the problem lies with his sperm can be tricky.

'We have to take three different measurements of a sperm sample to determine if it has flaws that could be leading to infertility,' said Pacey. 'First a doctor has to peer through a microscope and count the individual sperm in a sample to see if there are enough. Then he has to study the shape of the sperm to see if they are distorted. And finally, he has to measure how well they are moving. It is all very subjective.'

Such measurements can take days with highly variable results. By contrast, the spermodynamics counter takes only a few minutes and produces consistent results.

The idea for the Computer Aided Sperm Analyser (Casa) came to Green and his Glasgow aerospace engineering colleagues, Eric Gillies and Richard Cannon, while carrying out wind tunnel research. 'We use special techniques to measure airflow over surfaces, such as wing-flaps, in wind tunnels and were wondering how we could transfer this technology to other areas when we hit on the idea of using it to monitor sperm,' said Gillies.

To measure air flow, engineers fill wind tunnels with smoke particles. They then study the resulting flow by flashing laser pulses through the smoke to create photographic images. The particles on each picture are counted and their behaviour analysed by computer. This produces a measure of the flow of particles.

'All we have done is apply that technique in order to measure how a sperm sample is moving about,' said Green. Several laser pulses are flashed through a sperm sample, each individual sperm is measured and its movement automatically tracked. These data are then analysed by computer. 'In minutes you get a result, one that would take days by current techniques,' he added.

The team, backed by the Medical Research Council, have now built their first prototype sperm analyser and will spend the rest of the year testing its efficiency. Clinical trials will begin next year, it is hoped.

'The device is important because it means we can spot accurately and quickly if it is a woman or a man who is the source of an infertility problem and take action to help without them going through rounds of tests and inappropriate treatments,' said Green. 'In a sense, we are providing a man with a reading of his "vigourosity".

'In addition, the machine will be a major help for researchers studying the causes of male infertility, which are generally poorly understood. And there is considerable scope for its use in veterinary science.

'Breeding of dogs and cattle using artificial insemination is big business, and it is important that problems do not occur. Our machine should be just what is needed.'