Ventilation instrument helps cavers

Testo offers an affordable and accurate digital draught meter that is ideally suited for use underground.

Picture this scene: a promising passage ends at a collapse and two cavers are wondering if it might be worth trying to excavate a route through the boulders.

"Is it worth pushing? It looks hopeful.

We just need to follow the draught".

Without a draught, an underground dig loses direction and somewhere beneath the hills, deep inside a boulder choke, everyone gives up.

As long as there is a draught to follow, digging teams stay enthusiastic, sometimes committing years of effort to making a breakthrough.

Given all that is at stake, a non-caver would assume that detecting and measuring draughts in caves would be an exact science by now.

In reality, cavers use anything from candles and joss-sticks to the dust off their clothes to detect draughts, and hardly anyone has attempted to measure them.

Fortunately, Testo offers an affordable and accurate digital draught meter that is ideally suited for use underground.

Not surprisingly, this didn't come about because Testo took pity on cavers and their fruitless heroics in boulder chokes, but was designed particularly for monitoring ventilation ducts.

Rob Davies, president of the Gower Caving Group is very impressed: "I believe that the Testo 405 is an indispensable tool for anyone engaged in cave exploration.

Hopefully its use will become widespread and in future, instead of reporting prospects with 'good draughts', cavers will be able to quantify them in terms of metres per second or, better still, cubic metres per second".

The Testo 405 is very reasonably priced at GBP 72 and includes a 300mm flexible handle, easy-to-read swivel display, duct holder and multifunction clip for quick positioning.

Now it is also among a range of measuring instruments available from Testo on a 10-day trial offer, with no obligation to purchase, making it an even better proposition for anyone measuring airflow.

The Testo 405 is highly sensitive, with a resolution of 0.01m/s.

To put this in perspective, people can just about feel a draught if it blows at 0.25m/s.

The Testo 405 achieves its precision with a tiny hot-wire anemometer housed in the tip of a 300mm-long probe.

It works by heating a tungsten/platinum probe to above-air-temperature.

Using sophisticated circuitry, the instrument maintains the probe temperature at a constant level throughout the measurement.

The higher the velocity, the faster the rate of cooling, the more voltage has to be supplied to maintain the temperature of the probe.

In addition to measuring velocity, there is also a built-in volumetric calculator that converts velocities to volume rates.

This is useful because the best measure of a draught is really the rate at which a volume of air is moved; subtle draughts that blow in large passages only become strong where they are forced through constrictions.

Apart from measuring air movement, the Testo 405 doubles as a digital thermometer with 0.1C precision - a useful accessory for verifying that an outward draught at the surface is indeed at typical cave temperature and not some local wind effect.

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