Laser chillers and Dew Point

The English weather, although often talked about, is never extreme. It’s almost unheard of for hot weather to be a problem here in Cambridge for example.

If you run a laser which is cooled by a recirculating water chiller, you’ll probably have the operating temperature set to around 20 C. For 99% of the time in England anyway, this will be a long way above the local “Dew point”.

In simple terms the Dew point is the temperature at which water vapour in the air will condense into liquid. If that happens on a cooled laser lens or cooled laser mirror, then the water film will absorb the laser beam, and boil off.

That process will destroy the coating on the mirror or lens. Condensed water films can also damage water cooled components in electrical equipment such as the power supply as well.

When the damaged optic is investigated, all that remains is the damaged coating; the water layer has long gone.

For much of the year the Dew point near Cambridge, England averages around 13 C, but the last few days this summer has seen the Dew point as high as 18 C.

The water chiller operating temperature should be 2-3 C above the Dew point, but remain within the laser manufacturer’s limits.

Synrad for example recommend a coolant set point of 18 – 22 C. Just recently though 18 C would have been too low, and at times I’m sure below the dew point.  

In other parts of the world I’m sure this is not news at all, but it has caught out at least one laser user in the UK.   

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