[rcheshire (Rowland)]

Well, I've finally got this working.

Using a spare 1000D sensor, I've fitted a nichrome element in the recess reserved for the AA/UV/IR filter - resistance = 7R7 as it happens.

Testing - I started by cooling without the dew heater running and predictably the sensor face fogged up badly.

Powered up the dew heater, starting at 1V, incrementing by 0.5V to 5V. Dew began to clear at 3.5 - 4V, but not completely. At 5V no dew.

The heater element needs to be aligned with the inside edge of the plastic frame, as shown to be effective. Made a small adjustment on one side to move element closer to edge, which had an immediate effect clearing that side of dew.

Second test - started cooling with dew heater 5V - no sign of dew. To be expected. At 5V the element is working overtime to clear an accumulation... etc etc

Third test - started cooling with dew heater at 4V - no sign of dew. Speaks for itself really.

Given test observations and the high thermal resistances involved because of the use of plastics, condensation prevention seems to be due to radiant heat and to a limited extent, conduction. The heater was only successful when an edge of the element had line-of-sight to the sensor face.

No doubt there is a trade off with cooling, but not equivalent to heater power. Yet to do a temperature test.

The best way to install the element is to attach it to a very thin, rigid plastic frame, in place of the AA filter. The element hangs below the frame and is removable. If the AA filter is retained stick the element to that. But you will most likely need to remove the piezo device to get things to fit. Wiring is very tight.

To fabricate the element, which is 3mm wide and 1mm deep, wind nichrome around a 2mm drill shank - tight - no gaps in the coil. Drop the coil into a bench vice - again, keep the coil tight and support in the vice with a 1mm straight edge. Tighten the vice to clamp the straight edge. When released from the vice the element will twist. This all but dissapears when stretched. Depending on requirements, stretch to say, 0.5mm or 1mm spacing, depending on sensor size and target resistance.

I have aimed to keep voltages low. It might be necessary to strip the supply leads back far enough, to clear the sensor assembly - insulate with heat shrink to keep as thin as possible.

Nichrome is difficult to solder. I run a fine grit over the ends, then wind onto the ends of the supply lead and solder- minimal.

144 turns of 0.32mm nichrome = 12R - element width = roughly circumference of drill/2 and make an allowance for wire gauge.