Saturday, February 23, 2019

about curved notched plastic

I wanted to build a custom dew heater strap. Well, I've wanted to do that for a long time...

In 2006, I started collecting (harvesting?) large quantities of nichrome wire. I even built a dual-element accessory for my finder scope. But I shelved that after a couple of runs.

With the recent failure of the Kendrick 2-inch eyepiece strap, these thoughts were rekindled.

I found inspiration in Eteny's DIY dew heater DIY project page where he made simple heating elements by wrapping nichrome wire around a plastic sheet.

I liked his approach with the plastic core but I was worried about how flexible that might be. Would it coil around a small eyepiece? Sure, it depended on the thickness of the material. And then, it hit me. Start with curved plastic! As Rhonda and I discussed the matter, she handed me a pale blue transparent plastic bottle, one that held her favourite bubbly water. Look at that!

Again, I liked his approach with the wire wrapped around the plastic core as I saw that very little or no stress would be transferred to the wire element itself as the strap was opened or wound around something. Clever. But I didn't like how the wires could wander, possibly shorting. He taped down the wire but I wanted a better solution. In a moment of clarity, I saw that if notches were made along the side of the plastic strip, the wire could be held in place, and with a bit of tension, unable to wander.

heating element design in zigzag pattern

I made a scaled diagram in Visio considering regular notches in the plastic strip and then I counted the vertical and angled runs. Total length about 85 centimetres.

Later, I measured the bottle diameter and tallied everything in an Excel sheet. Given the physical length of wire, I would just need to find an appropriate resistance from the nichrome supply.

This was exciting!

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