Monday, 5 October 2015

Fixing a ferric chloride/aquarium heater - bodge warning!

We've had our aquarium heater for a while now. It's great for heating a vat of ferric chloride, prior to etching (there's a world of difference between etching "hot" and etching cold!). But every now and again it goes a bit wonky. Then, today, it stopped working altogether.

The heater itself is actually very simple. It's just a coil inside a large glass test-tube-shaped, um, tube. When in use, it sits inside the glass with a rubber bung in the top, to make it waterproof. It has a crude temperature controller on the top, which is a dial to set the temperature anywhere between about 45 and 70 degrees.

As we've had this thing for a while, it's unsurprising that the bi-metallic strip that makes up the temperature controller occasionally sticks, keeping the two contacts apart - and effectively stopping the heater from working.

We've actually repaired this particular heater before, and, at the time, jammed some extra padding in to keep the bi-metallic switch closed for longer. It looks like it's just about had enough, as the actual body of the heater is now so warped that the switch no longer closes properly.

No worries - we simply removed the bi-metallic strip and replaced it with a bit of wire, connecting the two points (the purple wire in the photo, above, is the "fix"). Now there's no temperature control at all - plug it in, and it just burns away, getting hotter and hotter!

This is a real nasty bodge. But it works. So we no longer have to wait hours and hours for a board to etch, cold. In fact, this afternoon I made a brew while the ferric was heating, answered a phone call, had a few biscuits, answered the door to the postie (yes, they only call in the afternoons, here in Hove!) and forgot that the ferric was still being heated, by an uncontrollable heating element.

Suffice to say that the ferric was rather warm when I returned to it. But it fully etched a 5" x 3.5" board in less than a minute! I'm still not sure whether this is a great hack, or a disaster just waiting to happen.