Secret Scotland

If it's secret, and in Scotland…

Mystery of the invisible leak finally solved

I’ve been puzzled by a persistent leak in my roof, which has gone on for some years with no obvious cause.

Background to this problem lies in my amateur radio days of old, when I operated a small TV transmit/receive group with some other amateurs. Our usual hop was Glasgow (east end of course) to Motherwell, with the occasional guest dropping in. Unlike today, where one can send and receive video around the World almost without trying (via the Internet), we had to use 70 cm transceivers, and steerable beam antennae. In my case, that meant a 3-metre long Yagi atop a pole that went through my roof, and was mounted on a rotator.

At the time, a temporary seal had been made to the roof using a horrible, black, non-setting mastic. As usual, this supposedly temporary seal worked so well, it was left in place for some years.

However, it did eventually harden, and the TV group broke up, so the Yagi came down and was replaced with a discone. The hardened mastic didn’t matter since the discone didn’t need to rotate… but in the fullness of time, it was withdrawn too, by which time the mastic had given up completely, and water ran down the pole whenever it rained, leading to some strange solutions.

After tidying away the pole, rather than restore the roof, I placed a clear plastic window over the hole – and it served as a window for a light switch I used for various lights which come on during the hours of darkness.

This was all fine, but… after some years of trouble-free operation (during which the original photoswitch was upgraded after it died), I found water kept dripping from the body of the photoswitch.

But no matter where I looked in the surrounding area, there was no visible water tracking towards it, or point where water could be seen coming in and reaching it.

I gave up, and just kept mopping up.

After a few more years passed, I had to move the photoswitch while working on some other wiring under the roof – and solved the mystery of the dripping sensor:

Roof plastic window leak

Roof plastic window leak

The original photoswitch had been a truly ancient device, and operated by means of a heating element, a light dependent resistor (LDR), and a bimetallic switch inside a heating coil.

When it was dark, the LDR was high resistance so little  current flowed, but when it was light, the resistance would fall, current would flow and the heater would heat the bimetallic switch, causing the contacts to change over – and turn of the lights.

What I didn’t realise was that over the years, the heat from this old switch had been enough to degrade the clear plastic on my little window, causing it to fail and split.

I never saw this because I had to work on toys such as this at night.

And during the day, when I could have seen the problem, the body of the photoswitch hid the evidence – but the rain came in the unseen hole, and dribbled off the bottom of the unit.

Just goes to show you should CHECK EVERYTHING!

I just assumed my window was intact and water could not be teleporting through.

It may not have been teleporting, but it was still coming through what I had assumed to be a piece of continuous plastic sheet.

WRONG!

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February 27, 2014 - Posted by | photography |

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