37: A makeshift oven to rate chronometers.

25 01 2020

On page 95 of “The Mariner’s Chronometer” I mentioned briefly that the amateur could rate a chronometer at various warm temperatures using a makeshift heating oven and use a domestic refrigerator for low temperatures. At the moment, my area of New Zealand is enduring a drought combined with something of a heat wave, with temperatures reaching 31 degrees (88° F) in the last few days. This coincided with my wish to rate a Mercer chronometer dated to 1907 at various temperatures, as its balance rim  is fitted with a device designed to compensate for middle temperature error. I will be writing about it at more length in my next post.

In the last few weeks, night time temperatures have never fallen below 18 degrees (64° F) and maximum day time temperatures indoors have often exceeded 25 degrees (77° F), so I determined to make an oven that could not only maintain high temperatures (which is relatively easy) but also maintain temperatures below the ambient temperature (which is not).

An old mechanical room thermostat  was not equal to the task as its response was very slow and it could not maintain temperature to closer than four or five degrees, so it found its way out of the scrap treasure chest into the waste bin. While waiting for a modern electronic replacement and  Peltier device to arrive, I gathered together other bits of treasure hoarded over the years and assembled them into the untidy lash-up shown below in Figure 1.

Oven 001

Figure 1: Power supplies and control unit.

The enclosure is simply a polystyrene box that once protected an antique sextant on its way across the world to me and I was very happy to re-purpose it and keep it from polluting the environment, at least for a time. A small motor salvaged from a photocopier drives a circulating fan inside the enclosure and is powered from a small 12 volt transformer and a rectifier bridge made up of four large silicone diodes that are rated very far in excess of what is needed. A rectifier converts alternating current to direct current, which some motors and most electronic devices feed on.

A large salvaged 12 volt transformer feeds through another bridge rectifier to the Peltier device. This is a bunch of n-p junctions that form a large thermocouple working in reverse. When voltage is fed in, one side of the junctions becomes hot and the other side cools down, provided the heat from the hot side is conducted away into a large heat sink which is force-cooled by a powerful mains-powered fan. If the fan should fail, the hot side becomes very hot indeed, with the cold side not very far behind, so it is protected from such a fate, by a thermal switch applied to the  heat sink.

The control unit or thermostat is an STC-1000 digital device bought from e-bay for a relatively small sum. It is powered by 240 volts a.c., has a wide temperature range, can handle 10 amps d.c and can maintain temperatures within a single degree. The instructions that came with it have a “Chinglish” flavour and are hard to follow, but the device itself does all that is claimed for it.

The Peltier device is sandwiched between two heat sinks, and, since the interior side gets cold, I have called it a cold sink (Figure 2). A small circulating fan keeps the air moving over the sink and around inside the enclosure. Both fans run all the time. If the temperature exceeds the set temperature by half a degree, the control unit switches on power to the Peltier device, which operates until the set temperature is reached, when it is switched off. If the temperature falls by half a degree, the Peltier device is switched off, the light bulb is switched on and  the temperature rises to the set temperature, when it is switched off . Thus the temperature inside the enclosure is held to the set temperature ±½ a degree and the average temperature is the set temperature.

Oven 002

Figure 2: Interior of enclosure.

The owner of the chronometer has kindly agreed that I may keep it to study its response to different temperatures, so I will do so for a week each at 30, 25, 20, 15, 10 and 5 degrees. I will report back when I cover the overhaul of the chronometer itself.


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