In the Oct. 87 Newsletter, Eddie Pearce gives details of components for a home-made electronic thermometer. Eddie offered to build me one at cost price. However, after reading about the Maplin Digital Thermostat by Area 20 member, Gordon Barrow, in Oliver`s local News, Eddie suggested that I get one of those as they work by battery, whereas his works off the mains and the price difference is marginal. Eddie enclosed an advert from Maplin`s catalogue. This is a colourful affair giving good quality photos and details of each type of digital thermometer in their range. I decided on the model, FD25C, at £9.95, as Gordon mentioned. The parcel arrived within a week. It came well packed inside a `jiffy` type postal bag. The instructions are good but printed very small.

The actual size of the thermometer is 64cms x 48cms but has a large digital LCD readout giving both C & F readings, at the flick of a switch. This can also be used as a 12 hour digital clock, by pressing a button on the front. It returns to temperature readings on releasing, therefore can`t be used as a permanent clock. But a very handy facility.

There is a switch for both inside and outside temperatures. Set at the 'IN' position you get the temperature of the room you are working in. Set at the 'OUT' position you can read temperatures of various things by means of a `probe` attached to a long extension lead, ideal for checking bottles, developing tanks and heated water baths etc. By being able to check both room and heating temperatures you will learn how well your heating unit sustains the working temperatures in Summer or Winter, with or without room heating being on.

Power is by a 'button' type A67 Battery as used in many digital watches and are easily available. It is held in a tiny compartment at the rear with a sliding cover. It comes with a plastic strip which must be removed before better connection can be made. There is no ON/OFF switch and this plastic strip can be used to break connection if you don`t use the thermometer all the time.

There are two methods of fixing or setting up the meter. A seperate plastic holder has a sticky back and this can be stuck to a permanent surface and the thermometer slipped on and off for use in other sights. There is also a pull-out stand at the back of the thermometer so that it can be stood on any flat surface. The probe also has a small metal clip attached and this can be screwed to a suitable surface, though you can still unplug the probe from this clip for further flexibility.

RUNNING TRIALS

I have two types of heating unit. One is the UNO Electronic Thermoemeter/Heater which you have read about and which we advertise in the Newsletter. The other is a home-made unit by Bob Lewis which has a mechanical method of switching on and off the power via a heat probe. Water circulation is by a discarded record player motor. This is an old unit now but still working perfectly. The UNO unit uses hs digital circuitry and probe similar to Maplin`s Thermometers.

I started off with the UNO heater as this has a slightly fidgitty temperature setting knob which sometimes leaves room for doubt using a mercury thermometer. The bath I set up is as for 300ml size kits. I use 13.

Bob's unit for 600ml or larger sizes, mainly for convenience as I am sure the 200wt UNO heater would also manage the larger sizes of bottles and tanks. It's just that I have one unit fitted to a standard washing up basin and the other to a larger one and I use them according to whether I process one film at a time or two or three in a multi tank.

I set up the UNO unit and attached the Maplin thermometer probe just submerged at the opposite end of the basin from the heating ellament, filled the basin with water to 300ml bottle hight with the water mixed from the hot and cold taps, then switched on the power and the 'fishtank' air pump that I use for water circulation. The following are the times and temperatures that followed:-

13.55 Start a dummy processing run as for 6½ mins development, giving TWO inversion agitations every 20 secs. It should be noted that I had a single film Paterson Sys. 4 tank standing in the bath water for 30mns with a weight to stop it floating (all it contained was an empty spiral). I checked the chemical temperatures at the end of 6½ mns. and found that it had dropped to 37C. As I don't usually check temperatures at this stage I don`t know if this is normal, but doubt if a drop of just one degree over the described time would make that much difference as all processing instructions give an allowance of plus/minus half a degree.

I have since started a fresh water bath and left for a full hour, as I would normally do and bath water reached 39.8C which I would expect as being ready for a processing run. My next effort will be to try the Maplin thermometer with the Larger Unit, But what this does seem to prove is that I have been doing things fairly well by allowing the water bath to reach 39C or so, to allow the bottles to stay at 38C (+/- half a degree). My old Mercury thermometer also came out well. But now I felt that I have the UNO heater set as close as I could possibly expect and I have no hesitation in recommending the Maplin FD25C as a very good buy.