No description available.

No description available.

No description available.

The development of magnox fuel cans at the Heston Laboratories of A.P.C. Ltd. required a non-contact method of measuring fuel can temperature during heat transfer tests. Infra-red techniques were examined and a two colour ratio pyrometer was selected as offering the greatest prospect of success. A design specification was compiled for an instrument having a lower temperature limit of 250°C with a target area of approximately 1mm². During heat transfer tests a pressurised rig is used and an observation window is necessary. This leads to some deterioration in performance which it is impracticable to remove by internal chopping and the pyrometer is designed for outside use. Operational wavebands around 1.8 and 2.3 microns are chosen and a lead sulphide infra-red detector operating at room temperature is selected. The properties of the detector lead to a design where the signal is chopped at 900 cps and modulated at 30 cps by the energy in each waveband. Conventional optical systems are considered and rejected in view of the large size of chopper wheel necessary. An alternative system is devised in which a commercially available pencil galvanometer is used to combine the function of chopping at both 900 cps and 30 cps. This enables an optical system to be designed which, because it employs reflecting optics, is compact and for the most part axially symmetric. The signal ration is extracted using an automatic gain control technique and a high signal to noise ratio is achieved with a phase sensitive rectifier at the output. The pulses which activate the galvanometer chopper and phase sensitive circuits are derived from a master oscillator operating at 900 cps. The performance of the system is assessed and it meets the specification in all respects except for a small surface geometry effect. The underlying cause for this is examined and a means for overcoming it is suggested. The performance when scanning a magnox can during bench tests is illustrated and it can be seen that although agreement to within 2°C is obtained with thermocouples placed at the fin roots, the geometry effect prevents the isolation of fin tip temperature. During the development of the instrument the possibility of wider application was an important consideration. Some of the applications which have arisen are discussed briefly and an indication of the performance of the pyrometer is given. Furthermore the system is capable of easy modification for use with a cooled detector and other regions of the infra-red spectrum.