This Liquid Nitrogen Bath is a lower cost alternative to the ITL-M-18205. It is a simple apparatus, open to the atmosphere, composed of a stainless steel dewar flask filled with liquid nitrogen, a layer which houses a metallic equalizing block, and a thermometer holder. Lastly, a split, insulated lid reduces evaporation and permits easy addition of liquid nitrogen.
From time to time, extra liquid nitrogen must be added to the temperature calibration bath, approximately every 30 minutes, to keep the dewar flask full.
The liquid nitrogen dewar flask has a 100mm inside diameter, and is 280mm deep. The standard equalizing block within the temperature calibration bath houses four SPRTs or industrial thermometers up to 8mm in diameter, giving ±0.002°C temperature uniformity.
Method of Operation
A standard calibrated SPRT is placed in the equalizing block with the sensors to be calibrated. The whole is allowed equilibrate.
The level of the cooling bath is checked and nitrogen added as necessary, and readings taken 10 minutes afterwards.
The Isotech Simple Liquid Nitrogen Apparatus is safe to use, having no internal glass dewar flask to explode.
A comprehensive handbook accompanies the liquid nitrogen bath apparatus, which includes an article by Henry E. Sostmann on the corrections required to convert the calibration to the ITS-90 value of the Argon Triple Point.
The simple Liquid Nitrogen Apparatus, because there is air access, will slowly condense oxygen from the atmosphere, increasing the temperature of the Boiling Point.
This is of small importance, provided a calibrated SPRT is being used as the reference, and simultaneous ratios of SPRT and unknown thermometers are being recorded, with a bridge such a the Isotech microK.
Liquid nitrogen is not supplied with the apparatus.