Similarly in the case of dupont equipment the slope

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Unformatted text preview: o f f o f t he l i q u i d n itrogen. Refilling during a r un disrupted t he s t a b i l i t y o f t he e quipment a n d d isturbed the measurements. T h e s mall l i q u i d n i t r o g e n v o l u m e ( 5 0 - 75 c c . ) t hat t he o ovling a ccessory w as d esigned f or w ould c ompletely evaporate in l e s s t han 10 m in. w hile t he time required to cool t he a p p a r a t u s to t emperaturtes below -100 C is g reater t han 10 m in. A l arger d ewar ( F i g . 19) c oupled to a c opper block using a g lass-to-metal s e a l w as b u i l t to e x p a n d t he l i q u i d n itrogen capacity o f t he D upont c oollng a c c e s s o r y . T he d ewar w as b u i l t to h o l d a pproximately o ne l itre o f l i q u i d n itrogen which would last 4 5 - 90 m inutes depending on the t ype o f o peration f cooling/heating) a n d t emperature ranges o f t h e s cans. T he t o p of t he d ewar h ad a s tyrofoam l id t o s l o w d own t he evaporation o f t he l iquid nitrogen. I I T h e d esign o f t he c ooling apparatus is s uch t h a t t he o utside surfaces o f t he cooling accessory a n d DSC c ell are exposed t o t he atmosphere resulting in t he c ondensation of w ater v a p o r . S ubsequent operation from -40 C to 20 C c a u s e s w ater t o a ccumulate a t t he outside b ase o f the D SC c e l l . T he b a s e o f t he DSC c ell is n ot w ater p r o o f ( as m anufactured) a nd , t he e l e c t r i c a l a nd t hermal insulation i n s i d e t he c e l l a b s o r b s I t he condensed water at the b a s e o f t he c e l l u ltimately shorting t h e h eater w ire c onnections w ithin t he c ell a nd P age -40a- Figure 19 Differential Scanning calorimeter cell. D IFFERENTIAL SCANNING CALORIMETER CELL E vacuated GlassDewar \ Copper Silver Lid H eater Aluminum Block Glass Jar Sample Reference Thermal Radiation Shield Fairing 0-Ring Cell Base P age - 4 1 - d estroying many of the electronic circuits within the DSC controller. The DSC cell was partially dismantled and completely sealed with a s ilicone compound around the outer stainless steel shell of the DSC c e l l . This protected the inner...
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