A Homemade, Heatable Gas Cell
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crucial or cot~linoaus.Harshaw Chemical Compa& markets a cell and power supply for approximately 8900, and the Perkin-Elmer Corporation h a s a combination for around $3300. Both units are specified toachieve 200 'C. I have constructed acell which operates over the same temperature range and which, while not the epitome of amvenience that crne would expect in a commercial cell, is functional, inexpensive, and easy t o construct, requiring only modest glassblowing and machine shop
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The eonstrurtim and assembly of the cell exclusive of the heating elements is indicated in the figure. Two glass O-ring joints are hlown together "hack-to-hack." I used 15 mm i.d. pieces (Corning No. 6560). which take a No. 116 O-ring. These are a l s , the appropritate size for 25 mm diameter windows. T h e windows are held in place an the O-rings by metal flanges, drilled thmugh Lhe renter for passageof radiation and recessedon the inner face to hold the windows securely in place. Three metal rods with threaded ends run the length of the eell and nuts on these rods are used t o tighten the flanges, which may he removed for window replacement or cell loading. Teflon O-rings seem t o be ideal, being unreactive, "on-volatile, and sufficiently elastic to allow a good seal without dangerous tightening of the flanges. (An attempt ta use harder O-rings resulted in the cracking ofseveml salt windows.) For lower temperatures, the more common Buna-N or Viton O-rings might be used without luhricatim. A metal valve is attached t o the cell, for evacuation or gas introduction, via a Kovsr-glass seal. For prolonged use, a packless valve is recommended. A Chromel-Alumel thermocouple is affixed t o the center of the cell body by fiberglass insulating tape. The main heating element is Niehrome wire wrapped with insulating tape (or it may he covered with ceramic heads) wrapped around the cell body and neck leading t o the valve. Over this are wrapped several layers of asbestos paper tape, which applies easily when dampened and forms a fairly sturdy cast on drying. I t is necessary to use an auxiliary heater for the ends of the cell and the neck t o prevent condensation of the vapor in these regions. A "Briskeat" heating tape, manufactured hy the Briseoe Mfg. Company, Columbus, Ohio 43216, is suitable. The major inconvenience of the. eell is the necessity of having it attached, when in use, t o two variaes and a millivolt potenliometer. While the temperature accuracy and control are inferior t o what is achieved in commercial devices, they are sufficient for many purposes. Tests have indicated accuracy o f f 5OC, and controllability of i2'C. University of Nevada, Las Vegas Las Vegss. NV 89154
Boyd L. Earl
Volume 57, Number 2 February 1980 / 105
