The 7196B consists of a vacuum-sealed stainless steel Dewar Flask, a high-purity copper block, and a precision-fit lid. The dewar is filled with LN2 and the copper block is suspended in it; an SPRT is inserted into the block and a calibration is performed against your own calibrated SPRT. The 7196B-4 includes four 8-mm wells. The 7196B-13 includes five 8-mm wells and eight 6.35-mm wells. Low-cost calibrations to -196°C, uncertainty less than 2 mK.