An Alternative Method for Manufacturing Water Triple-Point Cells

Water triple-point (WTP) cells are the most important standards in laboratories that calibrate standard platinum resistance thermometers (SPRT). Over the years, different methods for manufacturing WTP cells have been proposed; some quite simple, others more complex. This paper describes a straightforward method for manufacturing WTP cells. The method is based on the fact that the pressure present when water is boiling is its vapor pressure, and because of that the amount of residual dissolved gases, if any, is very small. This makes the use of vacuum pumps unnecessary. The bulb of the WTP cell is filled with pure water produced by multiple distillations or by deionizer systems. The water is then boiled inside the bulb by a furnace built specifically for this purpose. After boiling the water for at least 30 min, the furnace is switched off, and a valve connected to the filling port of the cell is closed, in order to ensure that only water vapor and liquid water are present inside the WTP cell. The cell is then removed from the furnace and sealed when its temperature decreases. The paper also describes the procedure used to evaluate the WTP cells manufactured by this method, through comparisons against other reference standards, manufactured by more traditional approaches. The results show that the temperatures of the WTP cells manufactured by this new method have similar values to the ones obtained in WTP cells manufactured by traditional methods.