Water, triple point

Fig. 2. A water triple-point ceU. A, handle B, water vapor C, Hquid vapor, D, soHd water (ice mande) and E, thermometer weU.

In use, a mantle of ice is frozen onto the outer surface of the thermometer weU. A common way to do this is to fiU the weU with cmshed dry ice until the mantle achieves a good thickness. Descriptions of the technique for doing this are given in several pubHcations and in manufacturers Hterature. The temperature of the water triple point is 0.01°C, or 273.16 K, by definition. In practice, that temperature can be realized in the ceU within 0.00015 K of the definition. In contrast, a bath of ice and water for producing the temperature 0°C is difficult to estabHsh with an accuracy better than 0.002°C. 

The cold-junction temperature can be fixed by immersing the cold junctions into some known thermal environment an ice bath or a properly maintained water triple-point cell. A temperature-controlled oven at a temperature above ambient may be used. 

ASTM El750-95. Standard Guide for Use of Water Triple Point Cells. American Society for Testing and Materials, 199.5. 

The I-Lw-V line that connects the quadruple point to the water triple point (I-Lw-Vw) (273.16 K, 0.62 kPa), denotes the transition between... 

The triple points for most substances do not occur under normal conditions. Take water as an example. The temperature of waters triple point is nothing unusual—just above the freezing point. But the pressure of waters triple point is extremely low— 0.006 atmospheres—a level that exists only in outer space. 

Equilibrium between the solid, liquid, and vapor phases of water (triple point of water) 273.16 0.01... 

Fixed-point calibration has the highest accuracy, but to achieve such accuracy requires great precautions and is very time-consuming. Most freezing- and triple-point cells are difficult to maintain and usually cannot accommodate more than one sensor at a time. Thus, fixed-point calibrations are usually carried out only for high-precision thermometers and usually only at national laboratories. However, easily maintained water triple-point (or ice-point) systems are used in many laboratories to correct thermometer drift, particularly for SPRTs. 

Calibration of the CTD temperature sensor to better than 2mK accuracy thus requires three units (i) a reference temperature bridge that is accurate to 1 mK at a resolution of at least 0.5 mK, (ii) a thermostat bath to obtain the two oceanic primary fixed-point temperatures with a water triple point cell and a gallium melting point cell to control the drift of the reference sensor and (iii) a temperature controlled bath that is stable to better than 0.5 mK while taking a calibration point with the reference thermometer and the immersed CTD. 

Figure 2.5 Cross-section of a water triple-point cell. The cell has cyUndrical symmetry. Pnre water of the same isotopic composition as H2O in ocean water is distilled into the cell. The air is pnmped out and the cell is sealed. A freezing mixture is placed in the inner well to canse a thin layer of ice to form next to the inner wall. The freezing mixtnre is removed, and some of the ice is allowed to melt to a film of very pure water between the ice and inner wall. The thermometer bulb is placed in the inner well as shown, together with ice water (not shown) for good thermal contact.

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