Why Use a Triple Point of Water?

Speaker: Mike Lawyer

Author: Ron Ainsworth

Fluke Calibration

799 East Utah Valley Drive, American Fork, Utah 84003 USA

E-mail:

Phone: +1 801 763 1700

Abstract

Have you heard that temperature primary standards are expensive and complex to use? There’s an exception to that - the Triple Point of Water Cell.

The use of TPW cells is more widely needed than you might think. PRT reference thermometers at any level of accuracy must be periodically checked to ensure performance and to maintain accuracy of temperature calibration systems. Why is using a TPW better than using some other methods of monitoring resistance of your reference standard? How can a simple calibration laboratory easily incorporate this method into their calibration systems?

  1. Introduction

There’s never been a less expensive, easier-to-use primary standard than the Triple Point of Water (TPW) Cell. Fluke cells achieve an expanded uncertainty of 0.0001°C or better and cost around $1,000. There’s not a better bargain in the temperature calibration business.

Fixed-points, sometimes called intrinsic standards, are used to define the International Temperature Scale of 1990 (ITS-90). The most commonly used fixed-point is the Triple Point of Water. A Triple Point of Water Cell is used to create athermal equilibrium between the three phases of pure water: liquid, solid and gas. This thermal equilibrium occurs at 273.16 Kelvin, or 0.01°C.

  1. Use the TPW to maintain your SPRT or PRT standard

If you use SPRTs, the TriplePoint of Water Cell is your most importantfixed-point because ITS-90specifies the interpolation equationsof an SPRT based on its resistanceratio stated as

W(t)=R(t)/R(0.01°C)(1), thus requiringyou to measure the resistance ofyour SPRT at the triple Point ofWater frequently. Directly measuringR(0.01°C) in a Triple Point ofWater Cellreduces most of the errorcaused from the instability of anSPRT.

PRT reference standards can also benefit from frequent resistance measurements at the TPW. In either case, recording this resistance value in a control chart will give the user a history showing the performance of the reference standard, proving its stability and continued in-tolerance status, or showing that it is drifting and in need of calibration.

  1. The Triple Point of Water Cell

Fluke’s TPW Cell is a cylindrical borosilicate glass (or quartz) container filled with water and having a re-entrant tube serving as a thermometer well. The water in the cell is high purity and gas free and has the isotopic composition of seawater. The equilibrium temperature is measured at the liquid-solid interface surrounding the thermometer.

The “triple point” is formed by freezing a portion of the water with dry ice or liquid nitrogen being placed in the re-entrant well. Ice is formed around the re-entrant well about 8mm thick. The TPW is then placed in an environment maintained at around the TPW temperature to keep the ice frozen and the TPW temperature realized for an extended period of time. The realized TPW temperature plateau is at 0.01°C.

  1. Ice Bath alternative

Triple Point of Water Cells are not the only means of achieving a near 0°C reference temperature. Ice baths are an alternative, and if done properly, produce a temperature of 0.0°C.

Ice baths are made with chopped-up ice in a dewar flask. The flask should be at least 36 cm deep and 8 cm in diameter. Shaved ice and distilled water are added to the flask and allowed to reach a constant temperature.

To reduce vertical or radial temperaturegradients, distilled watermust fill all of the crevices betweenthe ice chips. Ice chips should be 2to 5 mm in diameter and must befree of contaminants(2).

As shown from these steps, icebaths are relatively easy to makeand inexpensive. Ice baths, however,have several limitations, includinggradients throughout thebath, potential purity problems, repeatabilityissues and variances inindividual construction and measurement

techniques.

The difficulty of using ice bathsled to the inclusion of the TriplePoint of Water Cell in ITS-90. TPWCells address many of the problemsof ice baths. With the high-puritywater in a sealed cell, contaminationworries are eliminated. Thecell operates free of atmosphericpressure and forces proper operatortechnique for thermal equilibriumto occur.

  1. Delicate, but reliable instruments

Cells are delicate and must be handled with care. However, they are inexpensive and give confidencein measurement accuracy and repeatability. Newer cell designs make TPWs more convenient with wider openingre-entrant wells and rubbershock absorbers on the base make.

Newer quartz-case versions of the TPW extend the optimum performance life of the cell, making it more difficult for impurities to migrate through the glass into the purified water.

  1. Conclusion

The triple point of water cell is necessary for maintaining PRT or SPRT calibration reference standards. With relative low cost yet being extremely repeatable and accurate, the TPW is better still than the traditional ice point for providing a reference temperature source to record resistance valuesof temperature standards.

  1. References
  1. The Internet Resource of the International Temperature Scale of 1990 —
  1. ASTM Standard E563 - 11 “Standard Practice for Preparation and Use of an Ice-Point Bath as a Reference Temperature,”ASTM International, West Conshohocken, PA, 2011, DOI: 10.1520/E0563-11,