Did you know that you can get an ITS-90 primary standard for less than $1,100? 

You should be frequently using this low-cost reference to make sure your thermometer standards are in tolerance. (Why Use a Triple Point of Water?)

The TPW is an intrinsic standard that does not need recalibration. It is the most accurate standard you can use, and you can learn to use it in an afternoon. 

The ITS-90 assigns the TPW a value of 0.01°C (273.16 K). Hart cells achieve this temperature with an expanded uncertainty of less than 0.0001°C. If you own a reference thermometer, you really should have a TPW cell to get the most out of your investment. 

If you calibrate secondary standards, the TPW is one point you can calibrate by primary fixed-point method rather than less accurate comparison methods. If you’re using a reference thermometer to comparison-calibrate industrial thermometers, you can make sure your reference isn’t drifting by frequently checking it at the triple point of water. 

Triple Point of Water CellsHart makes five triple point of water cells to suit your needs. A NVLAP-accredited cell intercomparison option is available for each one. Intercomparison includes comparing the equilibrium value of the cell against that of a Hart reference cell. 

Hart’s original 5901 cell is used by many national temperature labs around the world. It has a wide mouth for facilitating freezing of the mantle using crushed dry ice. The rubber foot lets you rest the cell on your ice bath or holding fixture for extra stability and protection while you’re using it. 

The Hart 5901A is a full-size cell designed after the original NBS cell, which had a glass support arm. While the arm does not impact performance in any way, some users prefer this design because it facilitates lifting and carrying the cell. The arm can be used as a hook for supporting the cell in an ice bath. The handle is also used as a McLeod gauge for a strictly qualitative check of trapped air in the cell. 

The Hart 5901B is a smaller cell that’s easy to handle, accommodates shorter sensors, and can be maintained in an automatic maintenance device. Despite this cell’s diminutive size, it is made with the same materials and technology used to make the larger cells. The 5901B has an expanded uncertainty of 0.0002°C and a well diameter of 8 mm. 

The Hart 5901C is designed like Hart’s original 5901 cell with the exception of the well size, which is 13.6 mm rather than the standard 12 mm. It’s a full 380 mm deep and has an expanded uncertainty of 0.0001°C. 

All these cells are cylindrical borosilicate glass filled with highly pure water. Hart’s newest TPW cell is the 5931. It uses a stainless steel shell so it’s virtually unbreakable. It can be easily realized and maintained in a bath or dry-well calibrator and provides uncertainty better than ±0.001°C.

A simple, time-saving method can be used with each Hart TPW cell for forming the ice necessary to generate the triple point. The smaller 5901B and 5931 models use the supercool-and-shake method, creating an ice slush, which remains stable all day. For the larger cells, the Model 2031 Immersion Freezer creates an ice mantle virtually automatically. Just fill the condensing reservoir of the 2031 with dry ice and alcohol, insert it into the cell, and return in under an hour to find a well-formed ice mantle. (A convenient method using LN2 is also available.)

We also have baths for maintaining your water triple point cells. Not only will your cells be ready to use when you need them, but they’re much safer if you maintain them in one of our constant-temperature baths.