Hart Quartz-Sheath SPRT

	*Quartz-Sheath SPRT, Standard Platinum Resistance Thermometer Models 5681, 5683, 5684 and 5685*
	*Hart Scientific Calibration Products > Primary Temperature Standards >*
	*Quartz-Sheath Models 5681 to 5685 > Specifications > Ordering Information >*

Quartz-Sheath SPRT, Standard Platinum Resistance Thermometer

Description
Choosing the right platinum thermometer as your primary standard may be the most critical purchase decision in your lab. Unfortunately, other manufacturers are pretty secretive about how their SPRTs are made. They won't tell you much more than you can already see by looking at one. Many of the leaders of a few decades ago have lost their original craftsmen and design scientists. Hart Scientific has one of only a few active SPRT design groups in the world today.

So how do you know you're making the best purchase? Self-proclaimed expertise shouldn't convince you. You should expect some sound evidence that the company is qualified in the ongoing science of SPRT development. At Hart they will let you talk to the people who design, build, and calibrate SPRTs. Finally, when you buy one, if you don't like it, Hart Scientific will take it back and return your money.

Quartz-Sheath SPRT, Standard Platinum Resistance Thermometer

Features

Drift rates as low as 0.0005 K.
Proprietary gas mixture ensures high stability.
Most experienced SPRT design team in the business.

Hart has four quartz-sheath SPRTs, covering the ITS-90 range of -200°C to 1070°C.

The Model 5681 is used from -200°C to the Aluminium point at 660.323°C.
Model 5683 is used from -200°C to 480°C with greater long term stability.
The 5684 and 5685 cover higher temperatures up to 1070°C and can be calibrated at the silver point.

Model 5681: -200°C to 670°C This 25-ohm thermometer is the work-horse of the ITS-90 ranges. It can be calibrated for any sub-ranges from the triple point of argon to the freezing point of aluminum. The 5681 meets the ITS-90 requirements for resistance ratios as:	W(302.9146 K) ≥ 1.11807 and W(234.3156 K) ≤ 0.844235
Model 5683: -200°C to 670°C While SPRTs traditionally cover temperatures to the aluminum point (600°C), most measurements occur between -100°C and 420°C. The model 5683 SPRT covers this range and more, from -200°C and 480°C, and does so with long-term stabilities that extended range SPRTs can't match.	Typical drift is less than 0.5 mK after 100 hours at 480°C
Models 5684 and 5685: 0°C to 1070°C ITS-90 extended the use of the platinum thermometer from 630°C to 962°C. The 0.25-ohm HTRP sensor uses a strip-shaped support made from high-purity quartz glass. The 2.5-ohm model uses a quartz glass cross frame. Stability after thermal cycling is excellent, and the design is reasonably tolerant of vibration. Choose from 0.25-ohm or 2.5-ohm nominal R_TPW values. In addition to meeting the resistance ratio requirements shown, above, these thermometers meet the following additional criterion:	W(1234.93 K) ≥ 4.2844

These glass probes really are a notch above the rest!

World Class SPRTs
Yes, they have all the features you would expect in a world-class SPRT. They have gold-plated spade lugs, a strain-relieved four-wire cable, convection prevention disks, the finest quartz glass available, delustered stems, and the purest platinum wire available.

The purity of a thermometer's platinum wire is critical to meeting ITS-90 requirements. Platinum resistance is measured by the resistance ratio "W" at specified ITS-90 fixed points. Maintaining that parity over the life of the thermometer impacts long-term stability. The quartz glass tube of the SPRT should be properly sealed to prevent contamination of the platinum wire. Other manufacturers use mechanical assemblies and epoxy seals. These introduce additional materials to the thermometer's internal environment and can be prone to mechanical failure, risking exposure of the platinum to impurities.

Theoretically, the best seal would be a direct seal between the quartz glass and the platinum wire. However, the quartz glass used in thermometer sheaths has a very small coefficient of expansion while platinum has a much larger coefficient of expansion. If you simply sealed the sheath's glass to the platinum wire, these different rates of expansion would result in a poor seal as the assembly is exposed to changing temperatures.

The Hart scientific design group figured out a way to match the expansion coefficients of the glass sheath and the platinum wires. They did it by creating a graduating seal that's made of 18 separate pieces of glass, each with a different coefficient of expansion. The expansion and contraction rate of the innermost piece of glass matches that of the platinum, resulting in an overall seal that prevents gas leakage and impurity penetration for at least 20 years. Fusing each piece of glass to the next is a painstaking process. All these things cost extra but the results are well worth it.

There's More!
Hart Scientific use only pure quartz glass materials for the cross frames, disks, and tubes. They don't use mica or ceramic materials. Additionally, they have a special glass-treating process to increase the resistance of the quartz to devitrification and remove more impurities than the typical cleaning process.

Hart did some more research to find the best-performing balance of argon to oxygen in the tube. Some oxygen in the sheath is necessary to minimize the danger of platinum being poisoned by foreign metals at high temperatures, but too much oxygen at temperatures below 500°C accelerates the oxidation process affecting the integrity of the platinum. They got a balance that provides exactly the right protection for the platinum. Each of these seemingly small things adds up to a better uncertainties and less drift.