Thermocouple Measurements Below Zero

Looking at the specifications of a typical thermodynamic data mining system can be a bit confusing when defining the ability to measure temperature. For example, Figure 1 is a snapshot of our DI-245 model voltage, mV, and thermocouple measurement range for a thermocouple data recovery system, taken directly from the product datasheet. If you believe that subzero temperature measurements can be made using any type of thermostat as shown in the table, you can forgive it. One of the biggest secrets in the instrument world is that sub-zero temperature measurements require heaters specifically designed for this application.

The problem of measuring thermodynamics below zero

By “mineral” I mean any temperature measurement below 0 ° C. Generally thermodynamic thermocouple manufacturers are unable to supply alloys that operate above and below zero in the same heating cable. Additionally, since the vast majority of thermodynamic applications involve measurements greater than zero, this is where most thermodynamics are guaranteed to work.

 For this purpose, heat exchangers for well below zero applications must be specially ordered.

Therefore, while instruments are designed to take measurements below zero, thermodynamics is the weak link, especially if it is not specifically designed for such use. Unfortunately, the danger is that an unqualified heater gives less work than zero, but generates very large measurement errors. Similarly, a pair of heaters qualified for operation below zero may cause errors greater than zero. Therefore, if you are taking measurements above and below zero, make sure the thermocouple has a cryptographic procedure that distinguishes them from each other.

Thermal types for sub-zero operation

If you want to function at sub-zero temperatures, the application typically uses E- and T-certified heaters. Other types of heat exchangers have brittle properties such as brittleness, corrosion resistance or extremely low electromagnetic field generation at cryogenic temperatures.

Any reputable heating supplier should be able to provide price and availability of sub-zero certified heaters. If you need it, please let us know when placing your order to ensure accurate measurements using ours or someone else’s product.

Sensor construction

A coiled RTD wire consists of a coiled thin wire that is wrapped around a ceramic or glass part and then placed in a probe sheath for protection (see Figure 1). Thin film RTD sensors have a platinum coating on a ceramic substrate. Thin film RTDs are not as sensitive to vibration as wirewound RTDs, but are generally more limited in the appropriate temperature range.

Winding RTD sensors are typically made from alloys of copper, nickel, or platinum. Platinum is the most popular because it has better accuracy and works over a wider temperature range. The RTD can be taped or glued to a surface, mounted on the probe, or integrated into the device using threaded holes.

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