Before getting started one should have an understanding of how to accurately measure cryogenic temperatures within a budget. After all, how else will one know when the critical temperatures are achieved. I went through several probes and devices before settling on one that is fairly accurate.
Cryogenic temperatures require special thermometers. The probes need to be accurate and withstand the extreme cold. K-type probes work at a wide range of temperatures and are not expensive. However, they become less accurate below -200F. I found that a K-type probe only registered -282F when submersed in liquid nitrogen. The R-type probe is more accurate for cryogenic temperatures. It is also more expensive. This probe registered a temperature of -310F when I tested it in a nitrogen bath. The R and K type probes are thermocouples. A thermocouple consists of two dissimilar conductors in contact, which produce a voltage when heated. The size of the voltage is dependent on the difference of temperature of the junction to other parts of the circuit. In contrast to most other methods of temperature measurement, thermocouples are self powered and require no external form of excitation. The main limitation with thermocouples is accuracy; system errors of less than one degree Celsius (°C) can be difficult to achieve. I finally settled on using an RTD probe. Resistance thermometers, also called resistance temperature detectors (RTDs), are sensors used to measure temperature by correlating the resistance of the RTD element with temperature. Most RTD elements consist of a length of fine coiled wire wrapped around a ceramic or glass core. The element is usually quite fragile, so it is often placed inside a sheathed probe to protect it. The RTD element is made from a pure material, platinum, nickel or copper. The material has a predictable change in resistance as the temperature changes; it is this predictable change that is used to determine temperature. Platinum is the best metal for RTDs because it follows a very linear resistance-temperature relationship and it follows the R vs T relationship in a highly repeatable manner over a wide temperature range. The unique properties of platinum make it the material of choice for temperature standards over the range of -272.5 °C to 961.78 °C. I found that an RTD platinum probe registered a temperature of -321F when I tested it in a nitrogen bath, which was perfect.
If you need a highly accurate and precise cryogenic thermometer, I have made one for production for those serious about this project. The unit is very simple and measures in Fahrenheit and Celcius in increments of 0.1F and 0.1C. Below are the specifications. You can view a video on the link and see how much better my RTD device fairs against commercial devices. The digital thermometer costs less, too!
Power 120vac/dc adapter
Probe RTD stainless steel, waterproof, 3-lead
Accuracy Class A, increments 0.1F/C
Thermal drift 0.005F/F