When we think about temperature sensor applications, we generally picture something hot. But depending on the process, the degree of coldness may be just as important, and it probably needs to be measured with the same precision.
Emerson’s Natalie Strehlke wrote an article for Process Cooling magazine titled Selecting Sensors to Probe Cold Temperatures, and it takes us in the opposite direction of what we’re used to—how to evaluate a temperature sensor’s ability to reach down into negative numbers.
Natalie isn’t alone when she says we don’t always think about cold:
There are many resources to help with temperature sensor selection — provided the application involves moving into higher temperature ranges — because most situations involve some sort of heat source. But, what if the application is going the other direction? Do the same guidelines apply when moving into areas below room temperature or below the freezing point of water?
As she points out, the answer is complicated. For example when an application gets cold, a thermocouple runs backwards. She takes us back to high-school physics where we made a thermocouple with a glass of ice water. But think about what happens when the sensing junction is colder than ice water:
Now, returning to our physics class, imagine inserting the sensing junction into a block of dry ice so it is now colder than the reference junction in the ice water. What happens? The voltage polarity is reversed because the reference junction is now the warmest spot, reversing the heat and electrical flow. A look at a typical thermocouple table will show that voltages go negative at 32°F (0°C). This is not an electrical characteristic so much as a convention relating back to the traditional ice water reference junction.
So, is an RTD better for low temperatures? To cite the universal engineering answer, it depends:
While RTDs do not have the high temperature capabilities of some thermocouple types, they are comfortable working in subzero applications. The most common type of RTD is the 100 Ω platinum, which is linear over its useful range, changing by 0.39 Ω per ºC. Its reference resistance is 100 Ω at 32 ºF (0 ºC), so the theoretical lowest limit is -428 ºF (-256 ºC), where its resistance reaches zero; however, it does not make sense to run to the very limit.
But when temperatures get into negative triple digits, Fahrenheit or Celsius, things don’t always work like we expect. RTD signals can change based on their construction, and thermocouples lose linearity. The kinds of voltages and resistance values also get very low, which makes signals hard to transmit to the I/O card of whatever kind of automation system is being used. Natalie recommends using an appropriate temperature transmitter:
Whether working with a thermocouple or RTD, the signal coming from the sensor is going to be low in magnitude and thus not very robust. This leaves it vulnerable to degradation if sent over a long distance. For thermocouples, any mismatch with extension wires, corrosion or simply poor terminations will be detrimental to accuracy. RTDs are somewhat easier to work with, but at reduced temperatures, the resistance becomes low. Installing a temperature transmitter as close as possible to the sensor can reduce these problems. The transmitter can convert the signal from a few millivolts to a more robust 4 to 20 mA current loop or a digital signal such as Foundation Fieldbus, either capable of being transmitted over long distances without the need for special cabling.
Ultimately the ability to use conventional temperature sensors in cold applications depends on how cold. The most extreme applications may require special sensors, but more typical low temperatures can use the same thermocouples, RTDs and transmitters Rosemount uses for measuring heat.
If you’ve struggled with measuring cold temperatures yourself, it’s likely other users are too. You can probably find many of them right here in the Emerson Exchange365 community. Look for the Temperature Group to interact with your peers and our experts.