Temperature Measurement:
Thermocouples vs. Thermistors

In the following article, we will discuss some of the issues that surround thermocouples and give the argument for using thermistor-based temperature measurement in the appropriate environments. For a more detailed account, read our temperature measurement Case Study, "Thermistors & Thermocouples: Matching the Tool to the Task in Thermal Validation" as published in the Journal of Validation Technology.

Choosing the Tools in Temperature Measurement

Thermocouple-based temperature measurement is often used in pharmaceutical validation projects. While the use of thermocouples is somewhat entrenched, the process of using them is well known to be complex and costly.

Difficult and time-consuming to set up, thermocouples are also frustratingly error prone, which translates to frequent calibrations. This is why measuring temperature with thermocouples requires that personnel be experienced and knowledgeable with both thermocouple technology and temperature calibration methods and procedures. But, even with vigilence and care, the measurement is only as good as the device used.

In contrast, thermistor-based temperature measurement, such as the technology used in Veriteq's VL-series validatable data loggers, offers a number of key advantages over thermocouple-based temperature measurement systems — particularly for measuring temperature ranges in typical pharmaceutical validation applications where accuracy is critical.

Many of these advantages result from the inherent benefits thermistor sensors offer over thermocouples; while other advantages result from Veriteq's unique design of their thermistor-based data loggers.

Thermocouple-based Temperature Measurement

Thermocouples are the world's most popular temperature sensor. They are widely available, standardized, reasonably cheap and able to measure extreme temperatures. They are also capable of producing an electrical signal that has a fairly direct relationship with temperature, a requirement that was particularly critical in early (pre-microprocessor) temperature measurement instrumentation. Perhaps this is one reason the use of thermocouples has been so ingrained in the process and manufacturing industries.

However, for all their advantages, thermocouples are ill suited for many applications in pharmaceutical validation where accuracy is key. In applications such as chamber and storage area temperature mapping, measuring these mid-range temperatures with thermocouples is neither practical nor reliable, for a number of reasons.

But, in order to understand where thermocouples are a practical temperature measurment option, it helps to know how they work.

How Thermocouples Work

Thermocouples measure temperature by generating a small voltage signal proportional to the temperature difference between the junctions of two dissimilar metals. One junction (the hot junction) is typically encased in a sensor probe at the point of measurement; the other junction (the cold junction) is connected to the measuring instrument.

Thermocouple temperature measurement is based on reading the voltage signal and the cold junction temperature to compute the temperature using mathematical equations applicable to the type of thermocouple used.

Measuring Temperature: Managing Errors & Calibration

Signal Errors & Noise

In theory, thermocouple systems work very well. In practice, temperature measurement errors are common and difficult to control. Many of these errors are attributable to the very small electrical signal that is generated by the thermocouple and the dependence that small signal has on the junctions and imperfections in the system circuitry. Such factors include:

• Length, thickness, polarity and quality of the thermocouple wiring
  
  
• Complexity of installation
  
  
• Number of connections in the system
  
  
• Magnitude of thermal gradients along the length of the wiring runs

These factors can result in high levels of system noise that can often invalidate the readings.

Cold junction temperature errors

Another major contributor of temperature error is cold junction temperature. Many people are surprised to learn that thermocouples don't actually measure temperature—they measure the temperature difference between two points.

Other factors that can contribute to cold junction errors include:

• Stability of the instrument
  
  
• Accuracy and responsiveness of the cold junction sensor
  
  
• Accuracy of the instrument's calibration
  
  
• Capability of the instrumentation to adapt to all of these factors

Pre- & Post Calibrations: Time, Costs & Accuracy

In order for a thermocouple-based system to be reliable, all sources of error must be tightly managed. In most applications this will involve a pre-calibration before use and a post-calibration following use. It also requires careful preparation of thermocouple probes, wiring and connections.

Temperature gradients and kinking of wires must be avoided. Cold-junction temperatures need to be kept as stable as possible. Probes need to be attached to the exact instrument channels for which they were calibrated. And so on.

All in all, it can take a lot of work and careful attention to get thermocouple readings you can trust. It's instructive to note that the possible errors in a thermocouple system are significant enough that a leading supplier of thermocouple based systems recommends field-calibrating their systems before and after each validation application—and further recommends the use of two different calibration standards for each calibration.

The temperature calibration process is in itself somewhat complicated. If not managed properly and performed by trained personnel, can be a major additional source of error.

Thermistor-based Temperature Measurement

How does the Veriteq VL system compare to thermocouple-based equipment? Our data loggers use industry-best thermistors—temperature-sensitive resistors—which are built in to each device. Although more limited in temperature range (typically -80 to 150°C), thermistors come with a number of significant advantages as a tool for temperature measurement, such as:

• A large output signal that results in better precision
  
  
• Greater stability, providing accurate performance for longer periods of time
  
  
• Higher accuracy than thermocouples in mid-range temperatures

Veriteq's VL-system employs special circuitry and design features. For example, each data logger is completely self-contained and equipped with sensors, memory, power supply, clock, and microprocessor.

This is an important advantage because it eliminates external system errors—the data logger itself is the system. As a result, there is no wiring, connection, or environmental concerns that negatively impact accuracy.

In combination, these factors result in a system that is capable of producing consistently reliable results with minimal effort and expertise.

For more on our Veriteq VL system as an option for a thermistor system, please contact us.