What is a thermocouple?
Thermocouples are a popular and inexpensive way to measure temperature. The thermocouple is a simple device – two wires of different metals welded together at one end. But this "simple" device poses many challenges to a measurement system, including the need for amplification, filtering, and cold-junction compensation (CJC).
When connected in pairs, they are simple and efficient sensors that output an extremely small DC voltage proportional to the temperature difference between the two junctions in a closed thermoelectric circuit (See Figure 1).
Figure 1: Type T Basic Thermocouple Circuit. A classic thermocouple measurement system requires two sensors: one for the environment being measured and the other for a reference junction, normally held to 0 °C (32 °F).
One junction is normally held at a constant reference temperature while the opposite junction is immersed in the environment to be measured. The principle of operation depends on the unique value of thermal electromotive force (EMF) measured between the open ends of the leads and the junction of two dissimilar metals held at a specific temperature.
Thermocouple junctions alone do not generate voltages. The voltage or potential difference that develops at the output (open) end is a function of both the temperature of the junction T1 and the temperature of the open end T1'. T1' must be held at a constant temperature, such as 0 °C, to ensure that the open-end voltage changes in proportion to the temperature changes in T1. In principle, a thermocouple can be made from any two dissimilar metals, such as nickel and iron. In practice, however, only a few thermocouple types have become standard because their temperature coefficients are highly repeatable, they are rugged, and they output relatively small voltages. The most common thermocouple types are called J, K, T, and E, followed by N28, N14, S, R, and B. In theory, the junction temperature can be inferred from the Seebeck voltage by consulting standard tables. In practice, however, this voltage cannot be used directly because the thermocouple wire connection to the copper terminal at the measurement device itself constitutes a thermocouple junction (unless the thermocouple lead is also copper) and produces another thermal EMF that requires compensation.
The voltage difference that develops at the open end of the thermocouple is quite small in the uV range. This voltage cannot be accurately measured using a typical multifunction DAQ device with for example a 0-10 V range. This small signal requires gains in the range of x100, which is not typically provided on traditional multifunction DAQ devices. And, because thermocouples generate a relatively small voltage, noise is always an issue. The most common sources of noise are the AC power lines (50 Hz or 60 Hz). Because the bandwidth of most temperature systems is lower than 50 Hz, a simple filter in each channel can reduce the interfering AC line noise. Common filters include passive filters, using only resistors and capacitors, and active filters using these components along with op amps. While a passive RC filter is inexpensive and works well for analog circuits, it's not recommended for a multiplexed front end because the multiplexer load can change the filter characteristics. An active filter composed of an op amp and a few passive components works well in multiplexed systems, but it's more expensive and complex.
Cold Junction Compensation
Ice baths and multiple-reference junctions in large test fixtures can be difficult to set up and maintain, but fortunately they all can be eliminated. The EMF correction needed at the terminals can be referenced and compensated to the NIST standards through computer software. When the ice baths are eliminated, cold-junction compensation is still necessary to obtain accurate thermocouple measurements. The software has to read the isothermal block temperature. One widely used technique is a thermistor mounted close to the isothermal terminal block and connected to the external thermocouple leads. No temperature gradients are allowed in the region containing the thermistor and terminals. The type of thermocouple used is preprogrammed for its respective channel, and the dynamic input data for the software includes the isothermal block temperature and the measured environmental temperature. The software uses the isothermal block temperature and the type of thermocouple to calculate the temperature of the sensor using a polynomial equation. The method allows many thermocouple channels of various types to be connected simultaneously while the computer automatically handles all of the conversions.
Open Thermocouple Detection
Detecting open thermocouples easily and quickly is especially critical in systems with many channels. Thermocouples tend to break or increase resistance when exposed to vibration, poor handling, and long service time. A simple open-thermocouple detection circuit consists of a small capacitor placed across the thermocouple leads and driven with a low-level current. The low impedance of the intact thermocouple presents a virtual short circuit to the capacitor, so it cannot charge. When a thermocouple opens or significantly changes resistance, the capacitor charges and drives the input to one of the voltage rails, which indicates a defective thermocouple (See Figure 2).
Figure 2: Open Thermocouple Detector. The thermocouple provides a short-circuit path for DC around the capacitor, preventing it from charging through the resistors. When the thermocouple opens – due to rough handling or vibration – the capacitor charges and drives the input amplifier to the power supply rails, signaling a failure.
Thermocouple Measurement Devices
DAQ devices that are not specifically designed for thermocouple measurements lack the needed signal conditioning and CJC required to accurately measure thermocouples. Devices like the USB-TC feature a ±0.080 V range, 24-bit A/D per channel, and a NIST traceable calibration process. These devices provide the most accurate temperature measurement possible, since the internal measurement electronics accuracy exceeds the accuracy specifications of the thermocouple sensors.
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