Controlling the Reference Junction

The second junction explained in the previous section can be controlled so that it produces 0 volts. This is accomplished by forcing its temperature to 32°F or 0°C. In the 1800s this was accomplished by placing the junction into an ice bath, so this junction became known as the cold junction. Modern thermocouple circuits today can accomplish the same function with an electronic compensation circuit called cold-junction compensation. The cold-junction compensation circuit provides the equivalent voltage produced by the junction in the ice bath so that the actual voltage produced by the junction that is measuring the amount of heat can determine the temperature accurately. The original thermocouple junction used as the heat sensor is sometimes called the hot junction, and the compensation junction is sometimes called the cold-junction or reference junction. ill. 1a shows a diagram of the reference junction that is placed in an ice bath, and a diagram of the cold-junction compensation circuit is shown in ill. 1b.

Above: ill. 1 (a) Example of placing the second junction in an ice bath to provide cold-junction compensation. (b) Example circuit that shows a thermistor (thermal resistor) used to provide cold-junction compensation. This type of compensation is called software compensation because the reference voltage must be calculated with the hot-junction voltage to get an accurate reading.

In Figure 1b a special resistor that changes its resistance as the temperature changes is used to provide the cold-junction compensation circuit. This resistor is called a thermistor and it helps provide additional compensation that is needed if the compensation junction is located where the temperature may change. e.g., the temperature in a factory where the voltage reading part of the thermocouple circuit is located may be as warm as 98°F in much of the summer and be 70°F during the remainder of the year. This temperature difference would be sufficient to provide a constant error in the readings. Since the thermistor will automatically change its resistance as the temperature changes, it's an ideal component to use in the compensation circuit. The only drawback to using a thermistor for cold-junction compensation is that the effects of the compensation voltage must be calculated or computed with the hot-junction voltage to get an accurate reading. For this reason this type of circuit is called software compensation and is commonly used where the thermocouple is connected as a temperature sensor directly to a solid-state or electronic controller.

Cold-junction compensation could also be provided by using a small battery voltage that represents the reference junction temperature. This type of compensation is called hardware compensation and it's widely available on electronic temperature controllers and on the newer versions of digital voltmeters (DVMs). Since the small compensation voltage can be provided inside a digital voltmeter, it allows technicians to accurately check thermocouple temperatures with the voltmeter while troubleshooting. When better accuracy is required, an integrated circuit can be used in place of the battery to provide the hardware compensation voltage. The main component of this IC is a temperature-sensitive solid-state device called an integrated circuit temperature sensor. The electronic symbol for the temperature IC is two overlapping circles. The overlapping circles are generally shown inside a triangle that is similar to the outline of an op amp. Since this type of compensation is provided by electronics, it's commonly called the electronic ice point reference. ill. 2a shows an example of electronic compensation and ill. 2b shows an example of battery hardware compensation.

Above: ill. 2 (a) An example of a temperature-sensing integrated circuit used to provide electronic ice point compensation. (b) An example of a battery used to provide hardware compensation to the thermocouple circuit.