When thermocouples sensors experience disconnections, the receiving instrument indicates or records the incorrect value. For example, when a controller is used for part of a combustion plant, if a sensor becomes disconnected a lower-than-normal temperature value is output resulting in excessive fuel being supplied which could cause a major accident. If the controller were used for research, erroneous data could result.

To avoid these kinds of situations, the measurements can be taken of data by first using a normal sensor then using a disconnected sensor in order to distinguish between the two.

When detecting a burnout, receiving instruments have functions that display and output upscale (scale off the plus side) or downscale (scale off the minus side), or directly display and output Burnout. In examples such as the previously mentioned combustion plant, mechanisms are in place so that when a sensor disconnection is detected, upscale is output on the safety side, or a burnout code that is separate from the temperature data is output.

With the DX series, burnout settings for upscale and downscale are available for each thermocouple input channel (see Fig. 1). If a sensor burnout is detected from the input signal, the measured result is set to positive or negative over range, and the word “Burnout” is displayed. (see Fig. 2)

Figure 1: DX burnout setting screen

Figure 2: DX burnout screen

The computation function of Yokogawa DXAdvanced series and CX series instruments discards error data from channels in which a sensor burnout occurs, and continues recording. This function also helps you to secure data using multiple temperature sensors.

For receiving instruments to detect thermocouple burnout, generally the sensor passes a current through the circuit which causes a voltage drop, and the disconnection is detected from that value. When multiple receiving instruments are connected to a single sensor, each receiving instrument detects burnout individually and sends detection current to other receiving instruments. Since this not only causes inaccurate temperature measurement but can also have an adverse effect on the other receiving instruments, the signals should generally be bifurcated with double elements or converters.

Note: Some instruments can perform burnout detections for RTDs.