This invention relates to the use of thermocouples. A thermocouple is a temperature sensing element that operates on the principle that when two dissimilar metals are junctioned and the junction is heated, it produces a low voltage (millivoltage) which is proportional to the temperature.
Thermocouples have a predictable and repeatable relationship between temperature and voltage. They are used widely in higher temperature applications because they can withstand greater temperatures than resistance temperature detectors (RTDs), and are less expensive in most cases.
However, a significant problem has arisen with respect to the accurate temperature measurement of a thermocouple with high loop resistance (greater than 2,000 ohms).
One of the uses of a high resistance thermocouple is to take accurate temperature readings in downhole thermocouple applications. An investigation of these applications have discovered that many errors occur in the readings and that the errors are a function of the resistance of the thermocouple. In other words, the higher the resistance of the thermocouple, the larger the error either positive or negative.
Experiments have been conducted using many transmitters, PLCs to troubleshoot the problems with determining accurate temperature measurement.
Tests have been conducted using thermocouples of various resistances and at different temperatures. It was discovered that there is not any consistency other than the higher the resistance of the thermocouple, the larger the error.
The issue is the high loop resistance of the thermocouple and the inability of some instrumentation to process it accurately.
Most thermocouple measuring instruments will not correctly measure the voltage output of a high resistance thermocouple. Thermocouple resistance is normally low (<500Ω) and the instrument input impedance is normally high (>1 MΩ). The thermocouple voltage causes a current to flow in the input circuit. This causes a voltage change at the instrument terminals due to the voltage divider created by the two resistances, one of the thermocouple wires and the second of the instrument input circuit. The effect is negligible when the thermocouple resistance is low, but it becomes significant when the lead wire resistance increases (greater than 2,000 ohms).
The prior art has suggested to introduce a transmitter that has been tested and is known to work in the application between the wellhead and the input cards but in order to do so, 4-20 mA input cards rather than thermocouple input cards would have to be used. Retrofitting 4-20 cards for thermocouple input cards can be an expensive proposition. Not to mention the additional copper cable and the installation of it if not already present. It is further noted that not all transmitters will work.
Transmitters won't solve the problem. They are prone to the same problem of high input resistance as any other measuring instrument.
When a thermocouple is accessible, a transmitter can be installed near the hot end and the 4/20 mA output signal sent over a long distance using copper wire in lieu of more expensive thermocouple wire (also requires a current input signal conditioner instead of thermocouple at the receiving instrument). This is not applicable where the thermocouple location is inaccessible such as underground, or in an unfriendly environment such as an area that is exposed to radiation.