Virtual Junctions

 

  Virtual Junction Errors in Temperature Measurement

What Causes Virtual Junction Errors?

Nearly all thermocouples are manufactured using magnesium oxide (MgO) as an insulator.  As the temperature of a process increases, the insulation resistance of the magnesium oxide insulated thermocouple decreases.  Tests of MgO thermocouples show the insulation resistance may drop from 1 gig ohm or more at ambient temperature to 30 ohms or lower at the temperature ranges Type K or Type N thermocouples are operated.

When the MgO insulation resistance drops to such low levels, and the thermocouple passes through a zone at a higher temperature than the point you are attempting to measure (tube skin thermocouples are a good example), the measurement point can shift from the tip of the sensor to the hottest portion of the thermocouple and give you an erroneous reading.  This is a "virtual" junction.

AccuTru Solves the Virtual Junction Error Problem

AccuTru has solved the virtual junction error problem with its proprietary MI-Dry insulated thermocouples.  MI-Dry is a new formulation of mineral insulation with properties that significantly improve thermocouple performance. 

MI-Dry has several advantages over MgO.  It is significantly less hygroscopic, has better electrical insulating properties, and MI-Dry traps, slows, and/or prevents contaminating ion movement through the dielectric resulting in longer sensor life.

The MI-Dry dielectric exhibits significantly higher insulation resistance at elevated temperatures when compared to MgO.  In addition to providing longer life and greater signal stability, MI-Dry reduces or eliminates virtual junction errors in thermocouples.

Test Results

Test data in the graph below shows that virtual junction error was present for the MgO sensor almost immediately after the test started, but the MI-Dry sensor continued to read true temperature.

After 100 Hours

• The MgO sensor was reading 10%+ high.
• The MI-Dry sensor was reading correctly.

After 400 Hours

• The MgO sensor was reading 19%+ high.
• The MI-Dry sensor was reading correctly.

Test Protocol for Determining Virtual Junction Error

The test to determine the susceptibility of mineral insulated cable to virtual junction error is quite simple. It requires two furnaces and a means of measuring the temperature in each furnace and the temperature output from each test thermocouple.

The center sections of the test thermocouples are coiled and placed in the mid-section furnace, with the tips exiting the end of the furnace. A reference thermocouple is placed in this furnace to measure the temperature of the coils. The tips of the test thermocouples and a 2nd reference thermocouple are wired together to create a thermally conductive mass. These tips are placed in second furnace.

The temperature of the furnace that contains the tips of the sensors is then heated to a set temperature, in this example, 1500 degrees F.

The readings are then taken for the test and reference thermocouples. All the readings should be within the limits of error for that type of sensor.

Once the temperature of the tip furnace has stabilized, the mid-section furnace is started and the temperature increased to a setting higher than the furnace containing the thermocouple tips. If the insulation of a thermocouple fails, the temperature reading reported for that sensor will begin to increase, reflecting the temperature in the mid section furnace rather than the temperature reading from the tip of the sensor.

This test protocol was used for the test results reported which show that MgO insulation failed and MI-Dry did not.