TABLE OF CONTENTS Historically, the response time of RTDs and thermocouples has been characterized by a single parameter called the plunge time constant ( ?). This is defined as the time it takes the sensor output to achieve 63.2 percent of its final value after a step change in temperature is impressed on its surface. This step change is typically achieved by suddenly immersing the sensor in a rotating tank of water at 1 meter per second. The water must be at either a higher or lower temperature than the RTD. Measuring ? in this way is referred to as plunge testing.
Until 1977, testing temperature sensors' response times in nuclear power plants was almost always performed using the plunge test. In nuclear reactors, however, plunge testing is inconvenient because the sensor must be removed from the reactor coolant piping and taken to a laboratory for testing. Nuclear reactor service conditions of 150 bar (2,250 psig) and 300 C (572 F) are difficult to reproduce in the laboratory. Therefore, all laboratory tests are performed at much milder conditions, and the results are extrapolated to service conditions. The combination of manipulating the sensor and extrapolating the results to service conditions leads to significant errors in the measurement of sensor response times, sometimes by as much as a factor of three. [11] These drawbacks of the plunge test motivated the industry to find a better way to test the response time of nuclear plant temperature sensors. As a result, the following methods...
