Measurement of the Dynamic Response of Differential Pressure Transmitters Using a Response Time Tester

Abstract

AECL's response time tester (RTT) for pressure and differential-pressure (DP) transmitters provide a simple, fast and accurate means of measuring pressure transmitter step response time and response time characteristics as defined in ISA Standard ISA-S67.06. The step response method of measuring response time is used by all prominent sensor suppliers and provides a result that is readily understood. The measurement of response time characteristics, which includes the transfer function, provides a complete and sensitive characterization of the dynamic response of the sensor that can be used to predict the response to ramps or any other transient. The AECL Model 200 RTT uses standard instrument air as the source oftest pressure, and is suitable for testing transmitters that operate at pressures less than 650 kPa. It can accurately measure step response times in the range of 20 to 2000 ms. It is thus capable of measuring the response times of most of the pressure instruments in the CANDU safety systems. The Model 200 RTT is completely contained in a cabinet that can be easily moved on built-in shock-absorbent wheels, to facilitate testing transmitter while on the bench or mounted on the instrument rack. The RTT comprises pressurized components in the front end, and computerized data acquisition components and analysis software in the back end. Tests are conducted by switching the pressure applied to the transmitter between two values using a long-life, fastacting 3-way solenoid valve. The actual pressure applied to the transmitter is measured with a high-frequency reference transducer. Once the operator has connected the air supply and transmitter, and set the test pressures, the actual testing and analysis proceeds automatically. Four sets of tests are conducted. A number of comparisons, as required by ISA-S67 .06, are made to verify that: the transmitter is linear with respect to direction and rate of change; and the results are consistent between two diverse test methods. The broadband noise tests are used to find the transfer function (all the significant time constants) of the transmitter. Given the transfer function, the expected response to a step input can be computed and compared to the measured step response, thus verifying both measurements. The results are summarized and printed on the built-in printer. Files are saved to disk for later review.