Changes in the values output from a measuring instrument can lag behind actual temperature changes in the item under test. This is called response delay. When this delay is small, we say that the instrument has "quick response," or "good responsiveness." Two factors that contribute to overal response delay in temperature measuring systems are delays in detector/sensor response, and delays in response due to signal processing.
- Delays in Sensor Response
Generally, non-contact sensors (such as radiation thermometers) exhibit less response delay than contact sensors—that is, they offer quick response.
[Contact Sensors]
- The shape (such as the outer diameter) and thermal conductivity of the material used for the sensor’s protective tubes or thermowells affect responsiveness. Responsiveness can be improved by using sensors with protective tubes of high thermal connectivity and small outer diameter.
- Grounded sheathed thermocouples yield smaller response delays than ungrounded.
- Thermistor temperature detectors can be made smaller to achieve measurement with smaller response delay.
[Non-Contact Sensors]
- Response differs depending on the type of detecting element; photoelectric types are faster, and thermoelectric types are slower.
- Delayed Response due to Signal Processing
It is necessary to evaluate the origins of response delay in the various stages of a system that follow the detecting element such as signal processing and display. This also includes any converters, recorders, communication, and HMI that may be part of the entire temperature measurement system overall.
Factors involved in response delay include circuit-related delay in analog and AD conversion circuits, sampling and multiplexing (multipoint input converters) when digital processing is used, and software-related delays.
- Methods for Evaluating Response Delay
Normally, the step response time is used to evaluate response delay. However when a linear temporary delay is introduced, it can also be expressed as a time constant. In digital processors, the input sampling period, or the display/output update interval also constitutes an index of response delay.
- Response time: With step response, the time until the indicated value, displayed value, or output signal reaches a specified percentage of the final value. For example, expressed as "90% response."
- Time constant: In linear temporary delay systems, the time until the step response attains 63.2% of the final amount of change.
Realistically, the response delay of recorders can be considered to be related to signal processing as mentioned in item 2 above. However in the case of the Yokogawa µR10000 (pen model recorder), the response time relative to the input is approximately one second. Also, with non–chart recorders such as the DAQSTATION series, the response is generally quick since no mechanical mechanisms are used to display the input signals.
Reference: JEMIMA Temperature Measurement 100 FAQ |
