Stages of a generalized measurement system

A generalized measurement system converts the measured quantity (measurand) into a form suitable for observation. It has three main stages: sensing/transducing, signal conditioning, and output (readout).

 Measurand    +-----------+   +-------------+   +----------+
 ------------>| Sensor /  |-->|   Signal    |-->| Terminal |
(temperature) | transducer|   | conditioning|   | (readout)|
              +-----------+   +-------------+   +----------+
                 Stage I         Stage II         Stage III
                              (modify, amplify)
                                    ^
                         Power supply / calibration

Functions of the stages

1. Sensor-transducer stage (detector): senses the measurand and gives an output related to it. It should respond only to the measurand and should not load the source.
2. Signal-conditioning stage: modifies the transducer output into a suitable form. It includes amplification, filtering, bridge circuits, analog-to-digital conversion, linearization and modulation.
3. Terminating (readout) stage: presents the result to the observer or controller. It may be an indicator, recorder, display or data logger.

Thermocouple temperature indicator

• Sensor: the thermocouple produces a small emf (mV) proportional to the difference between the hot- and cold-junction temperatures.
• Signal conditioning: a cold-junction compensation circuit adds the reference emf, an amplifier raises the mV signal, a filter removes noise, and linearization corrects the nonlinear emf-temperature relation. An ADC converts the signal to digital form.
• Readout: a digital display or chart recorder shows the temperature in $^\circ$C.

Calibration against a known standard temperature links the readout to the measurand.