1. Measured medium
2. Measured quantity
3. Primary sensing element
4. Variable conversion element
5. Variable manipulation element
6. Data transmission element
7. Data presentation element
8. Presented data and
9. Observer
(A) Primary Sensing Unit (3)
This is an element which first receives energy from the medium to be measured and produces a proportional output. The output signal of the primary sensing element is a physical variable such as displacement or voltage. The primary sensing element, therefore, is a primary transducer (sensor) that converts (transduces) one physical variable into another. A primary transducer may be followed by an intermediate transducer, if a second transduction is desired. For the measurement to be faithful, the sensor should be designed so as to extract a very small amount of energy from the medium. In other words, the medium should not be disturbed appreciably when the sensing element is inserted. As an example, consider the measurement of temperature of a body. The heat capacity of the thermometer, if large, will lower the temperature of the body and hence the measured temperature will be considerably smaller than its actual temperature.
(B) Variable-Conversion Element (4)
The output signal of the primary sensing element may require to be converted to a more suitable variable while preserving its information contents. This function is performed by the variable conversion element and it may be considered as an intermediate transducer. An example of this is the conversion or displacement to voltage in a pressure transducer.
(C) Variable-Manipulation Element (5)
This element is an intermediate stage of a measuring system. It modifies the direct signal by amplification, filtering etc. so that a desired output is produced. The physical nature of the variable remains unchanged during this stage. An example of this is an AC amplifier tuned to the frequency of chopper in some spectrophotometers.
(D) Data-Transmission Element (6)
When the functional elements of a measuring system are spatially separated, it becomes necessary to transmit signals from one element to another. This function is performed by the data-transmission element. It is an essential functional element where remote controlled operation is desired.
(E) Data-Presentation Element (7)
Usually information about the quantity being measured is to be communicated to a human observer for monitoring, control or analysis purposes. This is, therefore, to be presented in a form recognizable by some human senses. If the information is to be presented to the computer, it can be done in the form of a binary scale on the punched tape or cards, or the measuring system may be suitably interfaced with the computer. An element that performs this 'translation' function is called a data-presentation element. The observer (9) receives the data (8) in a form he can interpret easily.
