Introduction 2 Instrumentation Page1

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Instrumentation is a field of study and work centering the measurement and control of physical processes such as: pressure, temperature, flow and so on.

Instrument is the device that used to measure and/or control the physical quantity.

Electrical-based instruments are used widely because of the easiness of measuring, manipulating, and transmitting for long distances.

Signal is a physical quantity that used to convey information; this text for example is a signal to convey information about some instrumentation principles.

Signals can be classified mainly into two types: analog and digital.

Analog signals are those types continuously vary to represent any small amount of change. They have a continuous scale. Their resolution is ideally infinite. Typical analog ranges are: 4-20 mA, 0-5 V.

Digital signals have specified steps to represent a restricted amount of values, the values between scale steps can not be measured. They are mainly used to indicate on-off controls such as valves.

? You may have a question about why we used 4-20 mA and not 0-16mA, why do we should complicate the problem!!
The answer will lead to an important concept in many (and not all) analog instrumentation signal systems, that is: "Live Zero". Suppose a working instrument with 0-16 mA that indicates a level in a tank, then 0mA will indicate an empty tank (0%) and 16mA will indicate a full tank (100%) and any mA between 0 and 16 will indicate its associated % of the liquid amount of tank, you may say: this is perfectly what we want. Now take a fault problem (e.g. a wire cut, transmitter malfunction… etc), of course a 0mA will be measured even if the tank is full 100%. This may mislead the control system and a problem (sometimes a disaster) may happen!!
So 4-20 mA range is used to distinguish the malfunction from the 0% physical quantity. And this is called: Live Zero to refer to an analog signal scale using a non-zero quantity to represent 0% of real-world measurement.

• Reference: Lessons in electric circuits, volume-I: DC,
By R. Kuphaldt.
 

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