Introduction of process measurement

Static and dynamic characteristics
The elements of instruments
Parts of instruments
process control

1. Introduction of process measurement

2. A measuring instrument is simply a device for determining the value of a quantity or condition. The value determine by the instrument is generally, but not necessarily, quantitative. Where the instrument only indicates the general value or a range of values of its measured quantity. The elements of instruments

3. An instrument provides some kind of calibrated scale and pointer . One of the most important function of an instrument is found in the automatic controller, every one of which includes a mechanism performing the function of measurement.

4. All instruments contain various parts that perform prescribed function in converting a variable quantity or condition in to a corresponding indication. Parts of instruments

5. The parts of an instruments are indicated in above fig. with a simple example of an indicating thermometer. The primary elements is the part of the instruments that first utilizes energy from the measured medium to produce a condition representing the value of the measured variable. The secondary element merely converts the condition produced by the primary element into a condition useful to the function of the instruments.

6. The manipulation element preforms given operation on the condition produced by the secondary element. The functioning element simply denotes the part of an instruments used for transmitting, signaling, registering, indicating, or recording.

7. Static and dynamic characteristics

8. Static characteristics: It is not change with time or very slowly change with time. D esirable 1. Accuracy : How close the measurement approaches the true value of process variable. 2. Sensitivity : Ratio of change in output of instrument to change of input. 3. R eproducibility: Ability of an instrument to reproduce certain set of reading within a given accuracy

9. Undesirable: 1. static error: The difference between true value of quantity and measure value. 2. Dead zone: Largest range of value of measured variable to which the instrument does not respond 3. D rift: Change in indicating reading of instrument over time when the value of measured in quantity remain constant.

10. Dynamic characteristics: It is rapidly varying with time. D esirable: 1 . speed of response : Rapidly with which an instrument response to change in measure medium. 2. Fidelity: Degree to which and an instrument indicate the change in measure variable without dynamic error.

11. Undesirable: 1 . lag: Delay in response. 2. Dynamic error: Difference between true value and value indicate by instrument.

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