Measurement is an estimation of a physical variable by a measurement device. An instrument is a device that converts a physical variable of interest (the measurand) into a form that is suitable for recording (the measurement). In order for the measurement to have broad and consistent meaning, it is common to employ a standard system of units by which the measurement from one instrument can be compared with the measurement of another. An example of a basic instrument is a ruler. In this case the measurand is the length of some object and the measurement is the number of units (meters, inches, etc.) that represent the length. Figure 1 General measurement system Sensors? A device which provides a usable output in response to a specified measurand. A sensor acquires a physical quantity and converts it into a signal suitable for processing (e.g. optical, electrical, mechanical). Nowadays common sensors convert measurement of physical phenomena into an electrical signal. Active element of a sensor is called a transducer. Transducer? A device which converts one form of energy to another. Actuators? Actuators are basically opposite of sensors. Actuators input is electrical and output is a physical quantity. Active and passive instruments: Instruments are divided into active or passive ones according to whether the instrument output is entirely produced by the quantity being measured or whether the quantity being measured simply modulates the magnitude of some external power source. In active instruments, the external power source is usually in electrical form, but in some cases, it can be other forms of energy such as a pneumatic or hydraulic one. The phenomena which are most commonly detected are: Biological, Chemical, Electric, Electromagnetic, Heat/Temperature, Magnetic, Mechanical motion (displacement, velocity, acceleration, etc.), Optical, and Radioactivity. The key functional element of the measurement system shown in Figure 1 is the sensor, which has the function of converting the physical variable input into a signal variable output. Signal variables have the property that they can be manipulated in a transmission system, such as an electrical or mechanical circuit. Because of this property, the signal variable can be transmitted to an output or recording device that can be remote from the sensor. In electrical circuits, voltage is a common signal variable. In mechanical systems, displacement or force are commonly used as signal variables. In a basic instrument, the signal is transmitted to a display or recording device where the measurement can be read by a human observer. The observed output is the measurement M. There are many types of display devices, ranging from simple scales and dial gages to sophisticated computer display systems. The signal can also be used directly by some larger system of which the instrument is a part. For example, the output signal of the sensor may be used as the input signal of a closed loop control system. Common physical variables are: force, length, temperature, acceleration, velocity, pressure, frequency, capacity, resistance, time etc. Commonly used typical signal variables are Voltage, Displacement, current, force, pressure, light, frequency. Calibration is the relationship between the physical measurement variable input and the signal variable (output) for a specific sensor is known as the calibration of the sensor. Typically, a sensor (or an entire instrument system) is calibrated by providing a known physical input to the system and recording the output. The sensitivity of the device is determined by the slope of the calibration curve. Accuracy and Error? The Accuracy of an instrument is defined as the difference between the true value of the measurand and the measured value indicated by the instrument. Typically, the true value is defined in reference to some absolute or agreed upon standard. For any particular measurement there will be some error due to systematic (bias) and random (noise) error sources. The total error results from both systematic and random errors.
A K V
Jss academy of technical education
IIT Patna, IIT kharagpur
N N B
NARULA INSTITUTE OF TECHNOLOGY