Intelligent instruments contain extra sensors that measure the value of environmental inputs and automatically compensate the value of the output reading. They have the ability to deal very effectively with systematic errors in measurement systems and errors can be attenuated to very low levels in many cases.
Manual correction of output reading In the case of errors that are due either to system disturbance during the act of measurement or due to environmental changes, a good measurement technician can substantially reduce errors at the output of a measurement system by calculating the effect of such systematic errors and making appropriate correction to […]
Instrument calibration is a very important consideration in measurement systems. All instruments suffer drift in their characteristics and the rate at which this happens depends on many factors, such as the environmental conditions in which instruments are used and the frequency of their use. Thus errors due to instruments being out of calibration can usually […]
The benefit of adding high-gain feedback to many measurement systems is illustrated by considering the case of the voltage-measuring instrument whose block diagram is shown in Figure. In this system, the unknown voltage Ei is applied to a motor of torque constant Km and the induced torque turns a pointer against the restraining action of […]
The method of opposing inputs compensates for the effect of an environmental input in a measurement system by introducing an equal and opposite environmental input that cancels it out. One example of how this technique is applied is in the type of milli volt meter. This consists of a coil suspended in a fixed magnetic […]
Careful instrument design is the most useful weapon in the battle against environmental inputs by reducing the sensitivity of an instrument to environmental inputs to as low a level as possible. For instance in the design of strain gauges, the element should be constructed from a material whose resistance has a very low temperature coefficient […]
The prerequisite for the reduction of systematic errors is a complete analysis of the measurement system that identifies all sources of error. Simple faults within a system, such as bent meter needles and poor cabling practices, can usually be readily and cheaply rectified once they have been identified. However, other error sources require more detailed […]
Systematic errors in the output of many instruments are due to factors inherent in the manufacture of the instrument arising out of tolerances in the components of the instrument. They can also arise due to wear in instrument components over a period of time. In other cases systematic errors are introduced either by the effect […]
In connecting together the components of a measurement system a common source of error is the failure to take proper account of the resistance of connecting leads. For instance, in typical applications of a resistance thermometer, it is common to find that the thermometer is separated from other parts of the measurement system by perhaps […]
Systematic errors can frequently develop over a period of time because of wear in instrument components. Recalibration often provides a full solution to this problem.
An environmental input is defined as an apparently real input to a measurement system that is actually caused by a change in the environmental conditions surrounding the measurement system. The fact that the static and dynamic characteristics specified for measuring instruments are only valid for particular environmental conditions. These specified conditions must be reproduced as […]
Disturbance of the measured system by the act of measurement is a common source of systematic error. If we were to start with a beaker of hot water and wished to measure its temperature with a mercury-in-glass thermometer then we would take the thermometer which would initially be at room temperature and plunge it into […]
Errors in measurement systems can be divided into those that arise during the measurement process and those that arise due to later corruption of the measurement signal by induced noise during transfer of the signal from the point of measurement to some other point. It is extremely important in any measurement system to reduce errors […]
The foregoing discussion has described the static and dynamic characteristics of measuring instruments in some detail. However, an important qualification that has been omitted from this discussion is that an instrument only conforms to stated static and dynamic patterns of behavior after it has been calibrated. It can normally be assumed that a new instrument […]
Dead space is defined as the range of different input values over which there is no change in output value. Any instrument that exhibits hysteresis also displays dead space, as marked on Figure. Some instruments that do not suffer from any significant hysteresis can still exhibit a dead space in their output characteristics, however. Backlash […]