Measuring device are the instruments and gauges which are used for measurement of dimensions of any object. The devices used for measurement of mass are called, Mass Measuring Devices.
Choice of use of a measuring device depends upon the following factors –
- Type of measurement i.e. Direct measurement or Indirect measurement.
- Accuracy of measurement i.e. Rough measurement or Precise measurement.
- Type of attribute to measure i.e. mass, length, time, current etc.
According to the type of activity involved in measuring practice, measuring devices are of following types –
- Direct Measurement Devices.
- Indirect Measurement Devices.
- Graphical Measurement.
- Precise Measuring Devices.
In direct measurement technique, graduated measuring instruments are used to measure the dimensions of objects by applying the instrument directly on the object. These measuring devices are called Direct Measuring Devices.
- This method of measurement is also called as absolute measurement. Graduated scales, Micrometers, Vernier, Pressure Gauge, Screw Gauge etc. are direct measuring devices.
- A direct measuring technique is shown in figure (A).
In this method of measurement, graduated instruments are not applied directly on the target object. The dimensions of the target object are compared with intermediate instruments such as dial gauges, feeler gauges, block gauges, calipers etc. These devices are called Indirect Measuring Devices.
This method is also known as comparative measurement. It is generally employed for following reasons.
- It is employed at the places where it is not customary to apply the direct measuring devices or instruments due to space constraint.
- In some applications, some allowances and tolerances are allowed for the dimensions of components. The size of the component should lie in between two limits called upper limit or maximum permissible dimension and lower limit or minimum permissible dimension. For measurement of such components, knowing the exact dimension is not essential. Hence, in mass production of such components “GO & NOT GO” gauges are employed to compare the dimensions of target component.
- An indirect measuring technique is shown in figure (B).
Indirect measurement may also use geometrical or trigonometrical relations to get the result using graphs, drawings etc. These are called Graphical Measuring Devices.
This type of indirect measurement, use geometrical and trigonometrical relations or wave reflection method using LASER, RADAR or ultrasonic waves to get the result of measurement.
- Indirect measurement of height of a mountain peak, depth of sea, diameter of planets etc. are some examples of this type of measurement.
Precision refers to the resolution or the limit of accuracy to which the quantity is measured. Precision is determined by the least count of the measuring instrument. The smaller the least count, the greater will be the precision and accuracy.
A precise measurement technique requires a skilled operator having knowledge of handling of the measuring device, reading skill of its graduations and least count.
A precise measurement result can be achieved by –
- Accurate measuring methods to obtain true relationship among all physical quantities involved in the event.
- By using accurate measuring device with zero error.
- Accurate techniques of measurement.
- Skill of the person making the measurement and taking readings.
- By eliminating errors such as instrumental error, imperfections and personal error etc.
Micrometers, Vernier, Diagonal Scale etc. are precision instruments.
Mass of a body is defined as the amount of matter content of that body. It is fixed quantity and will not change till a portion or part of body is removed from it or added to that body.
- Mass of a body is fixed and doesn’t depend upon earth’s acceleration due to gravity.
Various mass measuring devices are used for mass measurement of a body. These include –
It is based on the metric system of measurement. A common balance has has two pans. The body, whose mass has to measure, is kept on one pan and a standard known mass is kept on other pan for comparing. Here, earth’s acceleration due to gravity (g) is acting equally on both the pans. Therefore, the effect of earth’s gravitation pull is not significant.
These are the portable microprocessor based devices extensively used for mass and weight measurement. A person stands on a digital scale platform, which obtains body weight. A digital scale calculates the mass by taking the body weight and dividing it by gravity.
Electronic load cell instruments are used for very accurate measurement of mass. These are more sophisticated weighing machines. The strain gauge load cells have a typical accuracy of one part in 2,000.
In situations where a balance can’t be used for mass measurement, such as measuring the mass of a liquid in a tank, transducers are used. The transducer works by sending a signal to a processor, which then makes the mass calculations.
This is used for measuring of fluid mass. The sensor determines the buoyant mass of an object by using density of fluid in it. Absolute mass is then processed and determined by measuring the buoyant mass of the object in other fluids of different densities.
Gravitational interaction between objects is most reliable to determine the mass. To determine the mass of a star, scientists find the distance between it and another star and the time of their respective movements. Scientists also use the speed of rotation to measure the mass of galaxies.
Weight is defined as the gravitational force by which earth pulls a body towards its center. It depends upon mass of that body and earth’s acceleration due to gravity.
- Weight of a body depends upon earth’s acceleration due to gravity. Hence, weight is called gravitational unit of force.
- Earth’s acceleration due to gravity varies with the distance of body from center of earth and also varies with different locations on earth’s surface. Hence, weight of a body change with location on the earth’s surface.
Various measuring devices are used for weight measurement of a body. These include –
- Spring balance – Weight of a body is measured by using a spring balance. A spring balance works on spring force and has only one pan on which measuring body is kept or hangs to it. Therefore, effect of earth’s acceleration due to gravity is very significant in this case.
- Computerized Digital Scales – These are the portable microprocessor based devices extensively used for mass and weight measurement. A person stands on a digital scale platform, which obtains body weight.
- Load Cell Instrument – Electronic load cell instruments are used for very accurate measurement of weight. These are more sophisticated weighing machines. The strain gauge load cells have a typical accuracy of one part in 2,000.
- Weigh Bridge – The weighbridge is a measuring device for heavy weight. It measures the weight of loaded vehicle passing or standing on it.