Selection of Instrument for Ensuring Reliability in Measurement

There are many types of instrument available in the market but in calibration process we should select the instrument by which our all requirement for the particular measurement can be fulfilled and the instrument is most convenient for that measurement. So for selection of instrument following point should be considered.

Range

In selection of instrument first point will be what should be the range of instrument. In dimension calibration instrument the maximum range of the process is considered, by which the maximum dimension will be measured and according to that the instrument range will be decided (i.e. For the selection of Vernier the maximum dimension of the job is 140 mm than the Vernier will be of 0 – 150 mm range). But in selection of bourdon tube type gauges (pressure gauges, temperature gauges) the instrument range should be 1.5 to 2 times more than the process range. Because in bourdon tube type gauges it gives most accurate results on middle ranges (i.e. If the process requirement is between 3 – 7 Kg/Cm2 than the pressure gauge of 10 Kg/Cm2 will be better). Because of the hystersis related mechanism in the bourdon tube, it gives the perfection in the results only at the 20 – 80 % range of pressure gauge. Means the gauge of 0 – 10 Kg/Cm2 can gives perfect results only at the 2 – 8 Kg/Cm2. Hence the selection of range becomes very crucial at this stage.

Least count

After selection of range the other main criteria is of least count. The instrument cost also depends on the instrument least count. Instrument least count should be sufficient four our requirement. So the best suitable least count is decided based on the tolerances of the process. Generally the instrument least count should be 2 to 3 times better than tolerance requirements of process. I.e. The calibration process tolerance is 0.05 mm than the instrument least measurement capability should be at least 0.02 mm or less.

Working Atmosphere and personnel skill

In the selection of instrument the atmosphere where it is to be used or by whom it is to be used, is also equally important. In the plant where dusty atmosphere and there may be chances of touch with oil, chemical fumes and vibration. The stability and accuracy of electronic instruments does not remain constant for long time. Hence better way is to use specially designed instruments or mechanism type instruments. I.e. In measurement of chemical pressure, diaphragm pressure gauges is most reliable with glycerin filled, which can gives stability to the gauge during vibration.

Identification of instruments for calibration

For any industry there is a big question regarding calibration is, which instruments should be calibrated. All the instruments in the process / plant need not to be calibrated only some identified instruments are needed to be calibration. For identification of instruments for calibration following points to be considered.

• The instruments affects in terms of process control parameter. Means the instruments used to control the process parameters like temperature controller, timer, go no-go gauges.
• Affects process in terms of life of equipment. In some pressure vessels liquid is feed or stored at some pressure if the pressure increases than the rated (designed) there may chances of explore of leakage so that monitoring instruments must be calibrated.
• Affects quality of product. The instruments used in quality control must be calibrated because they decide the quality of product. If they will not have quality then you will not have quality product.
• In-house templates, jigs, fixtures are must be calibrated. Jigs and fixtures are used in mass production so timely calibration of that instrument must be needed because any deviation beyond tolerance in that may case rejection in a big lot.
• Referred / identified in ISO documentation for process monitoring or measurement that instruments must be calibrated.

Hence the above points needs to be considered during the selection of instruments for better measurement and providing reliability in the measurement.

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