Introduction

Introduction ( This is For personal preference only.) Read only.

In order to operate a shipboard plant, such as boiler, we need to know the physical states of process fluids (e.g. water or steam inside). We need to know such parameters as pressure, temperature, level, etc., of the fluid.Measurement provides us with means of describing natural phenomena in quantitative terms. When you can measure you know what you are speaking about and express them in numbers, Industrial instruments are developed over the years, based on physical laws to measure all these parameters. The subject of "Instrumentation" basically deals with this area. At times, the instruments are designed not only to measure but also to control and the related studies are then called Instrumentation and Control.Before we can use these instruments, like any tool, we need to know its capability (what it can do). It is therefore necessary to define limits of performance of these instruments, and to do that, the terminology used, should be known.


Process Variables

Consider some simple measuring instruments such as a pressure gauge or thermometer.Analyzing the function of these gauges, we find a) A process = for example, the steam in the boiler or water in a pipe (that the instrument is expected to process)andb) A Variable = the pressure or temperature that varies in the said process called parameter or variable.The measuring device must be capable of faithfully and accurately detecting any changes that occur in the said process variable be it steam pressure or water temperature or whatever.

Elements of an industrial measurement instruments

All measuring system consists of basic three elements in its construction, namely a) a Sensing element b) a Processing element and c) a Display element.In case of small instruments, it may be difficult to separate them, but for large industrial instruments, the differences are distinct. Sensing unit or sensor - is to detect change in process variable, in case of a pressure gauge it may be the Bourdon tube inside, or in case of a mechanical thermometer it is the mercury bulb. The sensing unit is also a transducer. The Bourdon tube (inside the pressure gauge) converts pressure energy to mechanical movement of its end plate or the mercury bulb (in a thermometer) converts heat energy to mechanical movement of mercury in the glass capillary tube.Processing unit - Some sort of processing is always necessary. The movement of end plate of Bourdon tube as above, because of pressure change is usually so small that it cannot be read directly. It is processed (amplified many times using gears) before it can be used. Display unit - Is the round dial on which the pressure is read or the marked glass stem.



CALIBRATION

Measurement:All measurements are comparisons. When a length is measured, it is compared with a fixed length, referred standard of length. Then the number of times the unknown length is greater than the standard is found. The standard is so chosen, that the number of times or numeric is not too large or too small. To ensure that the length, or any other measured quantity, as measured by one person, shall agree with that as measured by a second person, the standards must absolutely be fixed, and reproducible with precision. Once a standard of length has been established, all measuring instruments based on this standard are made to agree with the standard. In this way, the length of an object as measured by one instrument will agree with the length as measured by any other instrument. Accuracy can be obtained only if measuring instruments are periodically compared with standards, which are known to be constant; i.e. from time to time, the instruments are calibrated.Calibration, in other words, is the process of comparing an instrument with a known standard. In describing the performance of an instrument, four terms are frequently used i.e. accuracy, precision, sensitivity and rangeability. Accuracy: -The accuracy of a reading made with an instrument may be defined as the closeness with which the reading approaches the true value. However, the term closeness to true value is not measurable. Therefore accuracy of an instrument is usually ex pressed in terms of its inaccuracy; i.e. in terms of percentage error. For example, a thermometer having an accuracy of +2¿¿ C at boiling point of water will be expressed, as having a percentage error of + 2%. The accuracy of an instrument may also be expressed in a number of different way, such as, intrinsic accuracy (under ideal conditions), point accuracy (i.e. accuracy at any given point of the scale) or accuracy as percent of scale range or percent of true value. Precision:Precision of readings is the agreement of the readings amongst themselves i.e. if same value is measured many times or is repeated many times and all the results agree very closely, then the instrument is said to have high degree of precision. Sensitivity:The sensitivity of an instrument is usually taken to be the size of the deflection, produced by the instrument for a given change in the measured variable. In other words, it means, it is the smallest change in the measured quantity, to which the instrument responds. It is of interest to note, the largest change in the measured variable to which the instrument does not respond, is called dead zone. Errors and Adjustments:When calibrating, instruments, say a Bourdon Pressure gauge for example, one comes across many errors and adjustments commonly found with such instruments. Refer to Figures next page.(1) Zero error: Zero error adjustment changes the base point without changing the slope or shape of the calibration curve. It is usually achieved by rotating the Indicator pointer, relative to the secondary gear or element on which the pointer is mounted. (2) Angularity error: Angularity adjustment changes the curve shape without altering base point and alters scale calibration at the ends. This error is corrected by ensuring that link arms are perpendicular with the pointer at mid-scale.(3) Multiplication error: Multiplication or magnification adjustment alters the slope without changing the base point or shape. In Bourbon pressure gauge, it is affected by altering the drive linkage length ratios between primary element and indicator pointer.(4) Hysteresis:The hysteresis is the difference between the readings obtained when a given value of the measured variable is approached from below, and when the same value is approached from the above. It is usually caused by friction or backlash in the instrument movement or by changes in the controlling spring. Curves of this nature give a complete statement of the accuracy of the instrument, and may be used for correcting the instrument reading. Where the both curves coincide, there is no hysteresis error. Note that ¿¿Backlash¿¿ is the play or false movement in the meshed gearing. That is, for a small amount of primary movement, there is no corresponding movement.

CALIBRATION

How to Calibrate Pressure measuring instrument

Manometers: -Manometers are calibrated by checking the accuracy of the scale against a standard length and checking that the scale correctly indicates the difference in mercury levels over the full manometer range. Care should be taken to avoid errors arising from parallax when reading the manometer scales. Bourdon gauges: -High quality manometers with vernier scales can be used for calibration purposes at pressure upto 2 bar. A range of pressure can be accommodated by the use of different density filling liquids, and for good accuracy due consideration must be given to variations in both temperature and gravitation.Dead weight testers (refer to Fig. below), are used for calibration within a pressure range of about 0.7 bar to 400 bar. As can be seen from the figure, the dead weight tester is a hydraulic device, which uses accurately known weights W, acting on the piston P to produce a pressure which is transmitted via the hydraulic medium to the gauge under test. For accurate results, due consideration must be given to variations in the value of the local gravitational field.