PRESSURE MEASUREMENT

OBJECTIVE 

To understand the pressure measurement techniques and its accuracy. accuracy.

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To investigate the calibration system on pressure measurement unit.

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To differentiate absolute pressure and gauge pressure.

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To study on properties of gas pressure.

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To become familiar in handling the pressure measuring instrument in plant.

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SUMMARY

This experiment was conducted with the purpose of understanding pressure measurement and calibration system. It is important to analyze gas pressure properties and differentiate between Gage pressure, where the reference is atmospheric pressure (psig or kPag), Absolute pressure, where the reference is complete vacuum (psia or kPa) and Differential pressure, which represents the difference between two pressure levels. Calibration system is important in obtaining accurate readings. It compa res a measurement made by an instrument being tested to that of a more accurate instrument to detect errors in the instrument being tested. Errors are acceptable if they are within a permissible limit. Manometer used to obtain pressure while transmitter used to convert pressure value to electrical output. Three experiments was conducted, where Experiment 1 for pressure measurement and Experiment 2 and Experiment 3 for calibration. As for Experiment 1 seven Run c arried out with different suggested pressures each twice to obtain average values. As in theory, the results shows that the gage pressure is always lower than absolute pressure (gage pressure + vacumm pressure). Pressure deviation always exist throughout whole experiment. This is due to factors such as sensitivity of instruments and leakage from connecters. As for Experiment 2 and 3 which is related to calibration checking reveals that most of the percentage deviation is above the permissible limits. Desired output values unable to achieved, which cause most of them fail except calibration at 0.30/ 15.00 at 25.0% is pass for PT1 and for PT2 pass at 15.00 at 50.0% and 30.00 at 100.0% respectively. The lowest percentage deviation is 0.0625. This situation or error occurred during pressing external hand pump to generate desired pressure. Constant force have to applied to achieve the targeted pressure but unable to achieve due tired of pressing for long period.

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INTRODUCTION AND THEORY Pressure is measured as a force per unit area. P ressure measurements are important not only for the monitoring and control of pressure itself but also for measuring other parameters, such as level and flow (through differential pressure). Pressure measurement is one of the most commo n measurements made in process control. It is also one of the simplest in terms of which measuring device to select. One of the key items to consider is the primary element (i.e., strain gage, Bourdon tube, spiral, etc.). Primary-element materials should be selected to provide sufficient immunity from the process fluids and at the same time the required measured accuracy under the process conditions they will encounter.

Pressure-measuring instruments are really pressure transducers that convert the pressure energy into a measurable mechanical or electrical energ y. Pressure measurement is always made with respect to a reference point. There are basically three types of pressure-sensing configurations . 1. Gage pressure, where the reference is atmospheric pressure (psig or kPag) 2. Absolute pressure, where the reference is comp lete vacuum (psia or kPa) 3. Differential pressure, which represents the difference between two pressure levels (note that gage pressure is a differential pressure between a v alue and atmospheric pressure, 14.7 psia)

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Principle of Measurement (Pressure sensor device and calibration)

The manometer is based on the principle of hydrostatic pressure and on the relationship between pressure and the corresponding displacement of a column of liquid. The same principles apply to the U-tube, where the process pressure supports a column of liquid of known density. The height of the liquid column is then read on a graduated scale. Pressure applied to the surface of one leg causes a liquid elevation in the other leg. Generally, the unknown pressure is applied to one leg and a reference pressure (typically atmospheric pressure) to the other. The amount of elevation is read on a scale that is calibrated to read directly in pressure units.

Calibration of control equipment is a key maintenance activity. It is needed to ensure that the accuracy designed into the control system as a whole is maintained. Calibration is performed in accordance with written procedures. It compares a measurement made by an instrument being tested to that of a more accurate instrument to detect errors in the instrument being tested. Errors are acceptable if they are within a permissible limit.

Transmitters

A typical pressure transmitter consists of two parts: the primary element and the secondary element. The primary element (which includes the pressure sensor or pressure element) converts the pressure into a mechanical or electrical value to be read by the secondary element. It is the part that is most subject to failure since it faces the process conditions. The secondary element is the transmitter’s electronics: basical ly,

a transducer to convert the output from the primary

element into a readable signal such as 4-20 mA. Typically, electronicbased sensors such as strain gages have a better response and a higher accuracy than mechanical- based types such as Bourdons (which are still acceptable in many applications).

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RESULTS AND DISCUSSIONS Results TABLE 1: Pressure Measurement RUN I

SET 1

SET 2

Average

A

Suggested Pressure, psig

30.00

30.00

30.00

B

Actual Gauge Pressure, psig

30.35

30.20

30.28

C

Gauge Pressure, psig

30.00

30.00

30.00

D


30.30

30.50

30.40

E

Absolute Pressure, psia

44.70

44.85

44.78

F=B+ 14.70

Calculated Absolute Pressure, psia

45.05

44.90

44.98

0.35

0.20

0.28

0.05

0.30

0.12

0.35

0.05

0.20

SET 1

SET 2

Average

G1 = | B — C |

At Pressure Gauge Gauge Pressure

G2 = | B — D |

Deviation, psig

At Pressure Indicator, P12

H=|F—E|

Absolute Pressure Deviation, psia

RUN II A


25.00

25.00

25.00

B


25.45

26.10

25.78

C


25.00

25.00

25.00

D


25.70

25.75

25.73

E


40.20

39.80

40.00

F=B+ 14.70


40.15

40.80

40.48

0.45

1.10

0.78

0.25

0.35

0.03

0.05

1.00

0.48

G1 = lB — Cl

At Pressure Gauge Gauge Pressure Deviation, psig

G2 = lB — Dl

H = IF — El

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RUN III

SET 1

SET 2

Average

A


20.00

20.00

20.00

B


20.05

20.20

20.13

C


20.00

20.00

20.00

D


20.20

21.00

20.60

E


34.70

35.20

34.95

F=B+ 14.70


34.75

34.90

34.83

0.05

0.20

0.13

0.15

0.80

0.47

0.05

0.30

0.12

SET 1

SET 2

Average

G1 = lB — Cl


G2 = lB — Dl

Deviation, psig


H = IF — El


RUN IV A


15.00

15.00

15.00

B


15.20

15.50

15.35

C


15.00

15.00

15.00

D


15.50

15.75

15.63

E


30.00

30.20

30.10

F=B+ 14.70


29.90

30.20

30.05

0.20

0.50

0.35

0.30

0.25

0.28

0.10

0.00

0.05

G1 = lB — Cl


G2 = lB — Dl

Deviation, psig


H = IF — El

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RUN V

SET 1

SET 2

Average

A


10.00

10.00

10.00

B


10.10

9.95

10.02

C


10.00

10.00

10.00

D


10.00

10.30

10.15

E


24.50

25.00

24.75

F=B+ 14.70


24.80

24.65

24.72

0.10

0.05

0.02

0.10

0.35

0.13

0.30

0.35

0.03

SET 1

SET 2

Average

G1 = lB — Cl


G2 = lB — Dl

Deviation, psig


H = IF — El


RUN VII A


0.00

0.00

0.00

B


0.06

0.07

0.065

C


0.00

0.00

0.00

D


0.40

0.55

0.48

E


14.80

14.40

14.60

F=B+ 14.70


14.76

14.77

14.77

0.06

0.07

0.065

0.34

0.48

0.42

0.04

0.37

0.17

G1 = lB — Cl


G2 = lB — Dl

Deviation, psig


H = IF — El

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RUN VI

SET 1

SET 2

Average

A


5.00

5.00

5.00

B


5.74

5.35

5.55

C


5.00

5.00

5.00

D


5.70

5.65

5.68

E


20.20

21.50

20.85

F=B+ 14.70


20.44

20.05

20.25

0.74

0.35

0.55

0.04

0.30

0.13

0.24

1.45

0.60

G1 = lB — Cl

At Pressure Gauge Gauge Pressure Deviation, psig

G2 = lB — Dl


H = IF — El


The data or readings collected from Run 1 until Run 7 shows that, for each suggested pressure there is pressure deviation. The suggested pressure cannot be achieved normally due to some factors. The average reading is took to get accurate result and at the same time to minimize the pressure deviation.

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As for pressure measurement calibration , the checklist of PT1 and PT2 reveals that Output Pressure, psia (Column B) and Measured Output, mA (column C) is different when compared with Input Pressure Applied, psig/psia at% of Calibration Range (column A) and Theoretical Output, mA (column D) respectively. It shows that the targeted pressure and current unable to achieved which result in high percentage deviation than stipulated deviation limit. Only pressure calibration at 0.30/ 15.00 at 25.0% is pass for PT1 and for PT2 pass at 15.00 at 50.0% and 30.00 at 100.0% respectively. The lowest percentage deviation is 0.0625.

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Disscusion

Pressure measurement and calibration system is one of main aspect or measurement system under process instrumentation. This experiment was conducted with the purpose of study about pressure measurement tactics and calibration system to find percentage of error. Gauge pressure and absolute pressure is oftenly related to pressure measurement. Differential pressure, which represents the difference between two pressure levels (note that gage pressure is a differential pressure between a value and atmospheric pressure, 14.7 psia). As for this experiment, pressure measurement skills was tested and calibration system of pressure is performed by anal yzing differential pressure or percentage of deviation between desired output and calculated output results. The digital manometer, pressure gauge and p ressure sensor / indicator were used to measure the pressure. The pressure transmitter was used to convert pressure into electrical output. It consist primary element (which includes the pressure sensor or p ressure element) converts the pressure into a mechanical or electrical value to be read by the secondary element.

As for Experiment 1 (Measurement) the readings of pressure was measured at different gas pressure level by run or repeat the experiment under suggested pressure,psig 30.00, 25.00, 20.00, 15.00, 10.00, 5.00, and 0.00. The pressure is measuremed by take considerations on suggested pressure. The value of F is calculated by adding the 14.70 psia, standard atmospheric pressure plus with actual gage pressure. Experiment 1 shown that gauge pressure always smaller than absolute pressure (gauge pressure + vaccum pressure). From Run 1 until Run 7, the result obtained obviously describes the situations encountered during takin g the reading of pressure where it is very hard to achieved the targeted pressure. Not even at least one of Runs from Run1 to Run7 resulted in zero pressure deviation eventhough each Run is done twice by taking their average values. The deviations shown by gauge pressure deviation,psig and absolute pressure deviation,psia is higher. Besides that, the gauge p ressure deviation, psig shown at pressure gauge and at pressure indicator, P12 is quite different in terms of range of values obtained. The gauge pressure deviation almost higher for all Runs shown higher value compared to deviation shown at Pressure Indicator P12.

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This situations might be lead by some factors. One of the factor is the sensitivity of the pressure measurement instrument which is pressure meters and manometers. It always leads to trouble in terms of pressure measurement accuracy. As for example, in Run 2 the

absolute pressure

deviation is 0.40psia, which is smaller in value but actually high in terms of accuracy. Morever, the connecter from manometer to PT1 sometime unable to fixed well hence there is leakage occurs which affect the results that must be achiev ed.

Calibration system is another next aspect plays a vital role in pressure measurement. A best calibration system management will lead to obtain accurate results. Calibration of control equipment is a key maintenance activity. It is needed to ensure that the accuracy designed into the control system as a whole is maintained. Calibration is performed in accordance with written procedures. It compares a measurement made by an instrument being tested to that of a more accurate instrument to detect errors in the instrument being tested. Errors are ac ceptable if they are within a permissible limit.

As for calibaration system checking two Runs was conducted, Run 1( PI1 and Multimeter as reference) and Run 2 (PI2 abd Multimeter as reference). This experiments was conducted to study calibration techniques rather than gas pressure properties. PT1 Site Calibration Check and PT2 Site Calibration Check shows that Output P ressure, psia (Column B) and Measured Output, mA (column C) is different when compared with Inpu t Pressure Applied, psig/psia at % of Calibration Range (column A) and Theoretical Output, mA (column D) respectively. It shows that the targeted pressure and current unable to achieved which result in high percentage deviation than stipulated deviation limit. Only pressure calibration at 0.30/ 1 5.00 at 25.0% is pass for PT1 and for PT2 pass at 15.00 at 50.0% and 30.00 at 100.0% respectively. The lowest percentage deviation is 0.0625. Highest percentage deviation for Run 1 is 9.36 and for Run 2 is 0.75.

There are some factors which affected the output result which lead to large deviations and more percentage of accuracy. Error might occurred during pressing external hand pump to generate desired pressure. Constant force have to applied to achive the targeted pressure but unable to achieve due to tired or fatigue of pressing for long period. 13 | P a g e

CONCLUSION AND RECOMMENDATIONS Conclusion This experiment was designed to investigate the relationship between gauge pressure, absolute pressure, and diferential pressure. Pressure measurement and calibration system is mainly studied in this experiment. This experiment conducted in two way where Experiment 1 for pressure measurement and Experiment 2 and 3 for calibration system. Experiment 1 shown that gauge pressure always smaller than absolute pressure (gauge pressure + vaccum pressure). Eventhough, experiment 1 runs for seven time with pressure,psig 30.00, 25.00,20.00, 15.00, 10.00, 5.00 and 0.00 each twice to obtain average result but there is always present deviations. Value of deviations quite high. This due to some factors such as sensitivity of the pressure measuring instrument (manometer in this experiment) and the leakage in connecters which not well fixed to gas tank. As for Experiment 2 and 3, most of the percentage deviation is above the permissible limits. Desired output values unable to achieved, which cause most of them fail except calibration at 0.30/ 15.00 at 25.0% is pass for PT1 and for PT2 pass at 15.00 at 50.0% and 30.00 at 100.0% respectively. The lowest percentage deviation is 0.0625. This situation or error occurred during pressing external hand pump to generate desired pressure. Constant force have to applied to achieve the targeted pressure but unable to achieve due tired of pressing for long period.

Recommendations

First of all, the system must be checked regulary to be in good condition where it can used in future. Maintenance have to do on system if needed. Makesure the pressure measuring instrument present in good condition with correct accurac y, because it will give fault result if not checked well. There must be insulation made on the hand pump handle to reduce the friction between handle and hand palm. This will prevent the hand palm from getting injured and force can be applied continuously without tired fastly. Morever, makesure that the readings becomes stable become record any measurements. Connecter from manometer to gas tank must be in proper condition without any leakages.

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TUTORIAL

1) The pressure is higher at the bottom of a tank filled with gas than at the top. This is because higher pressure (mass of gas) is exerted at higher depth a nd surface area for molecules to contact becomes bigger. 2) Absolute pressure - pa - is measured relative to the absolute zero pressure - the pressure that would occur at absolute vacuum. All calculation involving the gas law requires pressure (and temperature) to be in absolute units. Gauge Pressure - A gauge is often used to measure the pressure difference between a system and the surrounding atmosphere. This pressure is often called the gauge pressure.Therefore Gauge pressure is measured from atmospheric and absolute is measured from 0 (as all absolute scales are measured from). They both use the same scal for measuring. Standard atmosphere at sea level is 14.7lb/in^2 of pressure absolute and 0 pressure gauge. 3) The gas pressure will decrease. When gas compressed to a small volume tank, the pressure will be higher due to high frequency collisions within gas molecules. But, when the gas is filled in big volume tank the pressure will be lower due to low frequency collision of gas molecules which have more free space to move. 4) If the gas tank is heated the gas pressure will be higher due to higher kinetic energy among gas molecules which hits with tank wall. 5) Manometer and pressure gauge were used as pressure measuring instrument in this plant. 6) Error might occurred during pressing external hand pump to generate desired pressure. Constant force have to applied to achieve the targeted pressure but unable to achieve due tired of pressing for long period. Morever, connection of tube from manometer to gas tank is very sensitive. Sometime, there is leakage and sometime it n ot well connected. Hence, reading accuracy will be affected.

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REFERENCES

INTERNET

1. http://www.efunda.com/formulae/fluids/manometer.cfm 2. http://en.wikipedia.org/wiki/Calibration 3. http://en.wikipedia.org/wiki/Pressure_measurement 4. http://global.britannica.com/EBchecked/topic/475455/pressure-gauge

E-BOOK

5. Introduction to Instrumentation and Control http://books.google.com.my/books/about/Introduction_to_Instrumentation_and_Cont.htm l?id=CMz41xHI8PkC&redir_esc=y

6. The condensed handbook of measurement and control, 3rd edition http://books.google.com.my/books?id=eajpAAAAMAAJ&q=ebook+condensed+instrum entation&dq=ebook+condensed+instrumentation&hl=en&sa=X&ei=2hMTULxAsLMrQfAioHgDg&ved=0CD0Q6AEwAA

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PRESSURE MEASUREMENT

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