13. calibration of platform scale

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13. Calibration of Platform

Scale

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1. OBJECTIVES

a. To be able to know the principle behind theprinciple the operation of weighing scale.

b. To learn how to calibrate the platform scale.

c. To determine the accuracy and sensitivity ofthe platform scale.

d. To determine the leverage ratio.

e. To be familiar with the commercial types of

weighing scale.

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2. THEORY / HYPOTHESIS

Weight and forces are measured bycomparison with known weights with the aid ofa system levers such as in a beam balance, or byreference to an amount they deform some

elastic object such as a spring, which has beenpreviously calibrated against standard weights.

Platform scales have been constructed to weigha million pounds or more, as in industrial and

power plants, while the chemical balance, atthe opposite extreme, will easily weigh amillionth of a pound.

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(cont..) The sensitiveness of a platform scale is

determined by placing a large weight on theplatform, weighing it, and then finding thesmallest additional weight that will cause a

deflection of the beam which can be solelybalanced by the rider.

Platform scales are probably used more inengineering work about a power plant than any

other measuring device, and usually youngengineers do not very well understand theiroperation.

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(cont..) They are consists essentially by a device by

which a load applied to a systems of levers of

long and short arms, arranged so that a load on

the platform can be balanced by weights

applied at the end of a final lever called the

beam and by shifting a poise along the length of

the lever. This weighing beam is usually place

on an upright post at one end of the platform.

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(cont..)

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(cont..) The platform scale is arranged so that a large

weight, W to be measure, may be balanced by a

small weight on the beam, B. The small weight

is either or both P on the poise or R, the rider,

acting at a variable distance from the beam

fulcrum. If P balances W, then by the principle

of moments,

7

c

g

f

eW

c

dWF

b

aP

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(cont..) It is arranged that

so that

And the leverage ratio is

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c

g

f

e

c

d

c

dW

b

aP

bd

ac

P

WratioLeverage


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(cont..) A better method of obtaining the leverage ratio

consists of balancing a standard weight on the

poise with a standard weight on the platform.

Then from these data, calculate the leverage

ac/bd.

Examinations of the weights are supposed to be

balanced on the platform. In case the scale do

not balance in making these test, a little shotcan be added to the poise or weight until a

balance is obtained.

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(cont..) In calibrating platform scales against known

weights, the scales should be first thrown out

adjustment that is the adjustingcounterpoise

should be moved from its present setting. To

adjust the platform scale, the adjusting

counterpoise is moved until a balance is

obtained. This should be done without any

weight on the scale. Standard weights nowshould be placed on the scale in 5 lb.

increments and the scale is calibrated.

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(cont..) The weight shall be removed in a similar

manner and the scale calibrated as the weights

are removed.

It is also well to calibrate the scales withweights placed on the four corners and

compare these values with those obtained with

weights at the corner or center.

The sensitiveness of the scale is determined byfinding the smallest added weight that will

affect the balanced position of the beam.

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(cont..) Sensitiveness = initial reading. final reading.

The sensitiveness of a platform scale is

determined by placing a large weight on the

platform, weighing it, and then finding thesmallest additional weight that will cause a

deflection of the beam which can be solely

balanced by the rider.

Effect on Position

12

5

54321 SSSSS

Valueave

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3. LIST OF APPARATUS:

a. Platform Balance

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(cont..) b. Set of Counter Weights

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(cont..) c. Set of Standard Weights

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(cont..) d. Screw Driver

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4. SET-UP OF APPARATUS:

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5. PROCEDURE:

A. Test for Accuracy 1. Start by setting the scale to zero reading and

set the center beam between the upper and lowerstops by making screw adjustments.

2. With an initial weight of 15 kg place on one ofthe corners of the platform.

3. Then, transfer it into the second corner to thethird, fourth corner and through the center each

time getting the reading. 4. Next, add 15kg and repeat procedure no. 2.

5. Make a total of 5 trials each time increasingload by 15 kgs.

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5. PROCEDURE: (cont..)

B. Test for Accuracy 1. Take 9 kg as the load.

2. Adjust the movable poise such that the centerbeam is midway the two stops.

3. Move the poise little by little to the left untilthe center beam just touches the upper stops andget the reading.

4. Then repeat procedure no. 2.

5. Move poise to the right until the center beam

just touches the lower stops and get the reading. 6. Reduce the load by 9 kg increment and repeat

the procedure for the rest of the trials.

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5. PROCEDURE: (cont..)

C. Leverage Ratio

1. Set the scale to zero reading.

2. Load 90 kg on the platform balance.

3. Add counter weights and adjust themovable poise until the beam is midway the

two stops and get the reading.

4. Remove the 90 kg load on the platform and

measure the counterweights used in step no. 3.

5. Divide the measured weights by the load to

obtain the leverage ratio.

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6. DATA AND RESULTS:

A. Accuracy

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6. DATA AND RESULTS: (cont..)

B. Sensitivity

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7. SAMPLE COMPUTATIONS:

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8. QUESTIONS:

1. A 1500 kg mass is accelerated to 10 m/sec2,calculate the force required to perform thisacceleration horizontally and vertically whereacceleration due to gravity is 9.7 m/sec2.

2. A body weighs 2000 lb force when exposedto the earths standard gravitationalacceleration. What is its mass in kg?

3. What will be the weight of the body in

Problem no. 2 if it is exposed to the moonsstandard acceleration of 1.62 m/s2? Expressyour answer in Newton.

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8. QUESTIONS: (cont..)

4. How fast is the body accelerating inproblem no. 2 if a net force of 1000 lb is applied

to it on the moon or earth? Express your

answer in ft/s2.

5. How would the sensitiveness of a platform

scale be determined?

6. With the aid of a diagram, define the

leverage ratio of the platform scale used in theexperiment?

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8. QUESTIONS: (cont..)

7. If there are 454 grams in a lb mass, whatwill this mass weigh in grams, at an altitudewhere g = 20.6 ft/sec2? What is the mass ingrams at the point?

8. If the platform balance is raised to anelevation of 5000 meters, will there be anychange in the scale reading? Explain.

9. Differentiate a platform scale from a spring

balance. 10. What is a torsion balance?

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9. ANSWERS:

10. DISCUSSION:

11. CONCLUSION / RECOMMENDATION:

12. REFERENCES:

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END

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13. calibration of platform scale

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