Thermocouples

Objectives

- To describe how the thermocouple functions.
- To describe the use o a thermocouple with a Wheatstone bridge circuit for the measurement of temperature.
- To know the different types of thermocouples.
- To calculate the rise time of a thermocouple.

Introduction:

The measurement of temperature in industrial processing is an important concern. Temperature is the most important effect on certain manufacturing processes as well as on the physical manufacturing environment. Without information about the temperature of materials & its environment, manufacturing technology would be crude by today's standards. Temperature transducers are utilized to convert temperature changes into electrical changes. The electrical variations can be displayed on a meter or recorder. The thermal transducers most often employed are the thermocouples, thermistors & RTD.
Apparatus:

- Master builder S300B.
- Insertion panel SIP380-1.
- Digital multimeter.
- Hand made type J (iron-constantan) thermocouple wire & commercial type J thermocouple wire.
- Electric solder.
- Stop watch.
- Connection leads.


Review & discussion:

- What is a thermocouple?

It is a thermal transducer or thermal sensor that converts temperature changes into electrical changes; it is formed by a junction of two dissimilar metals, such as iron & constantan (type-J) or chromel & tophel (type-K). The junction when heated it produces a small voltage & associated current flow, the voltage developed is directly proportional to the junction's temp.

- What determines the amplitude of change at the output of a thermocouple amplifier?

The type of the materials used in making a thermocouple & the supplied temperature.

- Refer to figures 11-9 & 11-10 in the manual. Which thermopile would produce the largest voltage deflection?

The thermopile in fig. 11-10 because the thermocouples are connected in series which causes a larger voltage difference than the connection in fig. 11-9.

- What are thermopiles?

Thermopiles are series & parallel groups of thermocouples that can be used to increase both the voltage & current outputs of a group of thermocouples.

- Explain the term "time constant"

Time constant (τ) is the time needed to reach 63.8% of the final value in a transient response device.

- What metals are used to construct a J-type thermocouple & K-type?

J-type: Iron-Constantan.
K-type: Chromel tophel-Alumel.

- Refer to figure 11-4 in the manual. Name a type of thermocouple which could be used to 4000o F.

According to the figure we can see that a (C-type) thermocouple can be used to 4000o F.

- Which metal is the positive element of a J-type & K-type thermocouple? (Use fig. 11-6 in the manual).

J-type: Iron.
K-type: Chromel.

- What type of thermocouple can be used to 2300o F & would yield a high output voltage?

According to figure 11-4 in the manual the answer is (K-type) thermocouple.

Sin Bar and Angular Measurements

Objectives

To get familiar with the use of bevel protractor, vernier protractor & clinometer for measuring angles.

Apparatus

• Plain bevel protractor.
• Vernier protractor
• Clinometer
• A metallic triangular specimen

Plain bevel protractor:

When two surfaces are at any angle other than 90o, the angle between them must be tested with some form of protractor. Instruments for this purpose may have a scale of degrees, enabling the angle to be read off, or they may consist of gauge which must be set to the angle before use. And the bevel protractor in fig. 1 is an example of this second variety of gauges, & must be set to the correct angle before use.



Vernier Protractor:

The main scale of this device is divided up into degrees from 0o to 90o each way. The vernier scale is divided up to that 12 of its division occupying the same space as 23o on the main scale, thus 1 vernier division = 23/12 = 1.92 degree on main scale. The instrument therefore allows settings to 5 min of angle to be obtained, so we use it to get accurate readings. This device has a movable arm, plate blade, fixed nut & vernier scale, see fig. 2.



The Clinometer:

The clinometer is a special case of the application of the spirit level. In this instance the level is mounted in a ratable body carried in a housing, one face of which forms the base of the instrument. A main use of the instrument is the measurement of the included angle of two adjacent faces of a work piece. Thus, in use, the instrument base is placed on one face and the rotating table body is adjusted until a zero reading of the bubble is obtained. The angle of rotation necessary to bring this about is then shown on an angular scale moving against an index.

A second reading is taken in a similar manner in the second face of the work piece, the included angle between the faces being the difference the first & the second readings. Depending upon the type of instrument used, readings direct to 1 min are obtained, & up to a range of movement of 90o, see fig. 3.

Main scale = 360o.

Vernier scale: Full revolution = 1o = 60 min, so each division on vernier scale = 1 min.



Procedure

First of all we clean the component & remove the burrs if there are any, also make sure that the surface plate is nice & clean. In order to avoid compound angle error care must be taken in aligning the work piece with the measuring tool.

Then we take the readings of angles 1, 2 & 3 on the specimen using each of the provided equipment then fill the results in table 1.


Discussion

The personal skills needed for taking angular measurements are to know exactly how to fit the desired angle on the specimen on the measuring device & holding it still, an additional skill is needed when using the vernier protractor which is to know how to take the reading precisely.

The overall time needed to take the readings depends on the personal skills of the one taking the reading.

Linear Measurement

Objectives

To familiarize the student with the types, applications of calipers, micrometers and measurements.

At the completion of this experiment, the student will be able to:

1. Get familiar to the variety of the linear measurement tools, and know the type of a measurement tool needed to achieve a certain measurement.

2. Students will seek more efficient means of measure.

3. Take linear measurements with a certain accuracy depending on the instrument being used.

4. Clean, care for and store calipers, micrometers and dial indicators.

Apparatus

a. vernier caliper

b. dial caliper.

c. External micrometer.

d. 2-point inside micrometer.

Theory

  General specifications and functions of a caliper:

1. All stainless steel construction.

2. used to measure inside dimensions.

3. used to measure outside dimensions.

4. used to measure step dimension

5. convenient thumbscrew to lock a measurement in place.

6. accuracy equation:

  accuracy = 1 division main scale/no. of divisions on vernier scale

Block Gauges

OBJECTIVE:

1. To be familiarized with the types, applications of block gauges.
   
2. To calibrate some measurement devices.
   

APPARATUS:

a) Set of block gauges.

b) Granite surface plate.

Theory and Background:

Block gauges are made of stainless steel,each block has 6 faces, 2 of them are opposite and parallel and are highly polished. They must be used at standard conditions, which are:

a. Atmospheric pressure.

b. Temperature between (20 – 25°C).

c. Relative humadity (62 – 75)%.

d. Pure atmosphere (no dust). 

Uses of block gauges:

1. To measure lengths.

2. Calibration of measuring devices.

3. To measure angles.

4. To measure the pressure drop ( the head Δh in case of manometer).

There are 2 types of length standards:

1. End standard:

In which the unit of the length is defined as  being the distance between the end faces of the standard, these take the form of either slip.

2. Line standard or engraved scale:

In which the unit length is defined as being the distance suitably engraved lines.

End standards:

Gauge blocks are the practical length standards of industry. Amodern end standard consists fundementally of a block (slip) or bar of steel or cemented carbide, generally hardened. Whose end faces are lapped flat and parallel within a few tenth of a micrometer.

Gauge blocks are a good example of end standards. The name end standards rightly indicate that these consist of sets of standard blocks or bars, which are used to build up a required length.

1) End standards are highly accurate and are well suited to measurements of close tolerances.

2) They are time consuming in use and provide only one dimension at a time.

3) Dimensional variations as small as 0.0005 mm can be obtained.

4) They are subject to wear on their measuring faces.

5) Groups of blocks are (wrung) together to provide a given size, faulty wringing leads to damage.

6) End standards have a built-in datum because their measuring faces are flat and parallel and can be positively located on a datum surface.

7) The accuracy of end and line standards is affected by temperature change and both are originally calibrated at 20  °C.

8) They are made in high grade cast steel.

Criterea of the specifications for the block gauges:

• According to the number of block gauges inside the box:

a) 87 pieces.

b) 48 pieces.

• According to the roughness of the block gauges.

a) Grade 1

b) Grade 2

c) Grade 0

d) Grade 00

e) Grade calibration.

Grade 00 Block Gauge set:

This grade is accompained by a calibration chart which lists the departure from nominal size of each gauge in the set.

Calibration grade:

This grade provides the highest level of accuracy required in normal engineering practice and is intended for calibrating other blocks in conjunction with suitably accurate comparators. They are used where the tolerances are 2 micrometer or less, and are not intended for general gauge inspection. This grade is cheaper than grade 00 because of broader length tolerances. (flatness and parallelism tolerances are same).

The essential difference between 00 and calibration grade gauge blocks lies in their length tolerances. Calibration gauge blocks have broader tolerance than 00 grade gauge blocks, where as the geometrical tolerances, that is flatness and parallelism of measuring faces are the same in both.

A calibration chart is supplied with the calibration grade gauge blocks, because of their broader length calibration gauges are cheaper than 00 grade gauge blocks.

A theory is borne out by the face that if two highly finished gauges are firmly wrung together and left for a period of time (3 days) they may become very strongly attached, so that the faces are damaged when they are seperated with sufficient forces.

Thickness of the ringing film has been measured at the N.P.L. among various liquids as paraffin, heavy oil and vaseline and is found to ahve nearly 6 nanometers.

Instuctions for wringing together two slip gauges:

1) Surfaces must be clean and free from burrs. They should be washed in petrol, benzene, carbon tetrachloride or other DE-greasing agents and wiped dry on a clean cloth. Then be wiped with clean soft chamois leather. It is advised to take the following points into consideration.

In order to prevent damage to blocks:

• Protect from dust, dirt and moisture.

• Avoid magnetization.

• Handle lapped faces as little as posible to prevent etching from finger acid. Wipe all finger marks with chamois leather.

• Always wipe faces immediately before use even when use it continuous.

• Always replace clean gauges in their box and close it after use. If gauges are not in frequent use they should be coated to prevent corrosion.

• Do not handle gauges above the open box, they may cause damage to other gauges if dropped.

2) Slip gauges then should be held across one another at right angles and wring them with a rotary motion. This reduces the amount of surface rubbing necessary.

3) A minute amount of grease or moisture must be present between the surfaces for them to wring satisfactory. Unless a very firm wring is obtained there is always the possibility that the wringing film may be a micrometer thick.

Discussion :

A. Why do we always choose the minimum number of block gauges combination?

• The minimum number of blocks is used in order to decrease the error introduced by the error of each block (0.016mm) where increasing the number of block gauges the uncertainty of each piece adds to the total uncertainty of the overall assembly.

• To decrease the surface irregularities and dirts.

• To minimize errors due to gap wringing thick film.

• To minimize wear due to repeated usage of component.

• Using too many block gauges is a redundant and time consuming.

B. Suggest other applications for block gauges.

Gauge blocks are used as end standards, so the are useful in making standard sizes for various parts, since the error is minimum, another use is with the sinebar for angle measurement, also for calibration purposes of other measuring devices, and finally for measuring the pressure drop.

C. In the comparator measuring method what do we compare with?

In the comparator measuring method, the standard size of the blocks is compared with the measured size of the specified part by other measuring devices. This method is used when accurate reading is essentially to be found, if it is not the case then it is recommended to use direct reading methods.

Conclusions:

• In this experiment, the block gauges were identified and used for calibration for the micrometer.

• From the table, we conclude that as the number of blocks increase, the tolerance increases due to the accumulating error for each block (0.016 mm)

• In order to prevent damage to gauge blocks:

1. Protect from dust, dirt and moisture.

2. Avoid magnetization.

3. Always wipe the faces before use.

4. Always put the blocks in their box after use.

5. Do not drop the blocks.

• End standards are highly accurate and are well suited to measurement of close tolerances.

• They are time consuming in use and provide only one dimension at a time.

• Dimensional variations as small as 0.0005mm can be obtained.

• They are subject to wear on their measuring faces

• Groups of blocks are “WRUNG” together to provide a given size; faulty wringing leads to damage.

• End standards have a build-in datum because their measuring faces are flat and parallel and that can be positively located on a datum surface.

• The accuracy of the end and line standards is affected by temperature change and both are originally calibrated at 20 °c.

• They are made in high-grade cast steel.

• Although a very firm wring is usually obtained, there is always the possibility that the wringing film maybe a micrometer thick.

Autocollimators

Introduction:

An autocollimator is an optical instrument that is used to measure small angles with very high sensitivity. As such, the autocollimator has a wide variety of applications including precision alignment, detection of angular movement, verification of angle standards, and angular monitoring over long periods.

Objectives:

1. To measure straightness of a beam with the use of Auto-Collimator.
   
2. To identify the principle of Auto-collimator device.
   
3. To be able to draw conclusions about straightness error using graphical methods and least square method.
   

Apparatus:

1. Auto collimator.
   
2. Straight edge with 100mm marked intervals.
   

Theory:

Autocollimators are sensitive and inherently very accurate optical instruments for the measurement of small angular deviations of a light reflecting flat surface. The autocollimator has its own target which is projected by collimated light beams on a remotely placed surface and the reflected target image is observed in the ocular of the instrument.

The autocollimator is stationed at the end of the bed with a rigid support base. The movement of the reflector along the bed will cause the reflected image of the target to deflect according to the angular error of the bed,

The autocollimator is a flat mirror mounted in a short tube made to fit a Newtonian telescope focuser, and set accurately perpendicular to the tube’s axis. Centered in it is a small peephole or pupil that you look through.

The autocollimator projects a beam of collimated “parallel” light. An external reflector reflects all or part of the beam back into the instrument where the beam is focused and detected by a photo detector.

The autocollimator measures the deviation between the emitted beam and the reflected beam. Because the autocollimator uses the light to measure angles, it never comes into contact with the test surface.

How to calculate tilt of 1 sec. of arc of the reflector:

 θ is the angle taken after adjusting the micrometer properly (sec)

 h is the vertical distance (to be found)

 L is the length of the reflector carriage is 0.1m in our case. 

 (These parameters are shown in the below figure)

tanθ = h/L

Assume θ = 1sec

tanθ = θ for small values.

1sec = (1/60*60)(π/180) = 4.8*10^-6rad

h = 4.8-10^-6m

i.e. in each 1sec indicates a vertical distance of approximately 0.5 micron “since L = 0.1m”

As we can see the relation is nearly linear. We connected the best fit line by connecting the first and last point by means of a straight line.

The vertical rise h as we discussed in the theory section was:

H = 4.8*10^-6 m = 0.5 μm

This rise is due to 1sec of reflection angle so 1 sec = 0.5 μm

As we can see from our results that the time needed for reflection don’t depend on the position of the carriage, this is shown by the curve above.

 

We have some human errors like the readings.

Also the beam o light may not been reflected one hundred percent.