800 range sensor 120ohms strain gauge

8/2/2019 800 Range Sensor 120Ohms Strain Gauge

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dataTaker120 strain gauge example Page 1

Strain Gauge 800 range logger example.

1. Materials Required.............................................................................................................................. 22. Programming a strain gauge to measure PPM.................................................................................... 2

3. Changing the scaling of a strain gauge from PPM to strain ............................................................. 74. Calculating the Strain value of a shunt resistor .............................................................................. 11

5. Scaling a strain gauge to a practical measurement. .......................................................................... 11

6. The effect a low precision resistor makes on the measurement........................................................ 19


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1. Materials Required PC with DeLogger loaded.

800 range dataTaker data logger

120 strain gauge 3x120 precision resistor

59880 shunt resistor

120 5% carbon resistor

Set of 50cent coins

Multimeter

2. Programming a strain gauge to measure PPM.

2.1. Objective

When presented with a strain gauge of unknown wiring and unknown resistance we need to be able todetermine how to wire the strain gauge and how to determine the wiring configuration. We then need to

program the logger.

2.2. Prerequisites

PC connected to a data logger.

DeLogger running and a connection established to the logger

2.3. Steps

Determine the number of wires

We need to simply count thenumber of connection wires forthe sensor

In this case because it has only three wires it can ONLY be a quarter or half bridge. A fullbridge MUST have 4 Wires

Measure the resistance between each pair of wires

Red-White=120.4Red-Black=120.4Black-White= 0.3


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Here is a full Bridge. The Vo is afunction of the V and the variousresistances

Full Bridge will use a strain

gauge for each resistor. We cansee that it must use at least 4wires.

If the resistors of the strain gaugeis nominally 120The resistance across the bridgewould be:

=

=+=

++

+=

120

008333..0041667.0041667.

1

120120

1

120120

11

R

R

R

The Resistance across one legwould be:

=

=+=

+++=

90

01111.002777.008333.1

120120120

1

120

11

R

R

R


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A Half bridge will require externalresistors when used in a fullbridge channel (i.e. R1 and R2will be required) alternatively itwill not require external resistors

when used in a dataTaker halfbridge channel. (The resistors areeffectively internal to the logger).The half Bridge will generallyhave 3 wires

If the strain gauge is nominally120. You will measureapproximately 120 between thecenter leg and the outer leg andapproximately 240 between the

outer legsThe supplied strain gauge isobviously not this configuration

A Quarter bridge will require 3external resistors (R1, R2 & R3)when connected to a dataTakerchannel. The quarter bridge willgenerally have 3 wires.If the strain gauge is nominally120. You should measureapproximately 120 between two

of the legs and a short circuitbetween another two of the legs

The strain gauge supplied isobviously this type


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There are a number of wiringoptions available. The 800 rangeloggers support voltage andcurrent excitation, 6 wire and 4wire bridges. When used with this

type of quarter bridge straingauge resistors R1, R2 & R3must be supplied externally. Inthis case we are going to use a 4wire current excitationconfiguration (see figure 123dt800 manual) . There are a 10different wiring and configurationpossibilities. (figures 116~125)

Wire the strain gauge using the 3supplied precision 120

resistors.

We have a 500 range loggerconnected to our PC andDeLogger is currently running.In the Prog1.dl8 window.Click R1-Signal and select Bridge


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Click the wiring Button

In this case the default wiring isappropriate.

Enter the Nominal resistance ofthe gauge as a FactorDoble click R1-Factor and enter120


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N= Number of active gauge elements (in this case 1)

If the gauge factor isunknown it is generallyassumed to be 2.Documentation supplied with

the strain gauge will include astatement of the gauge factoralso usually the temperaturecompensation the needs tobe applied.The factor given in this caseis 2.1000.5% (at 24 C).

The Gauge factor in this caseis therefore

1.904811.2

4

4

=

=

=

NG

The Scaling Tab

Click 1-Type

Select Polynomial

Set Coefficient 0 to 0Set Coefficient 1 to 1.9048

Set Text Units to uStrain


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Select Tab A

Click R1-Scaling

Select Y1-uStrain

Send the program to the

logger

In the text window we cannow see the strain figurebeing returned. We observethe current reading is 323strain. If the test piece is inan unloaded state we maywant to adjust this figure tozero. To do this we adjust thescaling factor accordingly.


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Select the Scaling Tab onProg1.dl8

Set Coefficient 0 to -323

Send the Program to thelogger

Program> Send toConnection

Switch to the Text windowand note the approximatelyzero micro strain reading


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4. Calculating the Strain value of a shunt resistor

4.1. Objective

When we are using strain gauges and we are required to calculate strain it can be aconfusing process. The ability to provide an offset of a known strain value can make things

simpler

4.2. Prerequisites

Changing the scaling of a strain gauge from PPM to strain

4.3. Steps

Using the 59880 shunt supplied put it in parallel to the gauge. If the gauge factor isassumed to be 2 this resistor represents 1000 Strain. (Refer to the calculation on theslides.

Note: when the shunt resistor

is place in parallel across thestrain gauge (i.e. between the+ and terminals of channelone) we obtain approximately1000 Strain. The differencebeing between the gaugefactor of 2.0 assumed for thecalculation of the resistancefactor of and the actualgauge factor of 2.1 is thereason for reading 952

instead of the 1000 expected.If we multiply the 1000*2 anddivide by 2.1 we obtain 952.

5. Scaling a strain gauge to a practical measurement.

5.1. Objective

We can use a strain gauge to scale many physical phenomena. If we place the gauge as a cantilever and

progressively place 50-cent pieces on the gauge we can rescale the gauge to measure in units of 50cents.

5.2. Prerequisites

5.3. Steps

Remove the scaling from the logger so the logger reports in PPM and increasethe number of samples.


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Arrange the strip with theload cell as a cantilever andin such a way coins can beadded to the end.

On Tab A in Prog1.dl8

Set the Scaling to None

Select the Parameters TabSet P46 to 200


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Send the Program to thelogger

Program>Send toConnection

Switch to the Text.dltwindow.Record the Current readingsto one PPM.In this case 105ppm

Make a table of the PPM vsCoins

PPM Coins

105 0

110 1

115 2

119 3

124 4

129 5134 6

138 7

143 8

Switch to the Text.dlt window

147 9


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Open Excel and input thedata.

Highlight the data.

Click the Graph WizardButton

Select XY Scatter

Click Next

Click Next


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Click Next

Click Finish

Select the data Points.

Right Click and select AddTrend Line

Select Linear (in this case itis the most appropriate.

Click the Options Tab


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Check Display Equation onchart

Record the equation

In this case

y = 0.2131x - 22.431

Please Note: Sometimesexcel does not display thesenumbers to an appropriatenumber of digits. To change

the number of digitsdisplayed. Right Click on theequation and select FormatData Labels


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Select the Number Tab

Select Category Number

Set the Decimal Places to an

appropriate number.

Click OK

Note the equation isdisplayed to a higherprecision i.e.

y = 0.213059x - 22.430632

In Prog1.dl8Set Coefficient 0 to

22.430632Set Coefficient 1 to 0.213059

Set the units to Coins

Select the A Tab and Set the

scaling to Coins

Send the Program to thelogger.


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Select Form1.dlt

Ensure the Form window is

in Edit Mode

Click Channel 1 Cell

Select the appropriatechannel

Dt500 Com1.JOB1.A.BGIIn this case

Click OK

Click the Value 1 Cell

Click the 0 decimal placessetting


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Click the Edit Mode Button

The Form will now displaythe correct number of coinsplaced on the cantilever.

6. The effect a low precision resistor makes on the measurement.

6.1. Objective

We will replace the precision resistor with a 5% carbon resistor to observe the effect this has upon themeasurement.


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6.2. Prerequisites

6.3. Steps

Remove one of the precisionresistors and replace it with

a 5% carbon 120 resistor.

Remove the scaling from theProg1.dl8Click Scaling and selectNone

Send the program to thelogger

Select the Chart window.Click the Select ChannelsButton


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dataTaker

Select Channel 1

Dt800 Com1.JOB1.A.1BGIfor the Y axis

Click OK

Click the Edit Mode Button

Click The Re-Scale Button

Use the Left or Right Mousekeys to modify the graphappropriately. Hold the 120resistor and note how thereading varies wildly basedupon the currenttemperature. These resistorsare simply not usable as abridge completion resistors

800 range sensor 120ohms strain gauge

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