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Abstract
The objective of these experiments was to calibrate the strain indicator by relating the actual strain in strain gauge with the indicated strain on the indicator when simulated using the strain simulator.
Introduction
Strain gauge indicator is an electrical device used to measure the strain produced in the strain gauge as the load is applied on it. Strain gauge indicator may have 2-wire or 3-wire system. For better accuracy in results the indicator is calibrated using strain gauge simulator.
Change in resistance of strain gage is related to strain produced in it through a factor called as Gauge factor. Gauge factor is unit change in resistance per unit change in length of strain gauge wire. G.F. = (āR/RG) / Īµ ------------------------------ (1) Where, G.F is gauge factor āR = change in gage resistance Īµ = strain produced
Simuator is nothing but a variable resistor as shown in fig 01. Using this simulator V/E-20A strain indicator is calibrated. This indicator is a direct readout type indicator using constant current for bridge excitation. Its range is Ā±1999(X1) with 1 count resolution and 19990 (X10) with 10 count resolution.
Fig 01
Experiment # 1: Strain Indicator Calibration
Procedure:
i. For Quarter Bridge arrangement of simulator with indicator following connection was done. a. Active wire was connected at its one end to simulatorās binding post no. 1 and other to binding post S+ on indicator using short leads. b. Two short common leads connected the simulatorās binding post 2 with P- and D120 on indicator. c. Applied resistance 120Ī© on the simulator. d. Using āInstrument zeroā knob the zero was attained on the indicatorās display with āfunction knobā on zero. e. Now the function knob was switched to read mode and using the balance coarse and fine coarse knobs zero was attained on the indicator display screen. f. Function knob was switched to Internal Calibration mode and desired calibration number (1000) for gauge factor 2 was set using span coarse and span fine knobs. ii. From Eq. 01, āR = 0.12Ī© when F = 2, R = 120Ī© and Īµ = 500 Ī¼. iii. With āR = 0.12 ohm corresponding strain was noted from the indicator. iv. āR is increased with an increment of 0.12Ī© and corresponding strain was noted from the indicator display screen till the range of indicator is exceeded. v. Using Eq. 01 the strain was calculated for corresponding value of change in resistance. vi. Readings were also noted for negative change in resistance. vii. Graphs were plotted between the actual strain (on y-axis) and indicated strain (on x-axis)
Observations and calculations:
R = 120 ohms
Table # 1: Table showing calculated and observed Strain for Positive ĪR
Sr no. R ĪR Īµ(indicated) Īµ(actual)
- 120 0. 12 500 499
- 120 0. 24 1000 998
- 120 0. 36 1500 1497
- 120 0. 48 2000 1998
Table # 2: Table showing calculated and observed Strain for Negative ĪR
Sr no. R ĪR Īµ(indicated) Īµ(actual)
- 120 - 0. 12 - 500 - 505
- 120 - 0. 24 - 1000 - 1005
- 120 - 0. 36 - 1500 - 1505
- 120 - 0. 48 - 2000 - 2005
Experiment # 2: Lead Wire Desensitization of Strain Indicator
2a) Procedure for Determination of Lead Wire resistance
Procedure: i. For Quarter Bridge arrangement of simulator with indicator following connection was done. a. Active wire was connected at its one end to simulatorās binding post no. 1 and other to binding post S+ on indicator using short leads. b. Two short common leads connected the simulatorās binding post 2 with P- and D120 on indicator. c. Applied resistance 120Ī© on the simulator. d. Using āInstrument zeroā knob the zero was attained on the indicatorās display with āfunction knobā on zero. e. Now the function knob was switched to read mode and using the balance coarse and fine coarse knobs zero was attained on the indicator display screen. f. Function knob was switched to Internal Calibration mode and desired calibration number (1000) for gauge factor 2 was set using span coarse and span fine knobs. ii. Now replaced active short wire Ls with long wire Ll. iii. As the resistance of the simulator was increased by the factor of RLl. There was a positive strain on the indicator screen display. iv. For display to be retained at zero the simulatorās resistance was manually changed and using following formula the resistance of long leads was calculated. Rl = 120 - Rāg(s)
Observations and calculations: For white wire:
Value of Rāg(s) with long leads Ll =115.40 Ī©
Value of Rāg(s) with short leads Ls = 120Ī©
Value of RLs = 120Ī© - 115.4Ī© = 4.6Ī©
For Black wire:
Value of Rāg(s) with long leads Ll =115.40 Ī©
Value of Rāg(s) with short leads Ls = 120Ī©
Value of RLs = 120Ī© - 115.4Ī© = 4.6Ī©
For red wire:
Value of Rāg(s) with long leads Ll =115.40 Ī©
Value of Rāg(s) with short leads Ls = 120Ī©
Value of RLs = 120Ī© - 115.4Ī© = 4.6Ī©
2b) Procedure of Determination of Lead Wire Desensitization
Procedure: i. For Quarter Bridge arrangement of simulator with indicator following connection was done. a. Active wire was connected at its one end to simulatorās binding post no. 1 and other to binding post S+ on indicator using short leads. b. Two short common leads connected the simulatorās binding post 2 with P- and D120 on indicator. c. Applied resistance 120Ī© on the simulator. d. Using āInstrument zeroā knob the zero was attained on the indicatorās display with āfunction knobā on zero. e. Now the function knob was switched to read mode and using the balance coarse and fine coarse knobs zero was attained on the indicator display screen. f. Function knob was switched to Internal Calibration mode and desired calibration number (1000) for gauge factor 2 was set using span coarse and span fine knobs.
ii. Now applied āR = 0.48 ohms (corresponding to a simulated strain of 2000 Ī¼Īµ with F=2) and note indicated strain.
iii. Replaced the all three wires with long leads.
iv. Corresponding to a simulated strain of 2000 Ī¼Īµ with F=2 again applied āR = 0.48 ohms and note indicated strain.
v. Indicated strain in this step was less than that in step (ii).
vi. Desensitization value for long lead was calculated using following formulae,
D = Rg/(Rg+RLl) = 120/(120+RLl)
Where, RLl is the resistance of āactiveā long wire.
D =
Observations and calculations: Value of simulated strain with short wires = 2000 Ī¼Īµ
Value of indicated strain with short wires and ĪR (0.48 Ī©) = 1998 Ī¼Īµ
For white wire:
D = Rg/(Rg+RLl)= 120/124.58=0.
Experiment # 3: Measurement of Installed Strain Gage resistance
Procedure:
i. For Quarter Bridge arrangement of simulator with indicator following connection was done. a. Active wire was connected at its one end to simulatorās binding post no. 1 and other to binding post S+ on indicator using short leads. b. Two short common leads connected the simulatorās binding post 2 with P- and D120 on indicator. c. Applied resistance 120Ī© on the simulator. d. Using āInstrument zeroā knob the zero was attained on the indicatorās display with āfunction knobā on zero. e. Now the function knob was switched to read mode and using the balance coarse and fine coarse knobs zero was attained on the indicator display screen. f. Function knob was switched to Internal Calibration mode and desired calibration number (1000) for gauge factor 2 was set using span coarse and span fine knobs.
ii. Now connect the simulator (with short lead wires) and given gage with indicator in āHalf bridge arrangementā.
iii. Upon completion of Half bridge connection a strain will be displayed. Displayed strain will be positive if Rg > 120 ohm and vice versa.
iv. Put suitable āR (to equilize resistance of both arms) from simulator to minimize strain displayed.
v. Now Rg = 120 + āR. It includes lead wire resistance as well.
Observations and Calculations:
Indicated value of strain= 4346 Ī¼Īµ
āR=2.21 Ī©
Rg = 120 + āR =120+2.21 = 122.21 Ī©
Fe = (āRg/Ra)/ Īµ = 2.
Experiment # 4: Large Strain Measurement with V/E-
We can measure large strains (beyond the range of indicator) in addition with more accurate
strains results than obtained in strain indicator.
Procedure:
i. For Quarter Bridge arrangement of simulator with indicator following connection was done. o Active wire was connected at its one end to simulatorās binding post no. 1 and other to binding post S+ on indicator using short leads. o Two short common leads connected the simulatorās binding post 2 with P- and D120 on indicator.
o Applied resistance 120Ī© on the simulator. o Using āInstrument zeroā knob the zero was attained on the indicatorās display with āfunction knobā on zero. o Now the function knob was switched to read mode and using the balance coarse and fine coarse knobs zero was attained on the indicator display screen. o Function knob was switched to Internal Calibration mode and desired calibration number (1000) for gauge factor 2 was set using span coarse and span fine knobs.
ii. Set gage factor according to the gage factor of the gage through which large strain will be sensed.
iii. Connect simulator (with R= 120 ohm, with short connecting wires) and gage (unstarined state) in half bridge arrangement to the strain.
iv. Because of mismatch of the resistance between two active arms of the bridge, there will be a displayed strain.
v. Minimize this apparent strain reading by the application of āRi from simulator. Resistance of the arm containing gage = 120 + āRi
vi. Now apply large strain to the structure at which the gage is attached such that the active strain exceeds the range of the indicator. Now go on applying āR through simulator, in increments, till minimum value of the strain is indicated on the indicator screen. Corresponding to this situation if reading of resistance on simulator is Rf ohms then Rf = 120 + āRf and actual strain Īµ = āRf - āRi+/F(120+āRi)]
Note: Practically this part cannot be perfromed in lab as there are no means to
produce an appreciably large strain,.
Experiment # 5: Non-Linearity of Quarter Bridge Circuit
(a) Bridge Initially Balanced Resistively
Procedure: i. For Quarter Bridge arrangement of simulator with indicator following connection was done. a. Active wire was connected at its one end to simulatorās binding post no. 1 and other to binding post S+ on indicator using short leads. b. Two short common leads connected the simulatorās binding post 2 with P- and D120 on indicator. c. Applied resistance 120Ī© on the simulator.
Table # 04: Observed and calculated strain data
Applied āR ohms Observed strain Ī¼Īµ Calculated strain Ī¼Īµ āĪµ
Experiment # 6: Shunt Calibration of Strain Indicators
As calibration of the indicator means having a curve between indicated and actual (simulated) strain. We can introduce resistance change (and hence cause strain simulated) by series addition/subtraction of resistance in the form of āR to R (nominal value). R can also be changed by shunting R with Rc (shunt calibrated resistor). When R is shunted through Rc effective resistance of the arm becomes
Reff = (RRc)/ (R+Rc)
Clearly Reff < R thus āR = *(RRc)/ (R+Rc)]- R is always negative.
Therefore if shunting is done across active gage then negative strains are simulated and if shunting is done across bridge completion resistor then positive strains will be simulated.
To calculate value of Rc required to simulate Īµs (negative strain) in the resistor R with gage factor Fi we use following relation
Rc =[ -R/(Fi Īµs)] ā R
Procedure:
i. For Quarter Bridge arrangement of simulator with indicator following connection was done. a. Active wire was connected at its one end to simulatorās binding post no. 1 and other to binding post S+ on indicator using short leads. b. Two short common leads connected the simulatorās binding post 2 with P- and D120 on indicator. c. Applied resistance 120Ī© on the simulator. d. Using āInstrument zeroā knob the zero was attained on the indicatorās display with āfunction knobā on zero. e. Now the function knob was switched to read mode and using the balance coarse and fine coarse knobs zero was attained on the indicator display screen. f. Function knob was switched to Internal Calibration mode and desired calibration number (1000) for gauge factor 2 was set using span coarse and span fine knobs.
ii. To simulate Īµs = -1000 Ī¼Īµ, Īµs = -2000 Ī¼Īµ
Rc values are 59,880 ohm and 29,880 respectively.
Apply these resistors across binding posts No. 1 & 2 of the simulator one by one, note the values of indicated strains.
ļ For producing positive simulated strain the shunt calibrated resistor may be placed across S- and D binding posts of the strain indicator.
Table # 05: Table showing the shunt resistance calculations
RC shunt calibration resistance parallel with R (k Ī©) Observed strain Ī¼Īµ^ Calculated strain Ī¼Īµ 59.88 - 998 - 1000 43.4 - 1377 - 1386
Discussion:
The experiments give an insight about the important points noted while measuring the strain from the strain indicator. E.g., when using long leasds instead of short leads the desentisization of gauge is also incorporated in calculating strain from indicator for actual results. Temperature compensation is also an important factor in measuring the strain and calibrating the indicator. Someone can measure the strain greater than the range of the indicator used in experiment as in experiment # 04.
