The George Washington University
School of Engineering and Applied Science
Department of Electrical and Computer Engineering
ECE 2110 - LAB
Experiment # 13
A Liquid Strain Gauge for Biomedical Application
Components:
Lab Components
(1 piece) Rubber Tubing
(2 pieces) Copper Wire
(1 piece) Resistor
(1 piece) Liquid Solution
(1 piece) Syringe
Part Description
6 inch piece of 0.0625’’ inner diameter rubber tubing
Pieces of 12 AWG (American Wire Gauge) bare copper wire
Resistor of initially unspecified value obtained from the drawers in
the hall
Dilute solution of copper sulfate
Hypodermic Syringe
Table 1.1
Objectives:
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To build and test a resistor made out of an ionic conducting liquid solution, copper
sulfate.
To understand and use the formula for the resistance of a conducting cylinder.
To test the above resistor for possible use as a strain gauge.
To devise a circuit to turn the changes in length into voltages that can be observed on a
scope.
Devise an addition to the above circuit to subtract out a constant voltage offset
To learn to use the digital oscilloscope to record slowly varying voltages.
Understand how the concept of the above device can be used to monitor human
respiration for lie detector and medical applications.
Prelab: (Submit electronically prior to lab meeting, also have a printed copy for yourself during lab)
1. Caution!!!
You will be dealing with water in this experiment, specifically, a solution of a very good
conductor, copper sulfate. The mere presence of water in an electronics lab is immediately a
safety issue. If the water solution is spilled, it will stain everything bright green. It is
important to be careful in the execution of this experiment.
One area in Room 301 will be designated as the “wet” area. All filling of the rubber tubing is
to be done in this area while holding the tubing and syringe over a large waste paper basket.
You must cap the tubing with the copper wire, test for leaks, and must dry off every wet
surface before you leave this area. Paper towels will be provided. If your instructor sees
anything wet in your experimental area, points will be deducted from your grade in this lab.
2. Assume that the cross section of liquid conductor is a circle with a diameter of 0.0625 inch.
Determine the formula that relates the resistance to the resistivity, length and cross sectional
area of the liquid conductor. Based on the formula, if the resistivity and cross sectional area
are kept constant then what is the relationship between resistance and length of the liquid
conductor? Is it a linear or nonlinear relationship? (hint read this wiki definition for electrical
resistance)
3. Device a simple circuit to turn the changes in length into voltages using a power supply, one
resistor and the liquid conductor.
4. Write down the definitions for mean, standard deviation, and fractional standard deviation.
(hint: mean, standard deviation, and fractional standard deviation)
GWU SEAS ECE Department ©2010
ECE 11 – Experiment #13
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Lab:
Part I – Build and test a resistor made out of an ionic conducting liquid
a. Carefully fill the rubber tubing with the copper sulfate solution. Do so over the “wet” trash
can, and avoid spilling any of the electrolyte solution. Insert the solid AWG copper wire in
each end of the tubing to serve as electrodes in contact with the solution, as well as to
seal the tubing and prevent leakage of the liquid.
Q: What would the effect of a bubble or gap in the liquid be on the resistance of the
cylinder of liquid?
b. Measure the resistance from one of the electrodes to the other using the DMM. Stretch
the tubing and let it contract back.
Q: Does the resistance come back to the same value each time? If not, why? Discuss
with your GTA (Ans: dissolved air coming out of the solution, external air leaking in
past the wire end caps, overstretching the rubber tubing, liquid on the outside of the
tubing or on the hands of the experimenter shoring out the sensors, etc.)
Part II – Resistance measurements for the liquid strain gauge
a. With the help of your lab partner, hold the liquid resistor over a meter stick and measure
and record its resistance as you stretch the tubing in 1 cm increments up to 10 cm longer
than its initial length. For each increment, measure the resistance 5 times. Enter the
data (50 resistance values) in a spreadsheet.
b. Calculate the mean resistance and the standard deviation of the resistance at each
length. Calculate the fractional standard deviation at each length.
Q: Do you think this device is accurate enough to have some uses as a strain (or length)
gauge? Discuss.
Q: Is the graph of resistance vs. length linear as expected?
Q: If this graph is not linear, does this prevent device from being useful as a strain (or
length) gauge? Discuss.
Q: Is there anything else you have observed that might prevent this device from being
useful as a strain gauge?
Part III – Device circuits to turn the changes in length into voltages that can be observed
a. Show your prelab circuit to your instructor. Be prepared to defend your circuit.
b. Design a circuit using a second output from the bench power supply and possibly another
resistor that will make the measured voltage close to zero when the strain gauge is not
stretched.
c.
This addition will make small changes to the length of your strain gauge much easier to
detect since they will be happening around zero volts instead of around some large value
of voltage.
d. Test this circuit to see that it works with your strain gauge. Measure the output of the
circuit using the DMM on a voltage scale instead of the previously used resistance scale.
Adjust your offset voltage until that there is less than 0.1 volts at the output when the
strain gauge is unstretched. Measure the maximum voltage output when the tubing is
stretched by 10cm.
GWU SEAS ECE Department ©2010
ECE 11 – Experiment #13
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Part IV – Device circuits to turn the changes in length into voltages that can be observed
a. Instructor will demonstrate how to manually adjust the “time base” and “triggering”.
b. Adjust the time base to 0.1 second per division. Select the correct triggering option so
that the scope trace is continuously moving whether a signal is present or not.
c.
Adjust the vertical gain on Channel 1 so that the maximum voltage recorded in step 3d,
above corresponds to full scale deflection of the trace on the vertical axis.
Part V – Using strain gauges to monitor human respiration for medical and lie detector
applications
a. With the help of your partner, stretch your strain gauge by a few centimeters. Tweak the
vertical scale of the scope so that such small changes in length are clearly visible on the
scope. Change the time base from 0.1 seconds per horizontal division to 10 seconds per
division.
b. Imagine a strain gauge of similar design wrapped around your chest. Have your partner
periodically stretch and release the liquid strain gauge about once per second to simulate
the effects of breathing and watch the waveform that is mapped out on the scope.
c.
Have him try to be as consistent as possible in rhythmically stretching and releasing the
strain gauge. Note that it is almost impossible to maintain a steady frequency and
amplitude, even with the absence of any distraction. It is even more difficult in the
presence of distraction such as attempting to tell a lie. Discuss this with your instructor.
Instructor will demonstrate how to manually adjust the “time base” and “triggering”.
Part VI – Analysis to consider for Lab write-up
In this lab, we investigated how ionic liquid solutions can be used as a resistor component in an
electrical circuit and the resulting resistance is linearly proportionate to the change in the length of
the conductor. This concept can be implemented into a strain gauge used for monitoring human
respiration system and lie detector applications. Review your measured resistance data and see
if it resistance is indeed linearly proportionate to change in length, including all calculations and
formulas in your final lab write-up. Calculate the percentages of error in each case.
GWU SEAS ECE Department ©2010
ECE 11 – Experiment #13
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