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MCG4150: Midterm 2006

Course: MCG 4150 - Bioinstrumentation
Instructor:             Andy Adler
Date: Mar. 8, 2006
Directions: You have 80 minutes to complete this exam. The exam has four questions; you are required to answer any three of them. Each question is worth equal marks. This is a closed book exam; however, you are permitted to bring a 8.5"×11" sheet of notes into the exam. You are permitted to use a calculator. You may not communicate with anyone during the exam except the instructor.

You may make assumptions to simplify the problems as long a they don't change the calculations by more than 10%. You may assume the following conditions and constants:

  • Atmospheric pressure, Patm: 101.3 kPa
  • Density of air at Patm: 1.21 kg/m3
  • 1 cmH2O = 98.5 Pa
  • 760 mmHg = 101.3 kPa
  • 1 Amp= 6.24×1018 e/s

1. Flow Transducer Designs

Consider the following types of air flow transducers:
−Pneumotachometer (which measures pressure differences accross a flow resistance)
−Hot-wire anemometer (which measures the current required to maintain a thermistor at constant temperature)
−Rotating-vane flowmeter (which measures rotation of a turbine in the air stream)

1A. Sketch each transducer type and describe how it works.

1B. Compare each type in terms of: 1) accuracy, 2) frequency response, 3) linearity of the underlying sensor eleement, 4) directional response (response to flow in different directions)

2. Flow Transducer Frequency Limits

In this question, we want to compare the frequency response of the hot-wire anemometer and pneumotachometer.

2A. The pneumotachometer can be modelled as a 15 cm long tube of inner diameter 3 cm. Atmospheric pressure is 101.3 kPa. Calculate the compliance and inertance of the gas in pneumotachometer.
Compliance = V/Patm = πr²L/Patm
= π*(0.015m)²*(0.15m)/101300Pa
=   1.0467e-009 m^5/N
Inertance = ρ*L/π/r²
= (1.21 kg/m³)*0.15m/(π*(0.015m)²) 
= 256 kg/m^4

2B. The hot-wire anemometer acts as a first order system with time constant τ=0.5s. The pneumotachometer acts as a second order system with natural frequency fn. Compare the frequency response of the hot-wire anemometer and pneumotachometer. Which is the highest?
ωn= (LcCc)−½
=( 1.0467e-009m^5/N * 256 kg/m^4)^(-.5)
=( 2.6876e-007 m-kg/N)^(-.5) = (2.6876e-007 m-kg-s²/kg-m)^(-.5)
=1929 1/s
fn = ωn/2/π= 307 Hz.

pneumotachometer is much higher freq than anemometer

3. Electrode Types

Consider the following two electrode types:
−Ag/AgCl electrode
−Stainless steel (SS) electrode.

3A. What is a polarizable electrode? Compare these two electrodes in terms of polarizablility.

3B. An ECG amplifier is connected to an Ag/AgCl electrode and has an input resistance of 1 MΩ. The QRS peak of the ECG lasts 50 ms and has an average signal level of +20 mV. How many CL ions enter or leave the body from the electrode during the QRS peak?
I=V/R= 20mV/1MΩ= 20nA
Electrons = (20e-9 A)*(6.24e18 e/s/A)*(50e-3 s)
          = 6.24e9 electrons
Valence of CL is one.
Thus 6.24e9 ions leave body

3C. Consider a body which touches an exposed live wire carrying 120V at 60Hz.. Rank the following in terms of danger.
−Touching an SS electrode lead to the wire.
−Touching an Ag/AgCl electrode lead to the wire.
−Touching dry skin to the wire.
−Touching wet skin to the wire.

4. Nerve Conduction Velocity

Consider the classic "knee-jerk" reflex test.

4A. Sketch a diagram and describe the nerve signal conduction pathway during this test. What accounts for the time delay between the hammer tap and muscle contraction?

4B. We wish to measure the time between hammer tap and muscle contraction. Design a system to measure this time delay. Sketch your system and describe how it works?

Last Updated: $Date: 2006-03-10 09:30:52 -0500 (Fri, 10 Mar 2006) $