LEC

1. Part 1. General Instrumentation
Concepts
Reading Assignment: Chapter 1 in
our textbook.

2. What is an Instrument?
• Instruments:
Devices that can be used to make a measurement
and give quantitative (or sometimes qualitative)
results
• Biomedical Instruments:
Devices that can be used to make measurements
of biological or medical quantities and give
quantitative (or sometimes qualitative) results

3. Examples of Familiar
Biomedical Instrumentation
Clinical Thermometer Stethoscope

4. Instruments abound in a Neonatal Intensive Care Unit!

5. General Instrument Block Diagram

6. Classification of Instrument
Operating Modes
1. Direct vs. Indirect
Measurement Methods
Example #1. Direct vs. Indirect
temperature measurement

7. Direct Temperature Measurement
Clinical Mercury Thermometer
Bulb is brought directly into contact with the body part or material whose
temperature is to be measured, and as the mercury in the bulb exponentially
changes to match this temperature, the mercury expands or contracts,
pushing the very thin column of mercury up or down along a calibrated scale.
Temperature measurement
range: 35 – 42 degrees C

8. Indirect Temperature Measurement via
“non-contact” Infrared (IR) Pyrometer)
Laser beam indicates middle of temperature measurement
“spot”. Radius of spot size is given by S = D/10.

9. IR Blackbody Radiation: Planck’s Law
• Planck's law describes the radiation spectral density “I”
at all wavelengths emitted from a black body at
temperature T. As a function of frequency ν, Planck's
law is written as:
Note: h is Planck’s constant, k is Boltzmann’s constant, c is
the speed of light
1
1
2
)
,
( 2
3


kT
h
e
c
h
T
I 




c


10. Planck’s Law (Radiation Spectral Density) Plotted for
Several Values of Temperature


2
1
)
,
(



 d
T
I
R
Total Radiation R in Watts/m2
Between band of wavelengths

11. Typical IR Pyrometer Features:
• Infrared IR Thermometer with Laser Pointer
• Non Contact Temperature Measurements in
degree F and degree C
• Temperature Range: -25 to 716 °F (-32 to 380
°C)
• Accuracy: 2%, Battery: 9V included
• Dimensions: 6" x 5" x 1.5"

12. Direct vs. Indirect Measuring
Instrument Example #2:
Direct vs. Indirect Intraocular Eye
Fluid Pressure Measurement

13. Intraocular Pressure (IOP)
• Intraocular pressure (IOP) is a measure of the
fluid pressure inside the eyeball (cornea).
• High IOP indicates glaucoma, which is a silent,
symptomless debilitating illness that results in
tunnel vision and eventually in blindness if it
goes untreated with pressure reducing eye
drops.
• We should all have our IOP checked regularly,
since this is the first indication of glaucoma!

14. • Applanation tonometry measures IOP by directly
contacting a probe with the cornea of the eye. It
measures the the force required to flatten
(applanate) a constant area of the cornea.
• Goldmann Applanation Tonometry is considered to
be the gold standard in tonometry as it is the most
widely accepted method of determining approximate
IOP.
• The Goldmann tonometer principle is based on
the ”Imbert-Fick Law”: “the pressure in a sphere filled
with liquid and surrounded by an infinitely
thin membrane is measured by the counterpressure
which just flattens the membrane.“

15. The Imbert-Fick Law appears in textbooks of optometry
but you will NOT find it in Physics textbooks! WHY???
• This is because this pseudo-scientific “so-called”
“law” was actually concocted by Hans Goldmann
himself to justify the operation of his tonometer!
• This law is really just Newton’s 3rd Law “you press
on a stone with a certain force, it presses back on
you with the same force.”
• The “applanation” requirement is necessary to
ensure that you are measuring only the force due
to the fluid pressure inside the eye and not also
the additional lateral forces exerted by the
tension on the spherical corneal membrane.

16. Goldmann Applanation Tonometer

17. Goldmann Tonometer in use

18. Conversion from probe force in grams to mm Hg
• Pressure is equal to force per unit area but one must factor in the density
of mercury (Hg) which is 13.6 gm/cm3.
• The probe diameter for a Goldmann tonometer is 3.06 mm. Why was this
strange diameter value chosen by Hans Goldmann? THERE WAS METHOD
IN HIS MADNESS, as we see below!
The cross-sectional area of the probe is given by
Aprobe = π(3.06E-3/2)2 = 0.7354 mm2
• Find the pressure in mmHg if the applanation force on the probe is 1 gm
Pressure = Force/Area
Pressure = 1 gm / .073541714 cm2 x 13.6 gm/cm3 of Hg
Pressure = 1 gm x cm3 of Hg / .07354cm2 x 13.6 gm
Pressure = 1 cm of Hg / 1
Pressure = 1 cm of Hg = 10 mmHg
• Therefore, one gram of force is needed to flatten an area equal to
.073541714 cm2 when the eye contains a pressure of 10 mmHg.
=> Each large number on the micrometer is in grams, where 1 gram of
probe force equals 10 mmHg of IOP

19. Goldmann Tonometer Operation
• A special disinfected prism is mounted on the tonometer head
and then placed against the cornea.
• The examiner then uses a cobalt blue filter in front of a slit lamp
(emits a thin sheet of light) to view two green semi circles.
• The force applied to the tonometer head is then adjusted using
the dial until the inner edges of these green semicircles meet.
• Because the probe makes contact with the cornea, a topical
anesthetic is introduced onto the surface of the eye in the form
of an eye drops. Patient must not rub eye until anesthetic
wears off to avoid hurting eye, since there is no feeling in eye!

20. Eyeball is applanated when semicircles (viewed
through a slit lamp) are brought into alignment so
that their edges touch. That is when
IOP = Fplunger/(plunger area)

21. Patients do not like having their
corneas anesthetized!
• Might there be a better way that does not
require contact with the cornea?
• Enter the indirect non-contact “Air-Puff
Applanation Tonometer”
• Not as accurate as the Goldman contact
tonometer, but useful for screening patients to
find which ones need to be checked more
carefully with the Goldmann tonometer.

22. Indirect Measurement of IOP: Non-Contact “Air Puff” Tonometry
Air stream velocity steadily increased until cornea applanates at which time all light rays
are collected by the lens and are focused onto the detector, causing detector output
voltage to rise above a predetermined threshold, turning off the airstream. IOP is related
to airstream velocity at time of applanation. All of this must happen quite literally within
“The blink of an eye”; the air puff measurement must be complete before the opthalmic
reflex kicks in and causes the patient’s eye to involuntarily close.

23. Problems with the Air Puff
• Abandon the Air Puff
Let’s face it, most patients dread
the air-puff tonometer. (The
cartoon at left from our colleague,
Scott Lee, O.D., in his entertaining
book, Sight Gags: Cartoons for Eye
Doctors and Their Patients, tells the
tale.)
• Although the air-puff has served as
a reasonable IOP “screening
device” for many years, it is not at
all patient-friendly, and patients
anticipate the air puff and often
close their eyes even before the air
puff arrives!
• We have waited nearly 30 years to
see a technology evolve that makes
the air-puff tonometer obsolete.

24. Back to Direct Measurement of IOP:
Rebound Technology
• The Rebound Tonometer was developed in Finland
circa 2006 http://www.icaretonometer.com
• Required a decade of development by Finnish MD
Antti Kontiola. “Icare” Company.
• Determines IOP by bouncing a small plastic tipped
metal probe against the cornea. Probe is DISPOSABLE,
so this eliminates the need for disinfecting the probe.
• The device uses an induction coil to magnetize the
probe and fire it against the cornea.
• As the probe bounces against the cornea and back in
to the device it creates an induced voltage waveform
from which the intraocular pressure is calculated.

25. Rebound Technology, Cont’d
• Software analyzes the probe deceleration, contact time
and other parameters of the probe while it touches the
cornea. The deceleration and other rebound
parameters of the probe change as a function of IOP.
In simple terms, the higher the IOP, the faster the
probe decelerates and the shorter the contact time.
• The device is simple, cheap, portable, and easy to use.
• Anesthesia is not needed since the touch of the probe
is so gentle; the measurement is barely noticed by the
patient!
• It is particularly suitable for children and non-
compliant (old and confused) patients.

26. The Icare Rebound Tonometer

27. Other Instrument Classification
Categories
Digital

28. 6. Null-mode vs. Deflection Mode
Null-mode Instrument
• The purpose of any null mode instrument is to act like a laboratory
balance scale, indicating when the two quantities are equal. The
laboratory scale balance beam doesn't actually weight anything;
rather, it simply indicates equality between the unknown mass
and a pile of standard (calibrated) known masses.
Balance beam acts as a “null detector”, its scale need not be accurately calibrated, but it
must accurately indicate the “null” or balance condition.

29. Null-Mode Voltmeter
The voltage across R2 is to be measured in the “high resistance” circuit consisting of the battery, R1
and R2. Adjust voltage source until no click is heard in headphones when switch is operated. At this
point the adjustable source voltage is equal to the voltage to be measured (the instrument is
“nulled”. The audio transformer increases the input impedance of the headphones, but note that
this audio transformer DOES NOT load down (alter the voltage in) the circuit being measured when
no click is heard, since at that point there is 0V across the transformer, and so NO CURRENT flows
through it, no matter how low its impedance!

30. Null detector can be any kind of voltage/current sensing device. It need NOT read
voltage accurately or linearly, but it must indicate when the voltage across it goes to 0.
If possible, the sensitivity of the null detector should be increased as a null is
approached to get the most accurate null possible.
Notes:
1. Impedance of null detector element need NOT be high, since it does not load
down the circuit whose voltage is being measured when it reads “0” (nulled
condition). The null detector need not read accurately.
2. Likewise, the voltmeter used to measure the adjustable voltage source value
need NOT be of high impedance, since it is across a voltage source, not
across the high resistance in the circuit. However, it must be very accurate.

31. Deflection Mode Instrument
• While a null-mode instrument is as accurate as
its known standard value that the unknown
quantity is balanced against, it is an iterative
process that can take time to complete.
• The deflection-mode instrument is faster but
less accurate. The best example of a
deflection mode instrument is a spring-loaded
scale that measures weight.

32. Deflection Mode Instrument Example #1: Hanging Spring Scale
Spring scale operation is based
upon Hooke’s Law for a spring:
Fspring = kx
Where k is the “spring constant”
(N/m), and x is the deflection of
free end of the spring from its 0-
force (equilibrium) position.
Approximate deflection is read
along analog scale

33. Deflection Mode Instrument Example #2:
D’Arsonval Ammeter Movement
Developed in 1882 Jacques-Arsène d'Arsonval with a stationary permanent magnet
and a moving coil of wire, suspended by coiled hair springs. The concentrated
magnetic field and delicate suspension made these instruments sensitive and they
could be mounted in any position. By 1888 Edward Weston had brought out a
commercial form of this instrument, which became a standard component in
electrical equipment. This design is almost universally used in moving-vane meters
today. PERHAPS NO OTHER INSTRUMENT THAT WAS DESIGNED A CENTURY AGO IS
STILL USED IN ITS SAME ORIGINAL FORM!

34. D’Arsonval Ammeter Internal Structure

37. Because the D’Arsonval meter movement consists of a spring and a mass, its governing
equations for needle movement are described by a 2nd order differential equation. The
meter movement is inherently underdamped without adding some sort of damping
mechanism. Thus the step response of the meter can be oscillatory about the final
needle resting place; when the meter is suddenly connected across a dc voltage level,
the needle will overshoot the final value, then undershoot, and gradually die down to
read the desired value, which is very annoying for the user of the meter. To sufficiently
damp the system so that the needle does not take long to settle, friction must be
introduced. However this slows down the meter response. A better solution is apply
more friction when the needle moves fast and less friction when the needle moves
slow. This can be done using “Dynamic Aluminum Vane Damping” shown on next slide.

38. Dynamic Needle Damping: Aluminum damping vane
As aluminum closed loop moves in the dc magnetic field, an
induced current flows in the loop, creating an opposing torque
that is proportional to velocity of the needle. This serves to
damp the speed of the needle when it is moving fast, but not
damp it so much when it is moving slow. This reduces the
chance of needle oscillation about its equilibrium point.

42. Interfering and Modifying Inputs, Cont’d

45. Temperature, Age, etc.

47. Practical Signal Notch (Band Reject) Filtering
Example
• 60 Hz ac power-line interference is always present in indoor
biopotential measurements.
• In special situations, this kind of interference can be
neglected, but this is not a general rule.
• In laboratory experiments and clinical analysis, it is hard
(and expensive) to isolate the subject of measurement
from electrical fields produced by a power line.
• In human biopotential recordings, it is common practice to
apply a 50/60 Hz notch filter to reduce this kind of
interference.
• In such cases, there is no considerable distortion observed
on the recorded signal

48. Bootstrapped High Q Twin-”T” 60 Hz Notch Filter
See Bootstrapped Twin “T” Frequency Response.
(Disconnecting wire from output of op amp to junction of C3and R3 and
grounding junction of C3 and R3 yields much broader (Low Q) Twin “T”
Response.

50. Noise-Cancelling Stereo Headphones
J1 is microphone jack that is directed out
Into the noisy environment – it picks up
Noise only, not the desired audio signal.
J2 is audio input jack
and J3 is the audio
output jack.
R23 is audio volume
control, R14 is noise
cancelling adjustment,
S1 selects between noise
cancelling mode where
external environment cannot
be heard, and noise amplifying
mode, where external
environment CAN be heard.

53. Static Characteristics of
Instruments (DC response)
• To measure static characteristics of an
instrument, user must wait for as long as
necessary for the instrument reading to settle
before the measurement is read.

54. Static Characteristics of Instruments
• Accuracy
– Accuracy = (True Value – Measured)/(True Value)
– True value is usually traceable back to a value archived at the NIST (National Institute of Standards and
Technology) at Boulder Colorado.
– Example: Time and Frequency Standards Division of NIST
WWVH (Hawaii) WWVB (Fort Collins, Colorado)

55. NIST Time and Frequency Division
• The Time and Frequency Division, part of
the NIST Physics Laboratory, maintains the
standard for frequency and time interval for
the United States, provides official time to the
United States, and carries out a broad
program of research and service activities in
time and frequency metrology.

56. • NIST Shortwave radio station WWVH broadcasts time and frequency information 24 hours
per day, 7 days per week to listeners worldwide. The station is located on the Island of
Kauai, Hawaii on a 12 hectare (30 acre) site near Kekaha at Kokole Point. The information
broadcast by WWVH includes time announcements, standard time intervals, standard
frequencies, UT1 time corrections, a BCD time code, geophysical alerts, marine storm
warnings, and Global Positioning System (GPS) status reports. WWVH operates in the high
frequency (HF) portion of the radio spectrum and radiates 10,000 W on 5, 10, and 15 MHz,
and 5000 W on 2.5 MHz. Each frequency is broadcast from a separate transmitter. Although
each frequency carries the same information, multiple frequencies are used because the
quality of HF reception depends on many factors such as location, time of year, time of day,
the frequency being used, and atmospheric and ionospheric propagation conditions. The
variety of frequencies makes it likely that at least one frequency will be usable at all times.
• NIST VLF (Very Low Frequency) radio station WWVB is located on the same site as WWV
near Fort Collins, Colorado. The WWVB broadcasts are used by millions of people
throughout North America to synchronize consumer electronic products like wall clocks,
clock radios, and wristwatches. In addition, WWVB is used for high level applications such
as network time synchronization and frequency calibrations. WWVB continuously
broadcasts time and frequency signals at 60 kHz. The carrier frequency provides a stable
frequency reference traceable to the national standard. There are no voice announcements
on the station, but a time code is synchronized with the 60 kHz carrier and is broadcast
continuously at a rate of 1 bit per second using pulse width modulation. The carrier power
is reduced and restored to produce the time code bits. The carrier power is reduced by 17
dB at the start of each second, so that the leading edge of every negative going pulse is on
time. Full power is restored 0.2 s later for a binary “0”, 0.5 s later for a binary “1”, or 0.8 s
later to convey a position marker. The binary coded decimal (BCD) format is used so that
binary digits are combined to represent decimal numbers.

57. NIST Development: Miniature Atomic Clock
• NIST researchers have demonstrated a minuscule atomic clock with inner
workings about the size of a grain of rice and potential applications in
atomically precise timekeeping in portable, battery-powered devices for
secure wireless communications, more precise navigation, and other
applications.

58. • Precision --- # of distinct alternatives from which
the result is selected.
– Example: 3 digit vs 4 digit digital scale
153
153.7
• Resolution --- Smallest change that can be
measured between two nearly equal quantities.
– Example: 2 pounds for 3-digit scale
0.2 pounds for 4-digit scale
• Repeatability --- Variation in measured values
when reading of the same
measurand is repeated again
and again.
– Example: Repeatablility might be specified as
+ or – 0.1 pound for 4-digit scale.

59. • Statistical Error Control
– Guaranteed error bounds on errors that are both systematic and random.
– Systemic errors can be controlled by calibration and correction factors
– Random errors can be controlled by averaging repeated measurements
Systemic errors are associated with a
particular instrument or experimental
technique (e.g., a faulty ruler, inaccurate
instruments, misaligned gun sights, etc.). And
these errors always bias the results in one
direction, either too high or too low.
Random errors on the other hand are
unbiased as they come from unpredictable
sources (e.g., quiver while holding the device,
lab technician getting tired, inconsistent
gunpowder quality, etc.). The results obtained
are EQUALLY likely to be too high or too low.

60. Static Sensitivity
–If just one input is varied, with other inputs
held constant (desired inputs, modifying
inputs, and interfering inputs), the static
sensitivity of the output (the measured
quantity) to that particular input is defined as:
int
_ Po
Q
Input
Output
y
Sensitivit 








--Evaluation about a Q point is important when
the output vs. input variation is not linear. If it
is linear, any Q point will yield the same
sensitivity.

61. Linear Least Squared Error Curve Fitting (Linear Regression) =>
Finds the “best fit” of the transducer’s output vs. input behavior to a straight
line, y = mx+b. In a linear instrument, b must = 0. Thus the bias, or “Zero Offset”
(b) that is read at transducer output may be removed by subtraction, and the
slope “m “ is the static sensitivity “k”

62. Q-Point must be specified for a
nonlinear transducer
NOTE: Sensitivity at Q-Point #1 > Sensitivity at Q-Point #2
Q
Q-Point
#1 Point
#1 Q-Point
#2
#2

63. Zero Drift and Sensitivity Drift
• Zero Drift – Change in modifying inputs such as change in dc offset at
electrodes
• Sensitivity Drift - Change in gain of instrument (transducer or amplifier)
due to temperature or pressure changes, DC Power supply change, etc.

64. Linearity
• Note: the behavior of a linear transducer is characterized by
y = mx + b
where m = slope = static sensitivity (gain)
b = zero offset = measurement “bias”.
Note that this bias must be made = 0 via subtraction (calibration)
in order to make the instrument linear according to the definition
above.

65. Three Linearity Specification Methods
• “±B% of full scale” (pipe) accounts best for linearity deviations near zero input
• “±A% of reading” (cone) accounts best for linearity deviations near full scale
• “±A% of reading or ±B% of full scale, whichever is greater” (funnel) accounts best
for all linearity deviations.

67. Transducer Impedance
• Indicates degree to which instrument “loads”
(decreases) quantity being measured.
• For any transducer input quantity xd1, there is a related input
quantity xd2, such that
xd1*xd2 = Power delivered to transducer
• Call xd1 the “Effort Variable” (quantities such as voltage,
pressure, force)
• Call xd2 the “Flow Variable” (quantities such as current, fluid
flow rate, velocity)
• We define transducer impedance as
Zx = xd1/xd2 = effort variable/flow variable
• The power absorbed by the transducer is
P = xd1*xd2 => P = (xd22)*Zx => P = xd12 / Zx
• We desire to minimize the power absorbed by the transducer
so the transducer does not disturb the quantity being
measured. Therefore to measure effort variables (xd1) we
want Zx to be relatively high; to measure flow variables
(xd2) we want Zx to be relatively low.

68. Typical Nonlinear Static Transducer Behaviors

75. Two kinds of potentiometers (pots): linear
slider and rotational (Carbon and Wirewound)

76. R1
R2
L
E
K
x
y
y
Sensitivit
Static
L
x
E
R
R
R
E
y
R
R
L
x
R
R
R
L
x
R













_
2
1
2
)
2
1
)(
1
(
1
)
2
1
(
2
Linear (fader) potentiometer used as a
displacement measuring transducer

95. Eq 1.33 in
text

98. Eqn 1.35 in
Textbook

99. Using MAPLE to solve this 2nd – order Differential Equation
Solution Equivalent to
Text Equation 1.33

103. τd > 0

105. Periodic Signal Bandwidth Determination
Any periodic signal can be expressed as a sum of harmonically related
sinusoids. This is known as Fourier Series Decomposition. If f(t) is a
periodic function with period T, then

110. Maple Fourier Series Analysis of Square Wave

112. General List of Instrument Specifications (Consider this
when designing a new instrument!!!)

115. Biomedical
Instrument
Design
Cycle