Transcript Metrology of Electrical Quantities

Laboratory of Legal Metrology, CTU in Prague
Athens 2008
Athens
Athens Programme
Programme
2008
2008
Metrology of Electrical Quantities
Laboratory of Legal Metrology
Department of Measurements
Faculty of Electrical Engineering
Czech Technical University in Prague
Responsible professor:
Prof. Ing. Jaroslav Bohacek, DrSc.
Phone +420 22435 2220
E-mail [email protected]
http://measure.feld.cvut.cz/...
Laboratory of Legal Metrology, CTU in Prague
Athens 2008
I often say that when you can measure what you are speaking
about, and express it in numbers, you know something about
it …
… but when you cannot measure it, when you cannot express
it in numbers, your knowledge is of a meagre and
unsatisfactory kind.
William Thomson, Lord Kelvin
Athens 2008
Laboratory of Legal Metrology, CTU in Prague
Far from being a dull area of science, high-precision
measurements are an increasingly exciting area in which to
work, for they push theory and experiment to the very limits of
which they are capable…
If we recall that the rich oil deposits under the North Sea
remained unknown until the advent of the very precise
geophysical measurements, or that the spin of the electron
was inferred from precise spectroscopic measurements, we
see more clearly the class of discoveries which can come from
precise measurement.
Brian W. Petley
Athens 2008
Laboratory of Legal Metrology, CTU in Prague
Laboratory
visits
Course
objectives
Lectures
What is
metrology?
Previous
intro
Laboratory
demonstrations
Please, select some topic above to continue
Laboratory of Legal Metrology, CTU in Prague
Laboratory demonstrations
1.
Thompson-Lampard's capacitance standard
2.
Frequency performance of resistance standards
3.
Calibration of capacitance boxes
Athens 2008
Laboratory of Legal Metrology, CTU in Prague
Athens 2008
Course objectives:

to present an overview of modern and perspective methods for
precision measurements of electrical quantities,

to demonstrate various techniques used in calibrations of
electrical measurement instruments and standards.
Laboratory of Legal Metrology, CTU in Prague
Lectures
Lecture 1
Introduction
Measurement units and measurement standards
Quantum standards of voltage and resistance
Thompson-Lampard's capacitance standard
Equivalent circuits of standard resistors, capacitors and
inductors
Resistors with calculable frequency performance
Transfer standards
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Laboratory of Legal Metrology, CTU in Prague
Athens 2008
Lectures
Lecture 2
Voltage and current inductive ratio devices and optimization of
their metrological parameters.
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Laboratory of Legal Metrology, CTU in Prague
Lectures
Lecture 3
Methods for precision measurement of dc current and dc voltage
Modern potentiometers
Measurement of voltage, power and energy in audiofrequency
range
Sample
lecture
Laboratory of Legal Metrology, CTU in Prague
Athens 2008
Lectures
Lecture 4
Bridges for dc and ac measurements of resistance
Transformer and current-comparator-based capacitance bridges
Metrological applications of the quantum Hall effect (QHE)
Laboratory of Legal Metrology, CTU in Prague
Athens 2008
What is Metrology?
Metrology is the science of measurement.
Laboratory of Legal Metrology, CTU in Prague
Athens 2008
What is Metrology?
Metrology covers three main activities:
1.
The definition of internationally accepted units of measurement,
e.g. the metre.
2.
The realization of units of measurement by scientific methods,
e.g. the realization of a metre through the use of lasers.
3.
The establishment of traceability chains by determining and
documenting the value and accuracy of a measurement and
disseminating that knowledge, e.g. the documented relationship
between the micrometer screw in a precision engineering
workshop and a primary laboratory for optical length metrology.
Laboratory of Legal Metrology, CTU in Prague
Athens 2008
What is Metrology?
Metrology is considered in three categories with different
levels of complexity and accuracy:
1.
Scientific metrology deals with the organisation and
development of measurement standards and with their
maintenance (highest level).
2.
Industrial metrology has to ensure the adequate functioning of
measurement instruments used in industry as well as in
production and testing processes.
3.
Legal metrology is concerned with measurements where these
influence the transparency of economic transactions, health and
safety.
Laboratory of Legal Metrology, CTU in Prague
Athens 2008

Systematic measurement with known degrees of uncertainty is
one of the foundations of industrial quality control and,
generally speaking, in most modern industries the costs
bound up in taking measurements constitute 10 -15% of
production costs.

Laboratory of Legal Metrology, CTU in Prague
Athens 2008
Laboratory visits:
Czech Metrology Institue
www.cmi.cz
Calibration of digital multimeters
Quantum Hall effect based calibrations of resistance standards
Laboratory of Legal Metrology, CTU Prague
in Prague
See you in Prague!
2008
Athens 2006
TOPIC 3
Current balance
IC1 = IC2 = IC3 = I
C1
C2
C3
IC1 enters C1 at the bottom and leaves it at the top,
IC2 enters C2 at the top and leaves it at the bottom. 
 Electromagnetic forces on the suspended coil,
produced by these two currents, are of the same sense.
Current balance
F13  IC1 IC3
M13
2
 I f13
z
 0

4
d 1 d 3
a13
M13
 
C1 C3
(Neumann´s formula)
Current balance
M 23
2
F23  IC2 IC3
 I f23
z
From symmetry,
f13  f23  f
Resulting electromag netic force on C3 is
F  F13  F23  2 f I
2
Current balance operation
• The mass of the suspended coil plus the
vertical electromagnetic force exerted on it
are counterbalanced by means of a mass m
on the left scale pan.
• After reversing the current in the suspended
coil, the change m of this mass, necessary
to offset the change F of the acting
electromagnetic force, is determined.
1 Δm  g
ΔF  4 f I  Δm  g  I 
2
f
2
Current balance operation
A separate moving experiment makes it
possible to avoid the troublesome calculation
of f from the dimensions of the coils:
Coil C3, which is threaded by the magnetic
flux produced by the current I flowing in C1
and C2, is moved with a constant velocity v
in the vertical direction, a voltage u(t) being
induced in it:
Current balance operation
If a voltage drop U = R I is produced by
the current I on a known resistance R
and if a velocity v is choosen for which
u t   U
in the moment of passage of the scale
beam through its equilibrium position,
M13 R
f 

z
2v
Electronic kilogram
In case that quantum standards of
voltage and resistance are used to
measure the current I flowing in the coils
in course of the weighing experiment, a
current balance with a known value of f
can be used to produce known values of
electromagnetic force and to monitor
variations of masses which are used to
counterbalance it.
Voltage balance
is an apparatus based on counterbalancing
the attractive force between electrodes of a
capacitor.
1 2 C
Fz  U
2
z
DC comparator potentiometer
I
I
s
p
Slave
PS
R
Constant
current
PS
Ux,
Ue
N
n
N
s
s
p
PD
Ns = 1000 turns
ns =  11 turns
in steps of
10-4 turns
OSC
Np  2000 turns,
steps of 10-4 turns
Realization of the equivalent
of fractional turns
SPS
Is
... 500 50
Ω
Ω
5.625 Ω
1turn / step
1000 turns
R
Primary voltage standard system
with closed-cycle refrigerator
Primary voltage standard system
with closed-cycle refrigerator
• 100 nV calibration accuracy
• -11 V to 11 V range available from 10-V
system
• typically 1 hour stability time at 10 V for
10-V system
• automatic voltage calibration in minutes
• automatic calibration of DVMs
• complete system diagnostics
RMS/DC thermocouple comparator
1 2
S1
Rs1
Ux
TE1
S2
R
Rs2
TE2
Ur
Automatic RMS/DC thermocouple
comparator
Ux
Ur
Rs1
TE1
Rs2
TE2
Thermocouple protection
A
to relay R
drives
A
to relay R
drives
R
R
RMS/DC thermistor comparator
Ua
R1
R
2
Rs1
Ux
Ur
A
T1
T2
Rs
2
Improved RMS/DC thermistor
comparator
Ua
R1
R
R
3
2
A1
A2
Rs
Ux
1
Rs
R
2
4
T1
T2
Ur
Electrodynamic method of
measurement of AC power
moving
coil
ic1
ic1
dM
Inst. torque  i c1 i c2
d
dM
mean torque  i c1 i c2
d
T
ic2
i c1 i c2 
fixed coils
1
i c1 i c2 dt

T
0
ic1 = i1 + I1
ic2 = i2 - I2
i1 i 2  I1 I2  mean torque  0    0
Electrodynamic method of
measurement of AC power
Uout
Ur
I2
Control
circuit
R
I1
ic1
electrodynamometer
ic2
R
uU
R
R
i2
i1 uI
i1
A2
A1
fixed coils
moving coil
Thermal wattmeter
uU
Uout
R
R
R
R
A4
R
A2
A1
R
uI
R
R
A3
Uout  k uU  uI 2  uU  uI 2  4 k uU uI