Download R is the measured resistance

GROUP-A (5 MARKS)
1. Transformer method
In this method a transformer is used to measure resistivity without any direct contact with the specimen.
The transformer consists of a primary coil which energises the circuit with an AC voltage and a secondary
which is formed by a toroid of the concrete sample. The current in the sample is detected by a current coil
wound around a section of the toroid (a current transformer). This method is good for measuring the
setting properties of concrete, its hydration and strength. Wet concrete has a resistivity of around 1 Ωm which progressively increases as the cement sets.[4]
2. Relation to corrosion
Corrosion is an electro-chemical process. The rate of flow of the ions between
the anode and cathode areas, and therefore the rate at which corrosion can occur, is affected by
the resistivity of the concrete.[6] To measure the electrical resistivity of the concrete a current is applied to
the two outer probes and the potential difference is measured between the two inner probes. Empirical
tests have arrived at the following threshold values which can be used to determine the likelihood of
corrosion.
• When ρ ≥ 120 Ω-m
corrosion is unlikely
• When ρ = 80 to 120 Ω-m
corrosion is possible
• When ρ ≤ 80 Ω-m
corrosion is fairly certain
These values have to be used cautiously as there is strong evidence that chloride diffusion and surface
electrical resistivity is dependent on other factors such as mix composition and age. [7] The electrical
resistivity of the concrete cover layer decreases due to:[8]

Increasing concrete water content

Increasing concrete porosity

Increasing temperature

Increasing chloride content

Decreasing carbonatation depth
When the electrical resistivity of the concrete is low, the rate of corrosion increases. When the electrical
resistivity is high, e.g. in case of dry and carbonatated concrete, the rate of corrosion decreases.
3. On-site methods
Four probes
On-site electrical resistivity of concrete is commonly measured using four probes in a Wenner array. The
reason for using four probes is the same as in the laboratory method - to overcome contact errors. In this
method four equally spaced probes are applied to the specimen in a line. The two outer probes induce
the current to the specimen and the two inner electrodes measure the resulting potential drop. The
probes are all applied to the same surface of the specimen and the method is consequently suitable for
measuring the resistivity of bulk concrete in situ.[5]
The resistivity is given by:
V is the voltage measured between the inner two probes (measured in volts, V)
I is the current injected in the two outer probes (measured in amps, A)
a is the equal distance of the probes (measured in metres, m).
Rebars
The presence of rebars disturbs electrical resistivity measurement as they conduct current much better
than the surrounding concrete. This is particularly the case when the concrete cover depth is less than
30 mm. In order to minimize the effect, placing the electrodes above a rebar is usually avoided, or if
unavoidable, then they are placed perpendicular to the rebar.
However, measurement of the resistance between a rebar and a single probe at the concrete surface is
sometimes done in conjunction with electrochemical measurements. Resistivity strongly
affects corrosion rates and electrochemical measurements require an electrical connection to the rebar. It
is convenient to make a resistance measurement with the same connection.[3]
The resistivity is given by:
R is the measured resistance,
D is the diameter of the surface probe.
GROUP-B (20 MARKS)
1. Constant current potentiometer
A potentiometer being calibrated and then measuring an unknown voltage.
R1 is the resistance of the entire resistance wire. The arrow head represents the moving wiper.
In this circuit, the ends of a uniform resistance wire R1 are connected to a regulated DC supply VS for use
as a voltage divider. The potentiometer is first calibrated by positioning the wiper (arrow) at the spot on
the R1 wire that corresponds to the voltage of a standard cell so that
A standard electrochemical cell is used whose emf is known (e.g. 1.0183 volts for a Weston standard
cell).[2][3]
The supply voltage VS is then adjusted until the galvanometer shows zero, indicating the voltage on R2 is
equal to the standard cell voltage.
An unknown DC voltage, in series with the galvanometer, is then connected to the sliding wiper, across a
variable-length section R3 of the resistance wire. The wiper is moved until no current flows into or out of
the source of unknown voltage, as indicated by the galvanometer in series with the unknown voltage. The
voltage across the selected R3 section of wire is then equal to the unknown voltage. The final step is to
calculate the unknown voltage from the fraction of the length of the resistance wire that was connected to
the unknown voltage.
The galvanometer does not need to be calibrated, as its only function is to read zero or not zero. When
measuring an unknown voltage and the galvanometer reads zero, no current is drawn from the unknown
voltage and so the reading is independent of the source's internal resistance, as if by
a voltmeter of infiniteresistance.
Because the resistance wire can be made very uniform in cross-section and resistivity, and the position of
the wiper can be measured easily, this method can be used to measure unknown DC voltages greater
than or less than a calibration voltage produced by a standard cell without drawing any current from the
standard cell.
If the potentiometer is attached to a constant voltage DC supply such as a lead–acid battery, then a
second variable resistor (not shown) can be used to calibrate the potentiometer by varying the current
through the R1 resistance wire.
If the length of the R1 resistance wire is AB, where A is the (-) end and B is the (+) end, and the movable
wiper is at point X at a distance AX on the R3 portion of the resistance wire when the galvanometer gives
a zero reading for an unknown voltage, the distance AX is measured or read from a preprinted scale next
to the resistance wire. The unknown voltage can then be calculated:
2. Electric current (current of charge)

Ammeter

Clamp meter

Galvanometer
The relation between electric current, magnetic fields and physical forces was first noted by Hans
Christian Ørsted who, in 1820, observed a compass needle was deflected from pointing North when a
current flowed in an adjacent wire. The tangent galvanometer was used to measure currents using this
effect, where the restoring force returning the pointer to the zero position was provided by the Earth's
magnetic field. This made these instruments usable only when aligned with the Earth's field. Sensitivity of
the instrument was increased by using additional turns of wire to multiply the effect – the instruments
were called "multipliers".
Types
The D'Arsonval galvanometer is a moving coil ammeter. It uses magnetic deflection, where current
passing through a coil causes the coil to move in a magnetic field. The modern form of this instrument
was developed by Edward Weston, and uses two spiral springs to provide the restoring force. By
maintaining a uniform air gap between the iron core of the instrument and the poles of its permanent
magnet, the instrument has good linearity and accuracy. Basic meter movements can have full-scale
deflection for currents from about 25 microamperes to 10 milliamperes and have linear scales.
Moving iron ammeters use a piece of iron which moves when acted upon by the electromagnetic force of
a fixed coil of wire. This type of meter responds to both direct and alternating currents (as opposed to the
moving coil ammeter, which works on direct current only). The iron element consists of a moving vane
attached to a pointer, and a fixed vane, surrounded by a coil. As alternating or direct current flows through
the coil and induces a magnetic field in both vanes, the vanes repel each other and the moving vane
deflects against the restoring force provided by fine helical springs.[2] The non-linear scale of these
meters makes them unpopular.
An electrodynamic movement uses an electromagnet instead of the permanent magnet of the d'Arsonval
movement. This instrument can respond to both alternating and direct current.
In a hot-wire ammeter, a current passes through a wire which expands as it heats. Although these
instruments have slow response time and low accuracy, they were sometimes used in measuring radiofrequency current.
Digital ammeter designs use an analog to digital converter (ADC) to measure the voltage across the
shunt resistor; the digital display is calibrated to read the current through the shunt. There is also a whole
range of devices referred to as integrating ammeters.In these ammeters, the amount of current is
summed over time, giving as a result the product of current and time, which is proportional to the energy
transferred with that current. These can be used for energy meters (watt-hour meters) or for estimating
the charge of battery or capacitor.
Picoammeter
A picoammeter, or pico ammeter, measures very low electrical current, usually from the picoampere
range at the lower end to the milliampere range at the upper end. Picoammeters are used for sensitive
measurements where the current being measured is below the theoretical limits of sensitivity of other
devices, such as Multimeters. Most picoammeters use a "virtual short" technique and have several
different measurement ranges that must be switched between to cover multiple decades of measurement.
Other modern picoammeters use log compression and a "current sink" method that eliminates range
switching and associated voltage spikes.
Application
The majority of ammeters are either connected in series with the circuit carrying the current to be
measured (for small fractional amperes), or have their shunt resistors connected similarly in series. In
either case, the current passes through the meter or (mostly) through its shunt. They must not be
connected to a source of voltage; they are designed for minimal burden, which refers to the voltage drop
across the ammeter, which is typically a small fraction of a volt. They are almost a short circuit.
Ordinary Weston-type meter movements can measure only milliamperes at most, because the springs
and practical coils can carry only limited currents. To measure larger currents, a resistor called a shunt is
placed in parallel with the meter. The resistances of shunts is in the integer to fractional milliohm range.
Nearly all of the current flows through the shunt, and only a small fraction flows through the meter. This
allows the meter to measure large currents. Traditionally, the meter used with a shunt has a full-scale
deflection (FSD) of 50 mV, so shunts are typically designed to produce a voltage drop of 50 mV when
carrying their full rated current.
Zero-center ammeters are used for applications requiring current to be measured with both polarities,
common in scientific and industrial equipment. Zero-center ammeters are also commonly placed in series
with a battery. In this application, the charging of the battery deflects the needle to one side of the scale
(commonly, the right side) and the discharging of the battery deflects the needle to the other side. A
special type of zero-center ammeter for testing high currents in cars and trucks has a pivoted bar magnet
that moves the pointer, and a fixed bar magnet to keep the pointer centered with no current. The
magnetic field around the wire carrying current to be measured deflects the moving magnet.
Since the ammeter shunt has a very low resistance, mistakenly wiring the ammeter in parallel with a
voltage source will cause a short circuit, at best blowing a fuse, possibly damaging the instrument and
wiring, and exposing an observer to injury.
In AC circuits, a current transformer converts the magnetic field around a conductor into a small AC
current, typically either 1 A or 5 A at full rated current, that can be easily read by a meter. In a similar way,
accurate AC/DC non-contact ammeters have been constructed using Hall effect magnetic field sensors. A
portable hand-held clamp-on ammeter is a common tool for maintenance of industrial and commercial
electrical equipment, which is temporarily clipped over a wire to measure current. Some recent types
have a parallel pair of magnetically soft probes that are placed on either side of the conductor.
3. Uncategorized, specialized, or generalized application
Checkweigher measures precise weight of items in a conveyor line, rejecting under or overweight
objects.

Densitometer measures light transmission through processed photographic film or transparent
material or light reflection from a reflective material.

Force platform measures ground reaction force.

Gauge (engineering) A highly precise measurement instrument, also usable to calibrate other
instruments of the same kind. Often found in conjunction with defining or applying technical
standards.

Gradiometer any device that measures spatial variations of a physical quantity. For example as done
in gravity gradiometry.

Parking meter measures time a vehicle is parked at a particular spot, usually with a fee.

Postage meter measures postage used from a prepaid account.

S meter measures the signal strength processed by a communications receiver.

Sensor, hypernym for devices that measure with little interaction, typically used in technical
applications.

Spectroscope is an important tool used by physicists.

SWR meter check the quality of the match between the antenna and the transmission line.

Time-domain reflectometer locates faults in metallic cables.

Universal measuring machine measures geometric locations for inspecting tolerances.
Fictional devices

Tricorder, a multipurpose scanning device, originating from the science-fictional Star Trek series.

Sonic Screwdriver, a multifunctional device used occasionally for scanning, originating from the
science-fictional Doctor Who series.
4. Laboratory methods
Two electrodes
Concrete electrical resistance can be measured by applying a current using two electrodes attached to
the ends of a uniform cross-section specimen. Electrical resistivity is obtained from the equation: [1]
R is the electrical resistance of the specimen, the ratio of voltage to current (measured in ohms,
Ω)
is the length of the piece of material (measured in metres, m)
A is the cross-sectional area of the specimen (measured in square metres, m²).
This method suffers from the disadvantage that contact resistance can significantly add
to the measured resistance causing inaccuracy. Conductive gels are used to improve
the contact of the electrodes with the sample.[2]
Four electrodes
The problem of contact resistance can be overcome by using four electrodes. The two end electrodes are
used to inject current as before, but the voltage is measured between the two inner electrodes. The
effective length of the sample being measured is the distance between the two inner electrodes. Modern
voltage meters draw very little current so there is no significant current through the voltage electrodes and
hence no voltage drop across the contact resistances.[3]