questions and their answers
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Questi ons and their answers
Q: what is heat ,temperature,triple point , ice point and BP
Answers: Heat
The energy transferred from a high-temperature object to a lower-
temperature object is called heat
OR Any spontaneous flow of energy from one object to another caused by a difference in
temperature between the objects is called heat
Temperature is a physical property that quantitatively expresses the common notions
of hot and cold . Objects of low temperature are cold, while various degrees of higher
temperatures are referred to as warm or hot.
, the triple point of a substance is the temperature and pressure at which
three phases (for example, gas , liquid , and solid ) of that substance coexist
in thermodynamic equilibrium . [1] For example, the triple point of mercury occurs at a
temperature of −38.8344 °C and a pressure of 0.2 mPa .
A thermometer (from the Greek θερμός (thermo) meaning "warm" and meter, "to
measure") is a device that measures temperature or temperature gradient using avariety of different principles. A thermometer has two important elements: the
temperature sensor (e.g. the bulb on a mercurythermometer) in which some physical
change occurs with temperature, plus some means of converting this physical change
into a value (e.g. the scale on a mercury thermometer). Thermometers increasingly use
electronic means to provide a digital display or input to a computer.
Thermomters are used in chemical industries, petrochemical,pharmaceutical,power
generation,food beverage,mining
Construction:The bimetallic thermometer consists of a bimetallic strip by bonding together two
thin strips of two different metals. The differential change in expansion of the twometals results in bending of the bimetallic strips with change in temperature.
Working:The bimetallic strip has one end fixed and the temperature changes causes the free
end to deflect. The range of linear relationship between deflection and temperature
depends on the combination of metal used. Invar alloy is a low expansion material.The other metals used can be steel, monel and brass.
The deflection of the free end is directly proportional to the temperature change and
the length of the strip and is inversely proportional to the thickness of the strip.
If one end is fixed, the deflection of the free end is direct indication of the
temperature measured. the sensitivity of the thermometer is increased by increasingthe length of the bimetallic strip.
The size pf the thermometer can be made small by having the bimetallic striparranged as the circular spiral or as an helix.
The temperature range that can be measured is -40 degree Celsius to +550 degree
Celsius. The accuracy is within +or-0.5% to +or-2%.
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Uses: used in ambient temperature measurement devices.
Used to air-conditioning thermostatsUsed in oil refineries.
used in Tyre Vulcanizers.Used in hot work wire heaters.
Used in tempering tanks.
Advantages
Simple, robust and inexpensive.It has good accuracy.
It can measure temperature in the range of (-40 to 550) in Celsius.It can withstand 50% overage temperature measurement.
DisadvantagesNot recommended for measurement of temperature above 550 degree Celsius.
The metals undergo permanent warp distortion.Usage is limited to local mounting.
thermocouple is a junction between two different metals that produces a voltage related toatemperature difference. Thermocouples are a widely used type of temperature sensor for measurement and control [1] and can also be used to convert heat into electric power. They areinexpensive[2] and interchangeable, are supplied fitted with standard connectors, and canmeasure a wide range of temperatures. The main limitation is accuracy: system errors of lessthan one degree Celsius (C) can be difficult to achieveThermocouples are widely used in science and industry; applications include temperaturemeasurement for kilns, gas turbine exhaust, diesel engines, and other industrial processes.In 1821, the German –Estonian physicist Thomas Johann Seebeck discovered that when anyconductor is subjected to a thermal gradient, it will generate a voltage. This is now known asthe thermoelectric effect or Seebeck effect. Any attempt to measure this voltage necessarilyinvolves connecting another conductor to the "hot" end. This additional conductor will then alsoexperience the temperature gradient, and develop a voltage of its own which will oppose theoriginal. Fortunately, the magnitude of the effect depends on the metal in use. Using a dissimilar metal to complete the circuit creates a circuit in which the two legs generate different voltages,leaving a small difference in voltage available for measurement. That difference increases withtemperature, and is between 1 and 70 microvolts per degree Celsius (µV/°C) for standard metalcombinations.
Cold junction compensation
Thermocouples measure the temperature difference between two points, not absolute
temperature. To measure a single temperature one of the junctions—normally the cold junction—
is maintained at a known reference temperature, and the other junction is at the temperature to
be sensed.
A thermocouple can produce current, which means it can be used to drive some processesdirectly, without the need for extra circuitry and power sources. For example, the power from athermocouple can activate a valve when a temperature difference arises. The electricalenergygenerated by a thermocouple is converted from the heat which must be supplied to the hotside to maintain the electric potential. A continuous flow of heat is necessary because the currentflowing through the thermocouple tends to cause the hot side to cool down and the cold side toheat up (the Peltier effect).
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Applications
Thermocouples are suitable for measuring over a large temperature range, up to 2300 °C. They
are less suitable for applications where smaller temperature differences need to be measured
with high accuracy, for example the range 0–100 °C with 0.1 °C accuracy. For such
applications thermistors and resistance temperature detectors are more suitable. Applications
include temperature measurement for kilns,gas turbine exhaust, diesel engines, and other
industrial processes.
[edit]Steel industry
Type B, S, R and K thermocouples are used extensively in the steel and iron industries to monitor
temperatures and chemistry throughout the steel making process. Disposable, immersible, type S
thermocouples are regularly used in the electric arc furnace process to accurately measure the
temperature of steel before tapping. The cooling curve of a small steel sample can be analyzed
and used to estimate the carbon content of molten steel.
Thermopiles are used for measuring the intensity of incident radiation, typically visible or infrared
light, which heats the hot junctions, while the cold junctions are on a heat sink. It is possible to
measure radiative intensities of only a few μW/cm2 with commercially available thermopile
sensors. For example, some laser power meters are based on such sensors.
[ edit ] Manufacturing
Thermocouples can generally be used in the testing of prototype electrical and mechanical
apparatus. For example, switchgear under test for its current carrying capacity may have
thermocouples installed and monitored during a heat run test, to confirm that the temperature rise
at rated current does not exceed designed limits.
[ edit ] Radioisotope thermoelectric generators
Thermopiles can also be applied to generate electricity in radioisotope thermoelectric generators.
[ edit ] Process plants
Chemical production and petroleum refineries will usually employ computers for logging and limit
testing the many temperatures associated with a process, typically numbering in the hundreds.
For such cases a number of thermocouple leads will be brought to a common reference block (a
large block of copper) containing the second thermocouple of each circuit. The temperature of
the block is in turn measured by athermistor . Simple computations are used to determine the
temperature at each measured location.
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Primary and secondary thermometers
Thermometers can be divided into two separate groups according to the level of knowledge
about the physical basis of the underlying thermodynamic laws and quantities. For primary
thermometers the measured property of matter is known so well that temperature can be
calculated without any unknown quantities. Examples of these are thermometers based on the
equation of state of a gas, on the velocity of sound in a gas, on the thermal noise (see Johnson–
Nyquist noise ) voltage or current of an electrical resistor, and on the
angular anisotropy of gamma ray emission of certain radioactive nuclei in a magnetic field .
Primary thermometers are relatively complex.
Secondary thermometers are most widely used because of their convenience. Also, they are
often much more sensitive than primary ones. For secondary thermometers knowledge of the
measured property is not sufficient to allow direct calculation of temperature. They have to be
calibrated against a primary thermometer at least at one temperature or at a number of fixed
temperatures. Such fixed points, for example,triple points and superconducting transitions, occur
reproducibly at the same temperature.
While an individual thermometer can measure degrees of hotness, the readings on two
thermometers cannot be compared unless they conform to an agreed scale. There is today an
absolute thermodynamic temperature scale. Internationally agreed temperature scales are
designed to approximate this closely, based on fixed points and interpolating thermometers. The
most recent official temperature scale is theInternational Temperature Scale of 1990 . It extends
from 0.65 K (−272.5 °C; −458.5 °F) to approximately 1,358 K (1,085 °C; 1,985 °F).
There are a number of uses for thermometers. Thermometers have been built which utilize a
range of physical effects to measure temperature. Temperature sensors are used in a wide
variety of scientific and engineering applications, especially measurement systems. Temperature
systems are primarily either electrical or mechanical, occasionally inseparable from the system
which they control (as in the case of a mercury thermometer ). Alcohol thermometers, infrared
thermometers, mercury-in-glass thermometers, recording thermometers, thermistors, and Six's
thermometers are used outside in areas which are well-exposed to the elements at various levels
of the Earth's atmosphere and within the Earth's oceans is necessary within the fields
of meteorology and climatology . Airplanes use thermometers and hygrometers to determine
if atmospheric icing conditions exist along their flight path, and these measurements are used to
initialize weather forecast models. Thermometers are used within roadways in cold weather
climates to help determine if icing conditions exist. Indoors, thermistors are used in climate
control systems such as air conditioners, freezers, heaters,refrigerators, and water heaters.
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[19] Galileo thermometers are used to measure indoor air temperature, due to their limited
measurement range.
Bi-metallic stemmed thermometers, thermocouples, infrared thermometers, and thermisters are
handy during cooking in order to know if meat has been properly cooked . Temperature of food is
important because if it sits within environments with a temperature between 5 °C (41 °F) and
57 °C (135 °F) for four hours or more, bacteria can multiply leading to foodborne illnesses.
[19] Thermometers are used in the production of candy . Medical thermometers such as mercury-
in-glass thermometers,[20] infrared thermometers,[21] pill thermometers, and liquid crystal
thermometers are used within health care to determine if individuals have a fever or
are hypothermic . Liquid crystal thermometers can also be used to measure the temperature of
water in fish tanks. Fiber Bragg grating temperature sensors are used withinnuclear
power facilities to monitor reactor core temperatures and avoid the possibility of nuclear
meltdowns
What are Thermistors?
Thermistors are semiconductor devices that are used to measure temperature.
The name comes from a combination of the words "resistor" and "thermal".
Thermistors have an electrical resistance that is proportional to temperature.
From a general physics course on electricity and magnetism, you may have
learned that this is a property typical for all conductors. For example, devices
such toasters, heaters, and light bulbs operate on this principle. Thermistors are
different in that they are created to deliberately exploit this effect, and hence aremore temperature sensitive than usual.
The Mathematical Model
The basic mathematical model used for thermistors is the Steinhart-Hart equation, discovered by oceanographers I.S. Steinhart and S.R. Hart. In itssimplest form it is:
1/T = a + b(ln R) +c(ln R)3
where T is the temperature, a, b, and c are coefficients that are measured, ln isthe natural log, and R is the resistance in ohms.
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Construction
Thermistors are used in science and engineering applications. They are also
useful in medicine as clinical temperature sensors or as probes during surgery.There are two types: PTC (Positive Temperature Coefficient of Resistance)and NTC (Negative Temperature Coefficient of Resistanc
NTC thermistors have temperatures that vary inversely with resistance such that as the temperature increases, the resistance decreases, and vice versa. They are very often used for temperature control and indication, and for current suppression. Common materials used in their construction include oxides of materials such as nickel, manganese, copper, iron, and cobalt. Some are alsomade from silicon and/or germanium. They are usually packaged in an epoxy,
and are the most common type of thermistor.
PTC thermistors are the opposite of NTCs in that they have a resistance that increases with rising temperature and decreases with falling temperature. They are used to protect circuits from overload, and can function as thermal switchesor as ordinary thermometers. PTCs are constructed using semiconductorscombined with ceramics or polymers.
Advantages and DisadvantagesThermistors are small, very stable, are long-lasting, and are usually accurate towithin +/- .05% to +/- .02% . This makes them superior to thermocouples and other devices that measure temperature. The disadvantage is that like typical semiconductors, they are non-linear, and so this effect must be compensated for when building circuits. Also, unlike thermocouples, they cannot be used at very high temperatures.