23-09-2014, 11:55 AM
THERMOMETERS.
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ABSTRACT
A thermometer is a device that measures temperature or temperature gradient, using a variety of different principles. The word thermometer is derived from two smaller word fragments: thermo from the Greek for heat and meter from Greek, meaning to measure. A thermometer has two important elements, the temperature sensor (e.g. the bulb on a mercury thermometer) 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). Industrial thermometers commonly use electronic means to provide a digital display or input to a computer
INTRODUCTION
A thermometer is a device that measures temperature.
Temperature is measured in a scale called Fahrenheit (F) and in Celsius or centigrade. The point where water freezes is 32 degrees Fahrenheit and 0 degree Celsius. The point where water boils is 212℉ and 100℃. Some scientific thermometers use the Kelvin scale, where 0K is called absolute zero- a place where there is no movement of any parts of matter, where substance have no thermal energy. It’s about -273.15℃ (below zero) or 459.67° below 0℉
Scientists have never been able to measure anything about absolute zero, though they have gotten very close. Thermometers help us to know what the weather will be like. If it will be 90℉ outside, we are not going to put on a winter coat. Or if it’s below zero, we will not be wearing shorts
CHARACTERISTICS OF THERMOMETERS.
• Glass Stems & Bulbs: Solid stem /enclosed scale mercury-in –glass thermometers, made from specially selected and treated, with uniform bore capillary and tubing.
• Permanent Accuracy: Each thermometer is skillfully annealed (toughen metal or glass by heating and slow cutting) artificially aged to avoid loss of accuracy with time.
• Safety Chamber:All thermometers are provided with safety chambers for precaution against breakage in case of overheating and to facilitate rejoining of mercury column in case of separation in transit or in use.
• Ranges: available from - 100° C to 600° C.
• Graduation: Thin clearly cut and deeply etched graduations and distinctive figures accurately done by the most up to date graduating machines.
• Accuracy: All the thermometers are guaranteed to be accurate within tolerances required when they leave our works & when tested under correct conditions.
• Least Count from 0.01° C onwards is available.
• Calibration:Each thermometer is individually calibrated against with accuracy of 0.001° C. Calibration for total/partial immersion can be carried out. Thermometers can be supplied with works certificate.
• Fiduciary scale is provided below contraction chamber for checking purposes.
• Fillings:Filled with triple distilled mercury or red/blue clear appropriate thermometric liquid.
• Nitrogen pressure filling: Thermometers are pressurized with nitrogen gas above the column wherever necessary.
• Thallium Filling: Thermometers of minus range with mercury, thallium needs to be mixed with mercury after certain temperature to maintain the proper functioning of thermometers as freezing point of mercury is -38.83° C.
• Casings: Thermometers are supplied in individual plastic/card board cases.
Advantages:
a. It is very accurate. In fact its accuracy allows it to be utilized to calibrate other thermometers
Disadvantages:
a. It is not easy to handle and read.
b. It tends to be highly sensitive to temperature change, and mechanical vibrations. In fact to give a reading it usually entails a lot of time.
c. Expensive to manufacture and keep
Modification of the Characteristics of Thermometers
Taking into account the various types of thermometer owing to their functions, industrial value/use and applications, here we seek to enumerate the various changes made to the differing types of thermometer. These are characteristics that were precipitated by the need to improve on the transducers, sensors, measurements, calibration, modifying inputs, static sensor characteristics, dynamic sensor characteristics, etc.
The parent type and basic function of thermometer started from the development of the liquid-in-glass thermometer. Consequently, the evolution of other thermometers for varying other applications of heat measurement came to the center stage upon the advancement of science.
Thermal image camera, Resistance thermometer, The Constant Volume Gas Thermometer Infra-red thermometer are examples of thermometers that were improved based on the developed basic form of thermometric principle. This was achieved by introducing a transducer (a device that converts a signal from one physical form to a corresponding signal having a different physical form e.g. Physical form: mechanical, thermal, magnetic, electric, optical, chemical…) into the system.
The Dual/Multiple sensor thermometer is also an example of one that was developed based on a specific need: to take simultaneous reading together. The Fixed wired thermometer has a sensor that makes it deliver instant temperature reading in food and liquid. This is especially modified for the food industry. K-type thermometer were built on the ability to withstand very high temperature readings. This type of thermometer is especially build for the high power laboratories.
Other type of thermometers developed on the premise of specific function includes the clinical thermometer. The measured temperature of the body is compared with the standard calibration of the average human temperature already embedded in the thermometer. The Data logging thermometer also serves to record and display measured values in real time.