22-08-2012, 05:08 PM
DIGITAL THERMOMETER
1DIGITAL THERMOMETER.doc (Size: 58.5 KB / Downloads: 41)
INTRODUCTION
Thermometer is a Greek word made up of thermo+meter. Thermo means ‘warm’ and meter means ‘to measure’. The thermometer has two important elements : the temperature sensor in which some physical change occurs with temperature and some means of converting this physical change into value.
Physical Principle of thermometry
Thermometers may be described as empirical or absolute. Absolute thermometers are calibrated numerically by the thermodynamic absolute temperature scale. Empirical thermometers are not in general necessarily in exact agreement with absolute thermometers as to their numerical scale readings, but to qualify as thermometers at all they must agree with absolute thermometers and with each other in the following way: given any two bodies isolated in their separate respective thermodynamic equilibrium states, all thermometers agree as to which of the two has the higher temperature, or that the two have equal temperatures. For any two empirical thermometers, this does not require that the relation between their numerical scale readings be linear, but it does require that relation to be strictly monotonic. This is a fundamental character of temperature and thermometers. As it is customarily stated in textbooks, taken alone, the so-called 'zeroth law of thermodynamics' fails to deliver this information.
Thermometric materials
There are various kinds of empirical thermometer based on material properties.
Many empirical thermometers rely on the constitutive relation between pressure and volume and temperature of their thermometric material. For example, mercury expands when heated. If it is used for its relation between pressure and volume and temperature, a thermometric material must have two properties: (1) its heating and cooling must be reversible; that is to say, the material must be able to be heated and cooled indefinitely often by the same increment and decrement of heat, and still return to its original pressure and volume and temperature every time; some plastics do not have this property; (2) throughout the the range of temperatures for which it is intended to work, at a given pressure, either (a) the volume increases when the temperature increases, or else (b) the volume decreases when the temperature increases; not (a) for some temperatures and (b) for others; at temperatures around about 4C, water does not have this property, and is said to behave anomalously in this respect; thus water cannot be used as a material for this kind of thermometry for temperature ranges about 4C. Gases, on the other hand, all have these properties; indeed they have just property (2)(a). Consequently, they are suitable thermometric materials, and that is why they were important in the development of thermometry.
1DIGITAL THERMOMETER.doc (Size: 58.5 KB / Downloads: 41)
INTRODUCTION
Thermometer is a Greek word made up of thermo+meter. Thermo means ‘warm’ and meter means ‘to measure’. The thermometer has two important elements : the temperature sensor in which some physical change occurs with temperature and some means of converting this physical change into value.
Physical Principle of thermometry
Thermometers may be described as empirical or absolute. Absolute thermometers are calibrated numerically by the thermodynamic absolute temperature scale. Empirical thermometers are not in general necessarily in exact agreement with absolute thermometers as to their numerical scale readings, but to qualify as thermometers at all they must agree with absolute thermometers and with each other in the following way: given any two bodies isolated in their separate respective thermodynamic equilibrium states, all thermometers agree as to which of the two has the higher temperature, or that the two have equal temperatures. For any two empirical thermometers, this does not require that the relation between their numerical scale readings be linear, but it does require that relation to be strictly monotonic. This is a fundamental character of temperature and thermometers. As it is customarily stated in textbooks, taken alone, the so-called 'zeroth law of thermodynamics' fails to deliver this information.
Thermometric materials
There are various kinds of empirical thermometer based on material properties.
Many empirical thermometers rely on the constitutive relation between pressure and volume and temperature of their thermometric material. For example, mercury expands when heated. If it is used for its relation between pressure and volume and temperature, a thermometric material must have two properties: (1) its heating and cooling must be reversible; that is to say, the material must be able to be heated and cooled indefinitely often by the same increment and decrement of heat, and still return to its original pressure and volume and temperature every time; some plastics do not have this property; (2) throughout the the range of temperatures for which it is intended to work, at a given pressure, either (a) the volume increases when the temperature increases, or else (b) the volume decreases when the temperature increases; not (a) for some temperatures and (b) for others; at temperatures around about 4C, water does not have this property, and is said to behave anomalously in this respect; thus water cannot be used as a material for this kind of thermometry for temperature ranges about 4C. Gases, on the other hand, all have these properties; indeed they have just property (2)(a). Consequently, they are suitable thermometric materials, and that is why they were important in the development of thermometry.