Heat & Temperature Sensors Flashcards
It is a measure of the thermal energy in a body, which is the relative hotness or coldness of
a medium.
Temperature
It is a form of energy; as energy is supplied to a system the vibration amplitude of its molecules and its temperature increases. The temperature
increase is directly proportional to the heat energy in the system.
Heat
It is defined as
the amount of energy required to raise the temperature of 1 lb of pure water by 1°F at 68°F and at atmospheric pressure.
British Thermal Unit (BTU)
It is the quantity of heat energy required to raise the temperature of a given weight of a material by 1°.
Specific heat
It is the flow or transfer of heat from a high
temperature region to a low temperature region.
Thermal conductivity
It is the flow of heat through a material. The molecular vibration amplitude or energy is transferred from one molecule in a material to the next.
Conduction
It is the transfer of heat due to motion of elevated temperature particles in a material(liquid and gases).
Convection
It is the emission of energy by electromagnetic waves that travel at the
speed of light through most materials that do not conduct electricity
Radiation
It is the change in dimensions of a material due to temperature changes. The change in dimensions of a material is due to its coefficient of thermal expansion that is expressed as the change in linear dimension (alpha) per degree temperature change.
Linear thermal expansion
It is the change in the
volume (beta) per degree temperature change due to the linear coefficient of expansion.
Volume thermal expansion
Methods of measuring temperature
- Expansion of a materialto give visual indication, pressure, or dimensional change
- Electrical resistance change
- Semiconductor characteristic change
- Voltage generated by dissimilar metals
- Radiated energy
The most common direct
visual reading thermometer (if not the only one). The device consisted of a small bore graduated glass tube with a small bulb containing a reservoir of mercury.
Mercury in glass
It is a type of temperature measuring device that is relatively inaccurate, slow to respond, not normally used in analog applications to give remote indication, and has hysteresis. It is extensively used in ON/OFF applications not requiring high accuracy, as it is rugged and cost effective.
Bimetallic strip
These thermometers are used where remote indication is required, as opposed to glass and bimetallic devices which give readings at the point of detection.
Pressure-spring thermometers
Three types or classes of pressure-spring devices
Class 1: Liquid filled
Class 2: Vapor pressure
Class 3: Gas filled
It works on the same principle as the liquid in glass thermometer, but is used to drive a Bourdon tube. The device has good linearity and accuracy and can be used up to 550°C
Liquid filled thermometer
It is partially filled with liquid and vapor such as methyl chloride, ethyl alcohol, ether, toluene, and so on. In this system the lowest operating
temperature must be above the boiling point of the liquid and the maximum temperature is
limited by the critical temperature of the liquid.
Vapor-pressure thermometer system
It is filled with a gas such as nitrogen at a pressure range of 1000 to 3350 kPa at room temperature. The device obeys the basic gas laws for a constant volume system [V1 =
V2] giving a linear relationship between absolute temperature and pressure.
Gas thermometer
These are either a metal film
deposited on a former or are wire-wound resistors. The
devices are then sealed in a glass ceramic composite
material. The electrical resistance of pure metals is positive, increasing linearly with temperature.
Resistance temperature devices (RTD)
Resistance devices are normally measured using a _____ type of
system, but are supplied from a constant current source. Care should also be taken to prevent electrical current from heating the device and causing
erroneous readings
Wheatstone bridge
These are a class of metal oxide (semiconductor material)
which typically have a high negative temperature coefficient of resistance, but can also be positive.
Thermistors
2 types of thermistors
NTC and PTC
It is formed when two dissimilar metals are joined together to form a junction. An electrical circuit is completed by joining the other ends of the dissimilar metals together to form a
second junction
Thermocouple
Three effects that are associated with thermocouples
Seebeck effect, Peltier effect, and Thompson effect
It states that the voltage produced in a thermocouple is proportional to the temperature between the two junctions.
Seebeck effect
It states that if a current flows through a thermocouple one junction is heated (puts out energy) and the other junction is cooled (absorbs energy).
Peltier effect
It states that when a current flows in a conductor along which there is a temperature difference, heat is produced or absorbed, depending upon the direction of the current and the variation of temperature
Thompson effect
It is a number of thermocouples connected in series, to increase the sensitivity and accuracy by increasing the output voltage when measuring low temperature differences.
Thermopile
It is a device that measure temperature by sensing the heat radiated from a hot
body through a fixed lens that focuses the heat energy on to a thermopile; this is a
non contact device.
Pyrometers
Normally the reference
voltage of a zener diode or the junction voltage
variations are used for _____.
temperature sensing
Application considerations (Heat & Temperature Sensors)
- Selection
- Range and Accuracy
- Thermal time constant
- Installation
- Calibration
- Protection
