Temperature Flashcards
o Thermocouple
§ Operation
A conductor experiencing different
temperatures at different locations will also
produce a voltage difference due to the
difference in electron density.
• Two wires of different conductance are joined
at one end which is at T1. The other end of
each wire experiences T2.
• A voltage can be measured between each of these ends
because the two wires have different voltages
thermopile
Thermocouple arranged in series to amplify the voltage
o Thermocouple
§ Advantages:
§ Disadvantages
•§ Advantages: Small, inexpensive and simple to use • Wide range • Rapid response § Disadvantages • Signals are small and hard to measure; Need an instrument amplifier to measure signal • Accuracy is hard to maintain • A reference junction is necessary
Thermistor
§ Operation
• A thermistor is a semiconductor resistive temperature sensor
made of sintered metal oxides. These materials exhibit a
strong non-linear and (usually) negative dependence of
resistance on temperature.
• T = 1/(a +b × ln(R) + c × ln^3(R))
• Four wire resistance measurement:
o Use two leads connected to each end of thethermistor. A small voltage drop occurs across the
non-negligible resistance of the current carrying leads.
o The other two leads carry no current but are used to
measure the voltage across the thermistor
• Wheatstone bridge with instrumentation amplifier
Thermistor
§ Advantages:
§ Disadvantages
Advantages
• High precision temperature measurement (~0.1 mK) – most
sensitive
• Can be made very small with very fast response time
• May be encapsulated in glass, making robust
§ Disadvantages
• Small range (~-200-200)
• Self heating
• Highly non-linear
RTD/Thermoresistive Sensor
§ Operation
• Resistor made from metal that has temperature dependent
resistivity
• Most common material used as the wire is platinum because
it is resistant to corrosion
• Rt = Ro`(1 + alpha T)
• Get RTD Probes (for probing) and thin film (for surface
temperature)
• Force a known current through the resistor and measure
voltage
• 4-wire to avoid errors due to resistance of the current source
leads
RTD/Thermoresistive Sensor
§ Advantages:
§ Disadvantages
• Force a known current through the resistor and measure
voltage
• 4-wire to avoid errors due to resistance of the current source
leads
Quartz resonance•
The quartz resonance thermometer is made from a suitably cut quartz crystal resonator where the resonant frequency has a temperature dependence
p-n junction diode
The voltage-current characteristic of the forward-bias p-n junction is given by
I= A exp( (qV - Eg)/ kT )
what are these parameters
where I is the forward bias current, A is a constant depending on the geometry of the junction, qis the electron charge, Eg is the band gap energy, k is the Boltzman constant, and T is absolute temperature.
If the p-n junction is driven by different forward currents, I1 and I2, and voltages V1 and V2 are developed at these currents then V1–V2= (kT/q)ln(I1/I2)
Thus ??
the difference in voltages, corresponding to two currents maintained at a constant ratio, is proportional to the absolute temperature.
A radiation thermometer (following Planck’s black body radiation formula) is an instrument that measures
thermal radiation emitted from an object surface. squiggle E is the emissivity of the skin surface
Thermoelectric heat pumps (often called TECs) take advantage of the ______effect to ?
Peltier
transfer heat between two surfaces.
Thermoelectric heat pumps (TECs) Operation
A typical thermoelectric heat pump consists
of an array of p- and n- type semiconductor
elements soldered between two ceramic
plates, electrically in series and thermally in
parallel.
§ As DC current passes through one or more
pairs of elements from n- to p-, there is a
decrease in temperature at the junction
(“cold side”) resulting in the absorption of
heat from the environment. The heat is
carried through the cooler by electron
transport and released on the opposite (“hot”) side as the electrons
move from a high to low energy state
Thermoelectric heat pumps (TECs)
Properties
Bidirectional
§ Important to use heat-sinks if we need to transfer head to
surrounding air; and grease to thermally couple the TEC surfaces
§ Often used to create temperature-controlled environments
Radiation Thermometer
The total power received by a radiation thermometer directed at the
skin is the sum of the radiation emitted from the skin and ambient
radiation reflected from the skin surface
Radiation thermometry can provide non-contact temperature
measurement in under 1s with accuracy better than 0.5 K
