《现代传感器》课件Lecture7

Department of Optical Engineering
Zhejiang University
Other traditional techniques
Bimetal
based on thermal expansion very popular even today. switches are fairly inexpensive, can operate reliably for many cycles, may still be the correct choice for temperature
sensing applications.
Shortcomings
not accurate enough, not allow operation over a broad temperature
range.
Department of Optical Engineering
Zhejiang University
The thermal expansion coeffiecient of fused quartz is about 150x smaller than that of mercury, so this approximation is roughly valid.
the mercury in the column expands linearly with temperature.
Department of Optical Engineering
Zhejiang University
Resistance Thermometry
Platinum wires are commonly used for resistance thermometry.
though expensive, it is favored for these applications for several very good reasons:
Emitting signal : Esin(2ft); Received signal Esin(2ft+), with =2f*2d/c By mixing r(t) by
phas(t)=sin(2ft) (In phase) and by quad(t)=cos(2ft) (in Quadrature) we get : Vi (t)= r(t).phas(t) = (1/2)E{ cos() - cos(2ft +)} Vq(t) = r(t).quud(t) = (1/2)E{ sin() + sin(2ft +)}
For narrower temperature ranges, what can
we use?
Department of Optical Engineering
Zhejiang University
Thermistors and the Steinhart-Hart Equation
NTC thermister
Modern Sensors Lecture 7
X. Wu
Department of Optical Engineering
Zhejiang University
A Review of Lecture 6
Diaphragm deformation equations
Linear limit:
center deflection < 5X thickness Corrugated diaphragm ~10X thickness
If we want 1 mm/K at room temperature, and we have a reservoir volume of 0.1 cm3, we need a 660m column:
the sensitivity depends very strongly on the diameter of the column.
Department of Optical Engineering
Zhejiang University
Basic Intent
Overview basic techniques for sensing temperature
Study some product examples Some techniques for the measurement of
Other traditional techniques
Thermocouple
A metal wire ~ a vessel contains electrons
Heating one end of the wire:
the effect of heat is to increase the average velocity of the electrons on the heated end of the wire
Department of Optical Engineering
Zhejiang University
Sensitivity and Resolution Concerns
thermal expansion coefficient (CTE) of mercury ~ 30 PPM/K
If we assume that dimensions of the container do not change appreciably
For measurements which require accuracy of +/- 10 K, and need to be carried out at temperatures near 1000K, thermocouples are definitely the way to go.
Signal not the same freq.-> zero Out-of-phase signal->zero
Department of Optical Engineering
Zhejiang University
Thermometers - traditional techniques
flow will be briefly highlighted
Department of Optical Engineering
Zhejiang University
Analog synchronous modulation/demodulation For example: Lasertreancghe nfinidqerue
Capacitive sensor or strain gauge Some silicon based example calculations carried
out,
An alternative technology: Kavlico sensor and learn as much as we can from the way it is designed, built, packaged, and priced.
reasonably large temperature coefficient,
not affected by most chemicals, mechanically stable
withstand very high temperatures,
few other metals offer such a favorable collection of long-term stability performance advantages.
The ratio at a specified temperature(T), of the rate of change of zero power resistance with temperature to the zero power resistance
The dc resistance value of a thermistor at a specified temperature with negligible electrical power to avoid self heating.
Zero Power Temperature Coefficient of Resistance (Alpha):
There are a number of wellknown historical technologies for the measurement of temperature. mercury thermometer, in which a reservoir of mercury is sealed in a glass container under vacuum. When the reservoir is heated, the mercury expands, rising through a long thin column, upon which a graded ruler has been etched. What sort of sensitivity can be expected for such a system?
Steinhart-Hart equation for thermistors
Department of Optical Engineering
Zhejiang University
Glossary of terms - Thermister
Zero-Power Resistance (Ro):
After low-pass filter:
I = (1/2)Ecos() , Q = (1/2)Esin() Or