Spring 2005ISAT 253 Transducers and Sensors II Monday, March 21

Spring 2005ISAT 253 The Sensors We’ll Study  Optical sensors  Thermal sensors  Chemical sensors  Strain gage sensors

Week 7 Thermal Sensors: Types 1. Fluid-Expansion Commercially available temperature range –36 to 761°F –38 to 405°C

Week 7 Types- cont. 2. Bimetallic Devices Application of Thermal Linear Expansion Steel Brass Thermal Switch

3. Change-of-State Devices Types- cont. Reversible Temperature Labels, Multi Temp Liquid Crystal Strips Non-Reversible Temperature Labels

Week 7 4. Infrared Temperature Measurement Devices Types- cont.

Week 7 5. Thermocouple Types- cont. Thermocouples consist essentially of two strips or wires made of different metals and joined at one end. Changes in the temperature at that juncture induce a change in electromotive force (emf) between the other ends. As temperature goes up, this output emf of the thermocouple rises, though not necessarily linearly. SEEBECK VOLTAGE

Week 7 Thermocouples- Cont. Different Sizes and Shapes Beaded and Washer types Veterinary Implantation Surface Cement-on Hypodermic

Week 7 Thermocouples- Cont. The voltage generated by a thermocouple depends on the composition of the wires used to form the junction.

Week 7 Thermocouples- cont Temperature (Fahrenheit) Millivolts Output of Some Common Thermocouples

Week 7 Thermocouples- Thermopile Why might you connect thermocouples in series? Sensing Junctions Reference Junctions

Week 7 6. Resistance Temperature Devices (RTD) Resistive temperature devices capitalize on the fact that the electrical resistance of a material changes as its temperature changes. Two key types are the metallic devices (commonly referred to as RTDs), and thermistors. As their name indicates, RTDs rely on resistance change in a metal, with the resistance rising more or less linearly with temperature. Thermistors are based on resistance change in a ceramic semiconductor; the resistance drops nonlinearly with temperature rise. Types- cont.

Week 7 Resistance Temperature (RTDs) Since the resistance of a metal wire is proportional to the resistivity, the temperature variation of the resistance of a wire can be written: where R  is the resistance at some temperature T (in o C) and R o is the resistance at the reference temperature T o (usually 20 o C) and  is called the temperature coefficient of resistivity.

Week 7 (RTDs) Cont. The most common RTD sensors are constructed from fine platinum wire or from a thin film of platinum.

Week 7 RTDs can be more accurate than thermocouples. RTDs have an output response that is more linear than thermocouples. RTDs tend to be more stable than thermocouples (less change in response over time). Some very sensitive thermocouples and thermopiles can detect smaller temperature changes than RTDs. (RTDs) Cont.

Week 7 As with the RTD, the thermistor is a device that has a temperature-dependent resistance. There are some important differences: Thermistors are made from semiconductor materials (not metals). This results in thermistors having a much larger change in resistance with temperature (~ 4%/ o C or higher) than RTDs (~ 0.4%/ o C). For thermistors (semiconductors), the resistivity decreases as temperature increases. Thermistors are highly nonlinear. Thermistors

Week 7 Thermistors are not as linear as thermocouples or RTDs. Thermistors are generally not as accurate as RTDs. Thermistors are more sensitive than RTDs. Thermistors are restricted to relatively low temperatures (< 300 o C, and for some, < 100 o C). Thermistors

Week 7 Comparison

Spring 2005ISAT 253 Chemical Sensors Chemical properties that serve as the basis for a sensor –Ionic charge –Absorption of light –Reactivity –Others? A common feature of chemical measurement systems is a prefiltering process within the sensing element –separates chemical of interest from background matrix –removes chemicals that interfere with detection –protects the sensor from harm

Spring 2005ISAT 253 Spectroscopy Spectroscopy: a field of scientific study that examines matter by passing electromagnetic energy through it. Optical methods use ultraviolet, visible or infrared radiation. Components common to a spectroscopic instrument: Sample container Detector Display Wavelength selector (a.k.a. monochromator)

Spring 2005ISAT 253 Measuring Absorbance with a Spectrometer sample Optical sensor Absorption mode Display Light shines through sample and is filtered to select wavelength Signal is detected with an optical sensor (lots of options!)

Spring 2005ISAT 253 Ozone Measurements from Aircraft Mozaic Programme funded by European Commission

Spring 2005ISAT 253 Ozone Measurements from Aircraft

Spring 2005ISAT 253 Beer-Lambert Law A = Absorbance (unitless) ε = molar absorptivity (liter moles -1 cm -1 ) b = sample cell pathlength (cm) c = chemical concentration(moles/liter)

Spring 2005ISAT 253 Small Group Problem One type of breathalyzer, the Intoxilyzer, detects alcohol by measuring its absorbance of infrared radiation at a wavelength of 3.39 microns. At this wavelength, ethyl alcohol has a molar absorptivity of 1540 L mole -1 cm -1. If the Intoxilyzer measures A = using a 10 cm sample cell, what is the alcohol concentration in the breathed air? For more information about the Intoxilyzer:

Spring 2005ISAT 253 Chromatography Chromatography: the science of separating mixtures into their components based upon differences in molecular structure and properties. Stationary phase vs. mobile phase

Spring 2005ISAT 253 Application: Gasoline-Contaminated Soil Each peak represents one chemical component of the mixture The peak height (and area) is directly related to the concentration

Spring 2005ISAT 253 Strain Gage Sensors Piezoresistive effect: Electrical resistance is proportional to an applied stress the physical principle used in strain gage sensors need to know more about stress

Defining Stress and Strain Two common types of stress Compression: Tension: A = cross-sectional area F F FF

Defining Stress and Strain Strain = Change in length per unit length L is the original length. Strain has units of m/m (called a “strain”). The strain value multiplied by 10 6 gives the strain in “microstrains” =  strain.

Hooke’s Law Stress and strain are related by Hooke’s Law (analogous to the Hooke’s Law for springs). E is the modulus of elasticity (also called Young’s modulus). E has units of Pa or Gpa (G   ).

Piezoresistance in a wire The resistance of a metal wire is given by: What happens as the wire is stretched or compressed?

Piezoresistance in a wire As the wire is stretched, L increases and A decreases. As a result, R increases. Compressing the wire has the opposite effect. These effects are exploited in strain gages.

Strain Gage Construction Most strain gages consist of thin film metal “wires” bonded to a plastic backing. This complete “chip” is usually glued to the structure whose strain you want to measure.

The starting resistance of the strain gage is R. As the strain gage is strained (stretched or compressed), the resistance changes by an amount  R.  R can be  or  The strain is given by Where S is a property of the strain gage called the “gage factor”. Strain Gage Properties

Application: Cantilever beam Together with a Wheatstone bridge, the cantilever beam becomes a “force transducer” Four strain gages mounted on a flexible beam

Application: Pressure sensor