Temperature Sensors

There are many types of temperature sensors. They are listed in two categories. The categories are Mechanical and Electrical temperature sensors. Examples of mechanical temperature sensors are the thermometer and Bimetal. Examples of electrical temperature sensors are the thermistor, thermocouple and resistance thermometer.

I’m quite sure that almost anyone, who has graduated the fourth grade, can use a basic thermometer. It is simple. Mercury expands and contracts according to the temperature and thus this expansion and contraction can be measured. A bimetal temperature sensor however, is a little less known to most people. Bimetal temperature sensors use strips and disks of metals which convert into mechanical displacement.

Electrical temperature sensors include thermistors, thermocouples and resistance thermometers to name a few. Thermistors are types of resistors which in turn measure resistance with ensuing changes in temperature. The basic equation of  controls the sensor. Where R is resistance, T is the temperature and k is a coefficient based on other attributes of the sensor. Thermocouples consist of two conductors that yield a voltage with differences in temperature. The governing equation is . Again the equation is based on the metals in the thermocouple. Tables are easy to look up for the coefficient in the aforementioned equation. A resistance thermometer, also referred to by the acronym RTD (Resistance Temperature Detectors), correlates resistance to changes in temperature. Again variations in metals used in the product dictate the subsequent equation:  , where  .

Typically a thermostat in a small residential home uses mechanical temperature sensors in the form of bimetal technology. As fellow student Mike S. points out it is the relative cheapness of these sensors that makes them very useful in residential applications. But as a corollary, for uses in commercial and industrial applications where high accuracy, low drift, wide operating range and precision are needed, you will typically find some sort of electrical temperature sensor. These sensors tend to be more expensive based on the fact that they have to be engineered and calibrated to handle the above-mentioned qualities.

I wouldn’t be surprised to see, that when the prices of these electrical sensors come down with advancements in technology, we will see them installed into small residential homes.

It was interesting to see from other posts like C. Meraz’s post, that other sensors such as pressure sensors typically measure something other than pressure, like resistance and capacitance and then translate these changes from the change in temperature that causes this change in resistance or capacitance.

Sources:

http://en.wikipedia.org/wiki/List_of_temperature_sensors

http://www.temperatureworld.com/shci.htm

Pressure Sensors

According to the National Instruments website, “because of the great variety of conditions, ranges, and materials for which pressure must be measured, there are many different types of pressure sensor designs” [1].

My fellow classmate, G. Carpenter, first addresses in her post the most common type of pressure sensor (strain gage, see Figure 1 for cross section) and what it measures and how it is translated to indicate pressure.  She then discusses different categories of pressure gauges (force collector and other) and their electrical outputs (millivolt, amplified voltage, and 4-20 mA).

Figure 1: Strain Gauge Pressure Sensor Cross Section [1]

I will now discuss the other two most universal types of force collector pressure transducers (according to National Instruments): variable capacitance and piezoelectric.

A variable capacitance pressure sensor measures “the change in capacitance between a metal diaphragm and a fixed metal plate” and the capacitance changes when the distance between the two plates changes and the degree of this change is converted into an electrical signal [1] (see Figure 2).  (Capacitance is described as the “the ability of a body to store electrical charge” [2].)  These types of sensors are also described as, “very stable and linear, but sensitive to high temperatures and more complicated to setup then most pressure sensors” [1].

Figure 2: Capacitance Pressure Sensor Diagram [1]

A piezoelectric pressure transducer utilizes the electrical properties of naturally occurring crystals such as quartz [1], and uses these stacks of crystal to convert motion into an electrical output as they become strained [3] (see Figure 3).  They require no external excitation and are "rugged" [1], however, are not effective with dc or steady-state conditions [3].  These sensors are also highly susceptible to shock and vibration [1], and also require special signal amplification as their output signal levels are low [3].

Figure 3: Piezoelectric Pressure Sensor Diagram [1]

Sources:

[1] http://www.ni.com/white-paper/3639/en

[2] http://en.wikipedia.org/wiki/Capacitance

[3] http://www.digikey.com/us/en/techzone/sensors/resources/articles/what-you-need-to-know-about-pressure-sensors.html

Temperature Sensors

There are many different types of temperature sensors, and many of them operate in completely different ways. The most basic type of temperature sensor is the classic mercury thermometer. This sensor works purely based on the expansion and contraction of mercury in a glass tube. The next step up technologically is the bimetallic strip temperature sensor. This sensor makes use of the different coefficients of expansion that different materials have. These sensors are usually composed of steel and copper, which expand and contract at different rates. This sensor takes the temperature change, and turns it into a mechanical displacement. This technique can be used in many different types of sensors.

There are also much newer sensors that instead of turning heat into mechanical energy, they read the heat change in a difference of electrical energy. The two most common sensors are a thermistor and a resistance thermal detector (RTD). These two objects work on very similar principles, they both measure the change in resistance and correlate it with the change in temperature. The main difference between the two is the materials used to make them. Thermistors use primarily ceramic, whereas RTDs use pure metals. This difference in materials usually results in thermistors having a more accurate reading in a smaller range of temperatures whereas RTD stay accurate even at extreme temperatures.

Most of the previous sensors all require contact, or to be within convection range of the heat source to get a viable reading, however there are also non-contact thermal sensors that use radiation. These sensors use Plank's Law to correlate the radiation given off by an object and its overall temperature. These sensors are really useful as you can measure the temperature of something while being far away from it, whether it be the temperature in an oven, or the temperature of the sun.

Of all these sensors, the most useful for our applications is the RTD. The sensors themselves are really just metal, so they have no moving parts, and never need to be calibrated. Because of the simplicity and cheapness of these sensors they can easily be applied almost anywhere in a building. Couple the readings of these sensors along with the HVAC system or even window/blinds control and, without any human interaction, a room could always be at an optimal temperature. This is just one example that these sensors could be used in, the fact that they are so small and cheap and easy means they could be used almost anywhere, in ovens, in fridges, in water heaters, anywhere that temperature levels are of interest, these types of sensors could be applied.

Because I was the first to post, I discussed the history of sensors and their many different types. It seems the other posters used mine as a base and talked about what the different sensors are used for now or what they could be used for in the future. David talks about thermography which, in simple terms, is seeing heat which I think is a really cool concept and, like David said, has many uses including assisting fire fighters to save lives. Nathan goes on to discuss how IR sensors are used a lot in cooking to ensure the quality of food. Not only is it being used to ensure the food is done cooking, but I just saw on the news the other day, police are pulling over food delivery trucks and using these IR sensors to ensure the truck is at correct temperature for food shipment.

Sources:
http://www.ehow.com/how-does_4928076_temperature-sensor-work.html
http://www.instructables.com/id/Temperature-Sensor-Tutorial/
http://en.wikipedia.org/wiki/Bimetallic_strip
http://en.wikipedia.org/wiki/Thermistor
http://en.wikipedia.org/wiki/Resistance_thermometer

Movement Sensors

Motion sensors are of course used to detect moving objects. The sensors are programmed to activate a command once a specific movement is detected. Motion can be be detected in various ways: infrared, optics, radio frequencies, sound, vibration, and magnetism. In some of these cases, motion sensors do not really detect motion but rather measures other factors that can expose movement such as passive infrared detectors. Passive Infrared Detectors are the types of sensors you see for residential burglar alarms. The data that the sensors gather are translated mechanically or electronically. Vibration sensors use piezospeakers. When it senses vibrations a voltage is released to give a signal or command. Optic motion sensors have nothing to do with sight. From what I could see, optic motion sensors use fiber optic lines as a part of the sensor module. I thought optic meant something to do with a camera that would recognize and track an object. Mechanical translations are like switches that turn on or off, which can give a command upon a certain movement. Electronically, if the sensor detects a change in say infrared patterns then an electric signal is turned on to activate a command. Currently motion sensing is very successful in gaming but has great promise in other industries such as military and medical applications. The farther sensor has to detect the worse the detection is as the area to be detected becomes larger. Reading Kayleigh's blog, I found the hair sensor to be interesting. Kind of surprised to see that we are using human hairs to use for the sensor industry. Reading Matthew Tedesco's blog, the positive displacement meter reminded me of those Japanese water barrels that are filled with water but then once the water reaches a certain point the barrel tips over and the water rushes out. I wonder if those things aren't just used for aesthetics and had a measurement purpose to them. So it seems that a lot of these sensors are dependent on some kind of component that measures a current or voltage, or uses electricity to measure. I think most of these sensors are well established with the exception of motion sensors. I think motion sensors have lagged behind because you have to identify and measure moving objects. I'm sure it's especially harder when the sensor is also moving.


http://www.pickar.caltech.edu/e103/Final%20Exams/Motion%20Sensing%20Technology.pdf
http://en.wikipedia.org/wiki/Motion_detection

Humidity Sensors

Of all the sensors used in a typical building, humidity sensors were what I knew the least about before writing this post.  I had a general idea of how the other types of sensors worked, but couldn't even really make assumptions as to how humidity sensors worked.  The first thing I learned with this post is that humidity sensors are called hygrometers.  I did figure that, like other sensors, hygrometers used a combination of temperature and pressure, or change in electrical properties for electronic sensors to determine the moisture in the atmosphere. 

When searching the internet I came across a list of typical types of hygrometers used in the industry; metal-paper coil, hair tension, chilled mirror dewpoint, capacitive humidity, resistive, and thermal conductivity. The metal-paper coil and the hair tension hygrometers seem to be the most "primitive" form of sensor, being that it uses physical properties of materials and calibration to display humidity level on a dial.  The metal-paper coil hygrometer works like a bimetallic thermometer, only instead of two different metals, a salt infused strip of paper is attached to a coil, causing the coil to change shape.  A hair tension hygrometer works by measuring the change in tension in a human or animal hair with the hair shortening as humidity increases.  Both of those types of hygrometers are calibrated to display relative humidity. 

The other types of humidity sensors use the change in a materials electrical properties to determine the relative humidity.  Most of the time a type of salt or electrical polymer is used in these hygrometers. 

Humidity Sensors

Before understanding humidity sensors, one must first understand what humidity is. “Humidity refers to the water vapor content in air or other gases.” A humidity sensor measures and regularly reports the relative humidity in the air. As Kayleigh's post says, "hygrometer is the proper name for an object which measures humidity or moisture content.” The most commonly used type of humidity sensor relies on the ability of two nearby electrical conductors to create an electrical field between them. A source describes the process; “A polymer film on the conductors collects moisture from the air, and the moisture causes minute changes in the voltage between the two plates. The changes in voltage are converted into digital readings showing the amount of moisture in the air.” Humidity sensors then use the information collected to help regulate the space.

Humidity sensors are helpful in everyday residential uses. For example, people who suffer from conditions that are affected by high humidity, such as allergies or asthma, can have humidity sensors installed in their homes to control humidity. Humidity sensors are also very useful for medical purposes. Hospitals generally use humidity sensors to carefully maintain humidity in spaces containing sick patients. These sensors can also be very useful in spaces that store objects damaged by exposure to moisture. Examples include wine cellars, humidors, greenhouses, or antique storage areas. 

Humidity sensors are also an important part of building design, especially as we move towards green and energy efficient buildings. In an article entitled “Putting Building Science Into Practice,” a retrofit project of Fraunhofer’s new Boston headquarters is discussed. The rehab project aims to test emerging build technologies. One of these technologies relies on the use of humidity sensors to measure its efficiency. “The Showcase building will try many types of insulation on the walls, including vacuum panels that use gas, rather than cellulose or fiberglass, as the actual insulator.” The humidity sensors will collect data that will be used to determine the comparative advantages and worth of the new technology.

When reading over other student’s posts, I found Matthew's post about flow sensors to be very interesting. The comparison he made between the displacement method and the bucket and the stopwatch helped to simplify the method and make it easy to understand.

http://www.technologyreview.com/view/510131/putting-building-science-into-practice/

http://EzineArticles.com/4695977

http://www.wisegeek.com/what-is-a-humidity-sensor.htm

http://www.sensorsmag.com/sensors/humidity-moisture/choosing-a-humidity-sensor-a-review-three-technologies-840

Pressure Sensors

Pressure sensors, as the name indicates, measure pressure.  The pressure that it is being measured is typically that of a gas or liquid although pressure applied by a human or animal is also available.  According to Omega.com, pressure transducers “converts pressure into an analog electrical signal”¹.  A strain gage is the most familiar, and common, engineering pressure sensors.  For a strain gage, an electrical signal is generated when “the physical deformation of strain gages which are bonded into the diaphragm of the pressure transducer” ¹.  The strain that is formed from deformation of the diaphragm produces “an electrical resistance change proportional to the pressure” ¹.  Other types of pressure sensors include sensors that measure fluid/gas flow, speed, water level, and altitude².  Some common examples of these are piezometers, manometers, and tire pressure gages.

There are two different categories of pressure gages: force collector types and other types².  Force collector types (e.g. pizoresistive strain gage, capacitive, electromagnetic, piezoelectric, optical, potentiometric) use an item of known area – typically a piston or diaphragm – to measure strain/deflection of the applied force over the applied area².  The other types of pressure gages use different properties (e.g. density) to determine the pressure of the medium.  Some examples of these pressure sensors include resonant, thermal, and ionization².  Wang Chunyi's post describes how resonant and thermal pressure sensors work.  According to his post, resonant pressure sensors have a wire that vibrates at its natural frequency.  As the pressure changes, the resonant frequency of the wire changes in response.  The magnets around the wire create electricity through the wire's vibration which is then transmitted to the readout.  Thermal pressure sensors determine pressure through measuring the heat of a wire.  Under high pressure and constant volume, temperature within the gas, and subsequently the wire, will increase.  This increase in temperature must be calibrated based on the type of gas that surrounds the wire.

There are three types of electrical outputs available for pressure sensors: millivolt, amplified voltage, and 4-20 mA.  Millivolt transducers are typically the most economical sensor and their output is directly proportional to the input power or excitation¹.  However, their output is typically low and so they are not recommended for noisy environments, and distance between the sensor and the readout equipment should be short.  Amplified voltage sensors include “integral signal conditioning which provides a much higher output than the millivolt sensor” ¹.  Because of the higher output level, they are able to be used in noisier environments, like industrial.  The 4-20 mA sensors are the “least affected by electrical noise and resistance in the signal wires” ¹.  This benefit allows the signal to be transmitted long distances (1000+ ft.)¹.

¹ “Pressure Transducers.” Omega.com.

http://www.omega.com/prodinfo/ pressuretransducers.html.

² “Pressure Sensor.” Wikipedia.com. http://en.wikipedia.org/wiki/Pressure_sensor