Temperature Sensors - Thermocouple

As the other groups mentioned, resistive temperature detectors (RTD), simple mercury thermometers, and more technologically advanced infrared sensors are all used to sense temperature. I would like to discuss a different type of temperature measurement – thermocouples. As Elda mentioned, thermocouples tend to be less accurate than RTDs, but they are still widely used in industry. They are simple and easy to understand. They have some advantages too which include a wide temperature range, robustness, rapid responsiveness, and lack of self heating.

Like most scientific inventions, the thermocouple was invented by accident. An Estonian physician accidentally discovered the ability to sense temperature by the effect of joining two different metals together. When two different metal wires are joined together and a temperature difference exists along them, they generate voltage, which is indicative of the temperature difference. Figure 1 presents a simple thermocouple diagram: the junction where the metals meet is called the measurement junction, or the hot junction. That point should be exposed to the temperature we would like to measure. The wires should then be placed in what’s referred to as the reference junction, or the cold junction. At that point the wires are generally inserted in a bath of ice water to maintain a constant 0 degrees Celsius. Thermocouples measure the relative temperature between the two junctions, and therefore the reference junction must be known, and is usually kept at 0 degree Celsius.

Figure 1: Thermocouple Diagram

The metals used are indicative of the sensitivity, temperature range, and voltage range measured by the thermocouple. Table 1 has this information about the common types of thermocouple. The types also indicate the error in measurements. As mentioned before, the error in measurement can be significant when using thermocouples. Figure 2 shows the possible error for four different thermocouples for the temperature range of 0 to 400 degree Celsius.

Table 1: Types of Thermocouples

Figure 2: Error in Thermocouple Measurements

What is actually measured when using a thermocouple is the voltage created by the difference in temperature. In order to interpret this date, one needs to know how to convert the voltage data to meaningful temperature data, which is, again, dependent on the type of thermocouple. The seedback coefficient is the voltage change per degree Celsius in μV per degree Celsius, and it is represented in Table 2 for the different thermocouple types at 25 degree Celsius. The seedback coefficient is not constant, though, which makes the fitted graphs used to interpret the temperature nonlinear. Software needs to collect the voltage data and have a function imbedded within it used to convert these measurements to useful temperature data.

Table 2: Seedback Coefficient at 25 Degree Celsius

References:

http://cds.linear.com/docs/Application%20Note/an28f.pdf

http://www.analog.com/library/analogDialogue/archives/44-10/thermocouple.pdf

Infrared Thermometers

Mike does a great job summing up a wide variety of temperature sensors.  Back in high school I learned that the bimetallic strip was used to control thermostats.  The expansion of the metal at a specific temperature would close a circuit, thus turning on/off the HVAC system.  Now we have NEST, which is more accurate and efficient. 

I would like to expand on Infrared (IR) Thermometers.  These thermometers are essentially a laser gun that can read the temperature of an object without any contact.  This type of thermometer is has an increasing popularity in the food industry.  Chefs are using them to read the temperature of food so that they don’t have to puncture the food and because it gives a fast and accurate reading. 

Every object emits an invisible infrared energy.  IR is located on the electromagnetic spectrum between visible light and microwaves.  There are three ways to transfer this invisible heat: reflected, transmitted, and emitted.  The emitted energy is the only type of energy that can be used to get the actual surface temperature.  This is a disadvantage to this type of thermometer.  As I mentioned before this thermometer is becoming popular in the food industry.  If the food is under a heating lamp the temperature measured will also include that of the lamp.  Therefore, when taking the temperature of the food it should be in low light, or the light should be covered with a cloth.

 

Depending on how advanced the IR thermometer the emissivity value can be altered based on the material.  These values can be looked up in charts.  As a comparison, emissivity of aluminum and water are 0.77 and 0.95, respectively. 

Again these thermometers are growing in the food service industry. They are also great for monitoring equipment.  For example, they can be used to find hot or cold spots detecting leaks in HVAC equipment. 

Sources:

http://www.allqa.com/IR.htm

http://www.thermoworks.com/emissivity_table.html

http://www.buzzle.com/articles/laser-thermometer-how-does-it-work.html

Thermography

I think that Mike did an excellent job of going over the history of temperature sensors and how they work. When I was assigned this post, I immediately though of a thermistor, however it is important to think about the history of temperature sensor, and that the analog approach has worked very well for a long time. I personally got very interested while reading Elda's post when she talked about the use of infrared thermometers. Elda talks about how infrared thermometers are used to detect temperatures without the need for surface contact. This brought me to the idea to discuss thermography as a type of "temperature sensor."

A paper written by Maldague shows there are two main types of  thermography, passive and active. These two types of thermography are discussed as a means of Non-Destructive Evaluation Techniques (NDT). The idea behind active thermography is to send a wave of energy (in the form of heat), and see how the heat moves in the observed medium. Passive thermography on the other hand uses energy already found in the observed medium such as the difference in temperature of a pipe or electric box that generates or takes heat to show how the materials around it act. Thermography can easily be used to determine if an electrical box is overheating and needs attention. It can also be used for firefighters to determine what is going on in a household in the event of a fire, and where people are located inside the burning structure.

The most basic description of thermography, is the use of an infrared camera to detect the radiation from a given material, that then is translated to a surface temperature. It is important (according to NDT sources) to know the emissivity of the material observed to get an accurate reading of what temperature is being observed. If the wrong emissivity is chosen, than the entire process of thermography can become a fruitless exercise.

One can see each different way of measuring temperature has its pros and cons, and there is no one solution fits all approach for measuring temperature.

Temperature Sensors

    A temperature sensors are devices that measure temperature of a medium. As Mike S. mentions in his blog, there are many types of temperature sensors used from simple home purposes to extremely accurate and precise scientific uses. Thermocouples, resistive temperature detectors, infrared thermometers, bimetallic devices, liquid expansion thermometers, and state-of-change devices are the some of the basic types of temperature sensors used for simple or more complex purposes.

    As Mitchell Butler and John Scanlon mention in their blogs, the most basic liquid expansion thermometers, especially mercury thermometers, are the most common temperature sensors that is very easy to use and accessible to everyone. The main elements of the liquid expansion thermometers are the mercury-in-glass sensor which expands and contracts when there is a temperature change and the means of converting this change into a temperature reading. Although accurate, mercury-in-glass thermometers are delicate and mercury is a hazardous material.

    Resistive temperature detectors (RTDs) are the most common sensors used in laboratory and industrial purposes. RTDs use resistors to record resistance values as the temperature changes. They are very accurate and have a wide temperature range which is why they are used for heavy duty purposes. They are preferred over a thermocouple or a thermistor sensor because of their high accuracy and stability for many years.

    Infrared thermometers are non-contact temperature measurement devices that detect the energy emitted by the medium and convert the energy factor into a temperature reading. Infrared thermometers are very useful for temperature measurements of moving objects, or when non-contact measurements are required due to hazardous material; more conventional thermometers do not seem useful in situations like that.

    As mentioned above, temperature sensors are used for multiple purposes. One use of distributed-temperature sensors is to measure the temperature profile of oil reservoirs, which has been done for many years now. The information provided by temperature sensors are very important for the process control of the oil tanks; they provide visual data to optimize the well performance as well as detect flow and viscosity behind the reservoir casing. Nowadays, there are many temperature sensors used in HVAC systems of  buildings in order to efficiently maintain comfortable indoor conditions.


Sources:
http://www.omega.com/prodinfo/temperaturemeasurement.html

http://www.omega.com/Temperature/pdf/RTD_Gen_Specs_Ref.pdf

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

http://www.accessscience.com.ezproxy2.library.drexel.edu/content.aspx?searchStr=Temperature+sensors&id=YB980600

Temperature Sensors

While initially, sensing temperature may seem like a simple concept (and it's been getting done for a very long time now) the actual details of what we are sensing can become quite confusing. In its most basic sense, temperature is a measure of the movement of molecules within a body of matter. That matter can be solid liquid or gas, and more frequent movements translate to higher temperatures (with absolute zero at the bottom of the scale, meaning no particle movement at all) The reasons we care about temperature are varied, but mainly we want to be comfortable or we want to ensure that a certain process is happening by manipulating the temperature. Of course our perception of comfort is based on our own sensing of temperature by biological processes far more complex than the processes we use to quantify temperature.

Obviously the thermometer is the simplest way to sense and understand temperature. We figured out pretty early on that certain fluids expand when they are heated, so that their volume is a function of temperature. By controlling and marking this expansion of volume we were able to quantify temperature. As John points out, we were later able to recognize the same principle applies to solids, and we began to use this to control the movment of thin metal strips, which would in turn control some basic circuitry, mainly for switching on and off some electrical process. Of course, this is the principle by which most early thermostats operated.

As we progressed in knowledge of electrical properties of materials, it was found that the resistance of certain conductive materials decreases as we increase the temperature, and so we developed thermistors made of ceramics or polymers to effectively convert a change in temperature to a change in resistance that could be easily measured. We also knew that the current changes between to metals at different temperatures, so a slightly more complicated Thermocouple circuit was developed, where this change in current can be measured across dissimilar metals. Furthermore we found that different metals behaved more predictably for certain temperature ranges, so a number of sets of metals were used for varying applications. Finally, the resistance in a coiled wire was found to be more accurately measurable, so RTD's (made of a homogeneous wire wrapped around a ceramic core) became prominent. These could be configured differently to accomodate different temperature ranges and provided high accuracy and repeatability.

Mike does a great job of explaining the usefulness of all of the sensors mentioned above. However, there are still some applications in which even RTD's and Thermocouples cannot be used. For example measuring the surface temperature of an object that is moving or one that cannot be disturbed becomes quite difficult with the sensors mentioned above. Rather than measuring a change in a specific material's response to a temperature, Pyrometers are able to measure a temperature directly as a function of an object's electromagnetic radiation. The methods by which this is acheived can become rather complex, but in their simplest form all pyrometers consist of a lense to concentrate radiation into a point or array of points and an absorber, which chemically translates radiation into terms of flux. It is at this point that the line between sensing and transducing becomes blurred for me. David gives a good explanation of these principles as applied to an array of infrared sensors (thermography).  By some means of transduction the radiation is translated to thermal energy and the thermal energy is translated to an electrical current. The figure below helps explain the process.

References:
http://drexel.summon.serialssolutions.com/search?s.q=thermal+radiation+sensor

http://site.ebrary.com/lib/drexel//docDetail.action?docID=10438390

http://www.electronics-tutorials.ws/io/io_3.html

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

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

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

Project: Physical Temperature Measurements

I am working with Tom Ben-David on creating a network on sensors in a room that will hopefully provide a better reading of the temperature in the room, and provide for a greater analysis of how a room acts. This type of strategy has not been widely implemented as there is not too much data on this subject. However, this has not been that practical of a solution for that long. A research paper by Lin details life in 2002, where one sensor per room was not that common, and they were exploring the use of a temperature sensor in each room as an alternative to one temperature sensor per zone. This is still how most homes operate, but as sensor prices drop, can we can more efficient use out of our HVAC systems. Can we also provide better comfort to ALL of the occupants, not just the occupants who live in the spaces with temperature sensors in them. If by adding more sensors to a room, can we then even adjust an HVAC system to make sure all occupants in one room are happy. Think of the many times you sat right under a vent and were too cold, while your companions were too hot just a couple feet away. With more sensors, hopefully an HVAC system will know how to respond to such an event happening.

Our idea is to create the physical sensors will be Arduino hardware. The Arduino hardware will report a humidity and temperature back to a central database where this information will be able to be complied over a time range. Knowing where these sensors are placed in the room, and what is near each sensor (ie: a window or diffuser that would account for a change in temperature). Hopefully with knowing the location of the 4 sensors, and the different types of data they provide, we will be able to make a general consensus as to wether a multi-temperature-sensor room is a viable option and would provide for a more intelligent building.

I found Jeanine's topic very interesting, and applicable to the same type of model we are looking to design. In her post, she talks about how they are going to be selecting sensors to improve the students comfort in the library. We are also interesting in achieving a similar goal, however not with such a specifica building. I am excited to see how well their findings coincide with the findings in our report about ventilation rates.