Card Set Information

2012-08-06 19:53:02

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  1. How does heat differ from temperature?
    • Temp is a number of how fast the molecules are moving
    • Heat is energy traveling from one object to another
    • Heat is energy
  2. What is the unit for temperature and what is the unit for heat?
    • Heat is joule
    • Temp is kelvin or Celsius
  3. How is heat transferred?
    In conduction, like electricity. Atoms passing next to next

    In convection, a movement of atoms or molecules conveys heat by picking it up and moving it away. 

    In radiation, thermal energy is carried away by electromagnetic energy
  4. Where does heat appear on the electromagnetic spectrum?
    Radiation. All objects that have a temperature greater than 0° K will radiate heat.
  5. Where do you find latent or hidden heat?
    The extra energy temperature needed to change a state of a substance
  6. 6. What is an RTD and how does it measure temperature?
    RTDs or resistor thermometers are sensors used to measure temperature by correlating the resistance of the RTD element with temperature
  7.  What is the most common metal that an RTD made from?
  8. 8. What is the typical measuring range of a RTD?
    They can measure temperatures between -200 to 850 °C
  9. 9. What is the resistance of a PT100 at 0 °C and what is it made from?
    • 100 ohms 
    • And platinum 
  10. 10. In relation to RTDs what is alpha?
    The rate that the resistance changes for a substance is called it's Alpha. 
  11. 11. What effect does increasing the temperature have on a RTD?
    The rate that the resistance changes for a substance is called it's Alpha. The most common value for platinum is 0.385 Q/°C. Values of 0.375 and 0.392 are also available. For values between 0 °C and 100 °C you can assume that the slope of the curve is linea
  12. 12.Why are there 3 and 4 wire RTD?
    An RTD can be 2 wire, 3 wire or 4 wire. The 3 and A wire RTD allow the lead resistance to be cancelled out.
  13. 13. What does the term negative temperature coefficient mean?
    A thermistor is a semiconductor material that has a negative temperature coefficient. Its resistance decreases as the temperature increases. 
  14. How does a NIC thermistor measure temperature and what is its measurement range?
    • A negative temperature coefficient (NTC) occurs when the thermal conductivity of a material rises with increasing temperature, typically in a defined temperature range. For most materials, the thermal conductivity will decrease with increasing temperature.
    • Depending on the materials and methods of fabrication, they are generally used in the temperature range of -50°C to 150°C, and up to 300°C for some glass-encapsulated units
    • Materials with a negative temperature coefficient have been used in floor heating since 1971. The negative temperature coefficient avoids excessive local heating beneath carpets, bean bag chairs, mattresses etc., which can damage wooden floors, and may infrequently cause fires.
  15. 15. How do you make a thermocouple?
    Thermocouples are devices that are used to measure temperature. A thermocouple is a set of two wires welded together at one end, where a junction is formed between the two wires. Because each wire is composed of a different type of metal, each wire reacts differently to the temperature of the junction. Because of this difference, a voltage is generated. The voltage varies consistently with changes in temperature, and so there are devices, such as digital thermometers, that measure the voltage and convert it to a temperature reading.
  16. 16.Why do you need at least 2 junctions?
    U need to have a cold junction and a hot junction to get the right reading
  17. 7. What happens when you put two or more junctions in parallel?
    Multiple thermocouple junctions can be connected in parallel with each other to generate a voltage signal representing the average temperature between the junctions. 
  18. Why do you need cold junction compensation?
    Cold Junction Compensation

    A thermocouple voltage/temperature chart is referenced to 0 °C. The thermocouple characteristics are non linear so it is not possible to add the ambient temperature to the measured value. You must determine the voltage that would be created at the ambient temperature and this is the cold junction voltage. The hot junction voltage will be the sum of the cold junction voltage and the voltmeter volts.
  19. f you plug a thermocouple into a meter of other measuring device where is the cold junction?
  20. . How can you tell what type of thermocouple wire you have?
  21. 21. What is shunt impedance?
    A break down in te resistance of the leads

    water can get into the probe and give false readings

    High temperatures can cause the insulators to loose their integrity. If the resistance becomes low enough then a virtual junction can be created and give us an improper reading.
  22. 2. What happens when you pass a current through a thermocouple?
    Starts to act like a battery and gives false readings
  23. 23. What is the Peltier effect?
    The opposite effect of a thermocouple where instead of receiving volts we put voltage in and get the reverse effect, eg cooling
  24. What are Peltier devices used for?
    • Used to cool down critical electronic components
    • Uses alot of energy and is not practical
  25. 25. How can an infrared sensor be used to measure temperature?
    • An object with temperature radiates a frequency and this can be measured 
    • The higher the frequency the higher the tempt
    • A device could be constructed to be sensitive to a particular wavelength and the intensity it detected could be converted to temperature.
    • Used is non accessible places or hard to get to
  26. .Whatisemissivity? Howdoesitaffectthereadingfromaninfraredpyrometer?
    An objects ability to transmit temperature 

    Infra red pyrometers measure the radiation that is being emitted from the surface of the object. The effectiveness of the object to radiate is known as its emissivity.

     An object with an emissivity of 1.0 would be a perfect radiator which is known as a "black body". Reflective objects such as polished metal
  27. 27. What is a bimetallic strip thermometer? What is its principle of operation?
    Bimetalic Strip 

    • get a a piece o metal that expands alot when heat
    • And a piece of metal that does not expand alto when heated
    • Weld then together
    • And measure the bending
  28. 28. What is a thermowell?
    Thermowell Are a pocket u put the thermostat into

     Thermo well extend into the process and isolate the temperature probe from the process. Thermowells allow the temperature probe to be serviced without making contact with the process material. The process does not need to be stopped during maintenance unless the temperatures are too excessive.
  29. 29. How does a thermowell affect the temperature reading?
    thermowells can introduce lag into the process because there is a time delay between the change in process temperature and the heat being transferred to the temperature measuring probe. The thermowell also has the effect of insulating the probe from the process and the reading will be different from the true value. Heat transfer compound should be used to maximise the transfer of heat from the process to the probe.
  30. Why are thermowells used?
    Thermowells allow the temperature probe to be serviced without making contact with the process material.