Me 224 Experiments

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ME224 Lab4
Introduction The purpose of this lab is to build a temperature monitor and controller for a small aluminum block. Techniques involved in this lab include using transistors as switches, calibrating transducers, and writing control programs in LabVIEW. The main piece of equipment used in this lab is an aluminum block apparatus; the block has several holes drilled into it and embedded in them is a resistance heater, a thermistor, and a thermometer. In addition, a simple circuit is attached to the aluminum block with breadboarding for connection to the circuits built in the lab. The embedded resistance heater is used to heat the aluminum block. The heater is essentially a resistor that generates heat by the principle of Joule’s Law in which a current running through a resistor converts electrical energy into heat energy. Joule heating can be expressed by the relationship Q = I2 * R * t where Q is the heat (J) generated by a constant current I (A) flowing through a conductor of resistance R (Ω) for a given time t. [1] Although electric resistance heating converts nearly 100% of the electricity to heat, the overall process is still inefficient since the electricity is usually produced from oil, gas, or coal generators that convert only about 30% of the fuel’s energy into electricity. [2] Due to the energy loss in electricity generation and transmission, electric resistance heating is often more expensive than heat produced using combustion appliances, such as natural gas, propane, and oil furnaces. Thermistors are temperature sensing elements composed of sintered semiconductor materials such as silicon carbide that exhibit large changes in resistance in response to small changes in temperature. [3] Unlike most resistors, thermistors decrease in resistance as temperature increases because of their negative temperature coefficients as derived from their material properties. This relationship between resistance and temperature is better described by the equation R = Roexp(To/T) where R is resistance (ohms), Ro is a resistance value corresponding to infinite temperature, T is the absolute temperature (K), and T o is the activation temperature (K). [4] Because of their great sensitivity, thermistors are used in a numerous temperature sensing applications including monitoring coolant and oil temperature inside engines, and measuring temperature of charging battery packs. One drawback to their increased sensitivity over resistance temperature detectors (RTD’s) and thermocouple circuits is that thermistors are extremely non-linear devices that are highly dependent on process parameters. [5] Because of this, manufacturers have not standardized thermistor curves to the degree that RTD and thermocouple curves have been. Furthermore, because thermistors are made of semiconductor

materials, they are more susceptible to permanent decalibration at high temperatures than RTD’s and thermocouples. Therefore, thermistors are limited to use at just a few hundred degress Celsius. Materials and Methods Activity 4.1 The purpose of the first activity was to determine the specific heat of the aluminum block and the power generated by the heater by manually controlling the heater. Specific heat is the property that describes the amount of heat per unit mass needed to raise the temperature of a substance by one degree Celsius. [6] Heat is defined as the transfer of energy from an object of high temperature to an object of lower temperature. The relationship between heat and temperature change is expressed by the equation: dQ = c * m * dT where dQ is the heat added, c is the specific heat of the substance, m is the mass, and dT is the change in temperature. The first step was to build circuit 4.1 shown below in Figure 1 to achieve manual control of the heater.

Figure 1: Circuit 4.1

Next the resistance of the heater was measured and recorded. Then with the heater on, the specific heat of the aluminum block was calculated. The rate at...
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