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Circuit for Real Time Temperature Readings

2082 words (8 pages) Essay in Sciences

08/02/20 Sciences Reference this

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Thermal Project Background Research

Introduction:

The problem we are facing in this project is to create a device that will take the real time temperature of whatever condition that the device is put in. The materials that are being used in this project are a circuit board with a voltage regulator, resistor, diode and a 9V battery. With these materials we need to create a circuit with its own software that will give real time temperature readings.

Concepts and Benchmarking:

 There are many different options that could be used to measure the temperature for this project such as thermocouples, RTD’s, thermistors and pyrometers. Thermocouples are a type of industrial thermometer that are used to take extreme temperatures like those between 200-2600 degrees Celsius (Figure 1.1). It does this by taking,” temperature differences result from thermoelectric voltage differences produced by the two dissimilar metals that make up a thermocouple” (Johnson). A RTD on the other hand stands for Resistance Temperature Detector and are often called resistance thermometers. They way that RTD’s work is that it uses the principle that the resistance of metals change with temperature (Figure 2). This means as the temperature of the metal increases so does the resistance to the flow of electricity. Now a thermistor similar in a sense to a RTD because it uses resistance to measure changes in temperature (Figure 3). It contrasts a RTD in the fact that with a RTD the resistance increases as the temperature increases, with a thermistor the resistance actually decreases as the temperature increases. The pyrometer is the one option here that does not use direct contact or resistance to measure temperature (Figure 4). A pyrometer actually measures temperature based on the heat radiation given off by an object which means that it can measure an objects temperature from a safe distance. For the given diode for the project, it would fall into the category of a thermistor.

Thermocouple

RTD

Thermistor

Pyrometer

Advantages

Temperature Range

Very Accurate

Quick Response

Fast Response

 

Direct Contact

Linear Change in resistance

More accurate than RTD

Inexpensive

Disadvantages

Corrosion

Expensive

Output is Non-linear

Fragile

Calibration

Less Sensitive

Fragile

Low accuracy compared to other methods

 For each of the methods shown above they all use something called the temperature coefficient of resistance to measure temperature. The temperature coefficient of resistance is basically the rate at which the resistance changes based on the change in an objects temperature from its starting temperature. A thermocouple uses this with the use of two different metals. As the different metals heat up they deform which cause a change in a resistance and the more the metals deform and heat up the greater the resistance becomes. A RTD and a thermistor both use this principle in opposite ways though, with a RTD as the temperature increases the resistance increases but with a thermistor as temperature increases the resistance decreases. Most pyrometers do not resistance to measure temperature but with a resistance pyrometer it uses a wire placed against the object that reads the change in electrical resistance caused by heat.

 For thermistors, there are two kinds that are used to measure temperature based on resistance and they are known as PTC and NTC. PTC stands for positive temperature coefficient and NTC stands for negative temperature coefficient. The reason for the different types is due to the materials that the thermistors are made of, in a PTC the metals used have a high positive temperature coefficient which means as the temperature rises the resistance rises as well. While a NTC thermistor is made of metals that have a negative temperature coefficient which means that as the temperature increases the resistance actually decreases.    

Thermometer Circuit Part 1:

 The circuit for this project is to be made up a 9V battery, voltage regulator, 4.7kilo-ohm resistor and a diode. The way it works is that the electrical current will flow through the voltage regulator from the battery to the resistor and then through the diode. The circuit will then be coded for the diode to read the temperature of the condition it is in by reading the volts flowing through one side of the diode to the other and based on the change of voltage it will be able to tell the temperature in real time. Diodes are able to measure temperature based on the property of electrical diodes. These diodes change the voltage going through it based on the temperature so if the temperature increases the voltage across the diode will decrease and vice versa. In this circuit a voltage regulator is used and the purpose of a voltage is regulator is to keep the voltage entering the circuit constant no matter what the input is. If there was no regulator for the circuit then there is a possibility that it could ruin the circuit by flooding it with more current than the other parts can handle. The resistor being used for this project is a 4.7kilo-ohm resistor and the purpose of this is to control the flow of current to other parts on the circuit like in the case the diode. For this diode there will be hardly any forward resistance to measure so it is hard to actually measure that quantity with the classroom multimeters. For the approximate current flowing through the diode can be calculated by using Ohm’s law which is V=IR but since the current is the variable we are looking for the equation changes to I=V/R. Since the voltage flowing through the circuit after the resistor was measured to be 4.37V and the resistor is 4.7kilo-ohms, the current flowing through the diode at room temperature would only be 0.0009 amps. For the forward resistance of the diode since there is very little current and about 0.6V running through the diode at room temperature, the resistance would only be about 670ohms.

Calibration and Filtering Part 1:

 For the thermometer device to work properly it needs to be calibrated so it can read different voltages and translate into the real time temperature. To calibrate my device the method I will be using multiple objects/fluid of differing temperatures, then placing the diode in the object/liquid and taking a reading of the voltage that is being given off by the diode. I will be using this method because by doing this it will allow me to gather a wide range of information of voltages based off of temperatures. With that information I gather I will be plotting them in an excel graph to look at the correlation and then write a equation for my device that will allow it to read any real time temperature it is exposed to. Since the diode being used is a type of thermistor it is expected for the resistance to change in non-linear way, so instead of a being a linear graph it will be more of a polynomial graph that I will be working with. When putting the data into excel and the getting a graph from that data, excel gives you an R2 value for a line of best fit for the data points. That value tells you how closely the line of best fit actually represents the data presented and the closer that R value is to 1, then the better that line of best fits the data points.

  When measuring data there is often data points that are either repetitive or just irrelevant to what you are looking for, data filtering can fix this. Filtering of data basically looks over all of the data points collected and takes out any of the points that don’t make sense or points that you don’t need to come to a conclusion. Some of the advantages of filtering is that it makes your data easier to read and also makes it the data correlate more to get more accurate results. One of the major disadvantages though is that it may also filter out a data point that may actually be relevant and skew that data a little. One time of data filtering technique is using a moving average filter and what this does is it takes a number of samples of input values over a certain time and takes the overall average of those inputs to produce one single output value. This is very helpful especially in a thermometer due to the fact that the voltage will fluctuate occasionally so the moving average filter would take those fluctuations and average them to give one single output value.

Test Plan:

 To test the thermometer device I will be using a hair dryer and a cold pack to make sure that the thermometer is working. With the hair dryer, the plan for that is to put it on the hot air setting and change the amount of heat being applied to the diode. From doing this test, the diode should read the change in heat and decrease the resistance and in doing that the voltage across the diode should increase which means that the diode is recognizing the heat being applied to it. For the cold pack, the diode will be sandwiched between the cold pack which will then cool down the diode. This test should show a drop in voltage across the diode because since the diode will be experiencing cold temperatures the resistance will rise which means there will be less current allowed to flow from one side to the other which will lead to a drop on voltage. For both of these tests I will be looking at both the transient response and also the steady state response. This means that I will not only be looking at the data as it changes but also as it steadies out. The reason for this is because I think it is important to see that change over time in the graph to make sure the thermometer is working right and also the time it steadies out at to make sure it is reading the right temperature.

References:

Appendix:

Figure 1

Figure 2

Figure 3

Figure

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