5-Mar-2015: Temperature Measurement System

QUIZ

PURPOSE

The purpose of this experiment was to build our skills in constructing a design and implementing it in an experiment.

PRE-LAB

Figure 1: Graph of the thermistor's resistance vs temperature

We began this experiment by looking at the graph in Figure 1. From this examining this graph, we estimated that the resistance of the thermistor was around 11 kΩ at 25°C and 7 kΩ at 37°C. We used these values and voltage division to calculate the theoretical values of Vout, shown on the top side of the red line in Figure 2 (click to enlarge).

Figure 2: Mathematical process of finding the desire R value

The resultant value was in terms of R, which was the value of the resistor that we had to select for our design. To decide which value of R to use, we had to consider the constraint of this experiment: Vout has to increase by a minimum of 0.5 V with the temperature increase. Therefore, we set the difference of the voltages at 25°C and 37°C equal to 0.5 V, and solved for R. This process is shown in the bottom half of Figure 2. When we solved for the quadratic equation shown in Figure 2 (circled in blue), we got two values for R: 17.633 kΩ and 4.367 kΩ. Since we did not have resistors that were close 17.633 kΩ, we decided to go with a 4.7 kΩ resistor in our design as it was relatively close to 4.367 kΩ. The measured value of the resistor is shown below in Figure 3.

Figure 3: Actual fixed resistance value

PROCEDURES

Figure 4: Measured resistance values of the thermistor at ~25°C and ~37°C, respectively

Before implementing our design, we measured the actual resistance values of the thermistor used in our setup. We first measured its resistance at room temperature, which we assumed to be around 25°C. Then, we measured its value after holding the thermistor in our hands until the number displayed on the multimeter stabilized. We believed that this was when the thermistor had reached the same temperature as our hands, which we approximated to be 37°C. The measured values are shown in Figure 4 above. The percent difference between these values and the theoretical values calculated in the pre-lab was 1.36 percent and 1.29 percent for the 25°C and 37°C resistances, respectively. Since the percent differences were not very large, we believed that the system was going to be effective in achieving our goal.

Figure 5: Setup of our circuit

After measuring the resistances of the thermistor, we set up the circuit as shown above in Figure 5. In Figure 5, the voltage source of the circuit is illustrated with the red circles and the multimeter used to measure Vout is marked with green ones. Then, we applied 5 V to the circuit and measured Vout for the two different temperatures. First, we measured Vout at room temperature. The measured value is shown below in Figure 6. Next, we held the thermistor in our hands to warm it up to approximately 37 °C. The resulting voltage is shown in Figure 7.

Figure 7: Vout at 25°C

Figure 8: Vout at 37°C

Percent error = |expected value - measured value|/expected value x 100%

Since the difference between these values was 0.53 V, our design met the minimum constraints. In fact, the percent error of this value was 6 percent. Considering the limitations that we had, we believed that the experiment was successful.

POST-LAB

After performing the lab, we did a post-lab exercise in which we attempted to design an experiment in which the output voltage increased by 0.1 V per °C change. Since the overall temperature increase was 12°C, we concluded that our design had to result in a voltage increase of 1.2 V. However, we when we tried to solve for R, we got imaginary values. Therefore, we were unable to come up with a design that met the design specifications. Our process is illustrated in Figure 8.

CONCLUSION

In this experiment, we learned how to come up with a design for a circuit and implement the design successfully. We also acquired valuable experience in dealing with variable resistors. In this particular case, we saw how a certain type of variable resistor reacted to temperature changes.

The design that we implemented in this experiment was successful in meeting the minimum constraints. The results could have been even more accurate if we had been exact in getting the thermistor to be exactly 25°C and 37°C.