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# Testing a rotary potentiometer for use as a depth sensor

Extracts from this document...

Introduction

Testing a rotary potentiometer for use as a depth sensor

Context: I will be investigating a sensor to measure the depth of water in a bath. The depth of an average bath is 25cm from base to over-flow pipe. Allowing for the increase in depth of water when a person is in the bath, yet ensuring it is deep enough for comfort, the depth of water should be run no higher than 15cm.

I will be testing my sensor between water depths of 2cm and 15cm, because these are the minimum and maximum levels of water that would normally be wanted. The sensor does not need o be able to measure water depths between 0cm and 2cm because it is unlikely this depth of water would ever be required, and if it was, then would run so quickly there would be no need for the warning system.

I will be taking measurements with the water increasing from 2cm to 15cm in increments of 1cm. I have chosen to investigate whether the sensor is accurate to  1cm as I feel this is how accurate the sensor would need to be. It

Middle

0.78

1.25

1.22

0.57

0.64

13.00

0.75

0.67

1.25

1.24

0.60

0.54

14.00

0.52

0.57

1.30

1.26

0.40

0.45

15.00

0.50

0.54

1.31

1.26

0.38

0.43

 Average Current (Amps) Average Resistance (   ) Average Voltage (Volts) 1.00 1.94 1.94 1.06 1.57 1.66 1.08 1.42 1.54 1.10 1.37 1.51 1.13 1.19 1.35 1.16 1.06 1.22 1.17 1.00 1.16 1.18 0.88 1.04 1.21 0.80 0.96 1.21 0.73 0.88 1.24 0.60 0.75 1.25 0.57 0.71 1.28 0.43 0.55 1.29 0.40 0.52

I have also drawn two graphs, one plotting voltage against the depth of water and one plotting resistance against the depth of water. I have does this so I can see more easily the relationship between the two, and therefore evaluate more easily whether my sensor is suitable. I have also done this as it means it is easier for me to see what pattern my results follow and whether there are any anomalies.

From looking at my results in graph form I can see several large jumps between results and it is hard to tell whether these are part of the pattern or whether they have been caused by experimental error or are anomalies. Therefore I have decided that I will improve my evaluation of the sensor by using a more accurate way of measuring voltage and resistance. Also, having plotted the error bars, it shows that the accuracy of the experiment was, in the greatest instance, only slightly less than  2cm. As this is the minimum accuracy level I have specified for the sensor, I shall attempt the experiment again, using a Wheatstone Bridge, rather than a voltmeter.

Conclusion

The sensitivity of the sensor to the changes in water depth is high, shown by the definite negative correlation of the points on the graph, and the clear gaps between the resistances for each depth. This makes the sensor suitable for the job of measuring bath water as it needs to be sensitive to small changes in the water depth, of between 1 and 2cm. However, there does appear to be some element of random variation, particularly in the repeated sets of results, most likely to have been due to the causes above. Due to this random variation the high sensitivity is sometimes negated because the lowest resistance for one depth overlaps with the highest resistance for the next. Because of this, the sensor is not appropriate for use in the context of monitoring the depth of bath water, as it is not sensitive enough on a reliable level.

Due to the combination of these factors, it has been shown that although in terms of response time and sensitivity the sensor is appropriate to be used to measure the depth of bath water, in terms of accuracy and variation, it does not fit the criteria. Therefore, overall, the sensor is not fit for the purpose that I have tested it for – that of sensing the depth of bath water.

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