Does Cable Length Make a Difference?

Burning on the back of our minds one morning was just how our thermometers would perform with different lengths of sensor chord.  The new Lego thermometer is limited, we’re told, by its IIC chords.  But because our thermometer line is analog, the sensor should be able to reach much further. 

To test this, we took three open thermometers with different lengths of cable: 60 cm, 100 cm, and 190 cm.  We then attached them side by side and started measuring the temperature of the room.  We gave the probes two quick burst of temperature by grabbing them, just to widen the temperature range. 

The temperature data for the three probes are graphed below. 

To make the data a little simpler, we went ahead and took the differences of the temperatures (for example, the immediate difference in temperature between the 190 cm thermometer and the 60 cm thermometer), and then found the maximum difference, minimum difference, and the average difference between all three probes.  Data is tabulated below. 

Data showed a maxiumum average difference of 0.40 degrees C, which is within the 0.50 C accuracy listed.  To search for any systematic differences in temperature, we graphed the average difference in temperature against the difference in length.  You can see the results below.

If we had systematic changes in sensor reading introduced by the sensor cable, we’d see an increasing error (or difference) with the increasing length of the sensor cable.  By graphing the average differences in temperature against he differences in length, we can see there’s no systematic difference in measurement . . . the maximum difference stays almost the same with increasing length (with no measured difference between the two greatest length differences), and the average difference (error) decreasing with length. 

So we’re left with the question: how long of a sensor cable could you actually need? 


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