Anemometer Experiments - Part II

What about ambient

In part 1 of this series, we looked at the test setup, and the results we achieved. We determined that it would be PLAUSIBLE to use a heated element, and associated heat loss through air movement to measure wind velocity. The were many assumptions implicit in our experiment, and this second article will examine a key oneof those

An astute reader of the first article may have noticed that we spoke of two sensors, and alluded to the possible use of the second, when all the data we collected related to only one sensor. In this article we will talk to the second sensor.

In the first article we used the chart below to talk to the fundamental premise of the anemometer, if the air moves across an object the temperature will change by an amount related to how much air is moving, and the capacity of the air to remove heat... in fact we used Exhibit 1 below to demonstrate this point

Exhibit 1

This chart seeks to demonstrate the changes due to airflow, but as is demonstrated on the chart assumes the ambient temperature remains constant. Any reasonable person would question the validity of that assumption. This article examines the role of ambient temperature

Ambient temperature changes

For the moment if we are to stay with a likeness of the original chart, we could redraw it per Exhibit 2, and attempt to show what would happen if the ambient temperature changes > This chart is titled the 'basic' chart because whilst it addresses some of the variables, there are still others to be considered.

Exhibit 2

Exhibit 2 shows that whilst the flow of air across the sensor will alter the temperature in the system, so will changes in the ambient temperature. As illustrated here the difference between no air, and low air reduces the elevated temperature, BUT the change in ambient actually lifts the temperature in the entire system. Hence it might not be valid just to take point temperature measurements and have a ready correlation to airflow.

So what about ambient temperature changes.

Test Setup and Measurements

For this test we use the same sensors, and test setup as the configuration for Part I with two minor exceptions. Firstly we will not be using the fan at any speed, as we are attempting to measure the effect of ambient changes, and secondly we will disconnect one heater, so that one becomes the sensor under test, and the other the 'control'

Exhibit 3

Table 1

For this test we disconnected the red wire, in the right hand tube … and hence only one side is heated to the elevated temperature. We used the Fluke 87V meter as a control, just to reassure ourselves the readings we were seeing seemed reasonable.

Measuring the temperature of the Ambient Dashboard, and the Fluke, Table 1 records the results we achieved. The columns Diff HtoA, and Diff HtoF are the derived readings between the Heater Sensor (on the right), and the Ambient and Fluke respectively. Note this is not a test of temperature accuracy, the fluke and the Ambient Dashboard use entirely different algorithms to calculate the instantaneous temperature. In our case we were looking for consistency in the differential, rather that alignment between the three sensors

Results Achieved

To interpret the results, we need to understand the extent to which when the ambient temperature changes, so does the elevated temperature of the sensor. If the ambient changes +1 degreeC, does the heated element change +1 degreeC also, in which case we may be able to subtract the changes and just deal with the delta ..

The following chart plots our results

Exhibit 3

From the outset, you can see what looks like quite a high correlation between the measure 'elevated' temperature of the heated element, and the ambient temperature. It becomes more apparent looking the the difference chart, barely 2 degreesC over the entire range, and the biggest gaps occuring at the first reading, may simply be reflecting the heater still yet to stabilise. Doing the math, reveals a Pearson Correlation Coefficient of 0.910... quite high.

Conclusions

So a reasonable conclusion to say if we subtract the ambient variations, we WILL have a more meaningful result of the change in temperature reflecting the change in Airflow...

Acknowledgments

With special thanks to our visiting Physicist for the creativity and sensible head to keep us right whilst undertaking this somewhat esoteric project ...many thanks Orlando !

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