Sunday, February 03, 2013

Scale results, round 2

I made a second run on my scale tester.  This time I used a 10KOhm thermistor as my temperature sensor.  This gave me around 0.15 degrees (F) of resolution, compared with the 1 degree resolution of the previous temperature sensor I used for the last test run.

Figure 1: Weight and Temperature vs. Time

The first graph this time is the usual weight and temperature over time.  You can see that the weight and temperature appear to be highly correlated just like the first test run. The other thing is that the temperature appears much less noisy then in the previous run.

Figure 2: Nominal weight minus weight vs. Average temperature
In Figure 2, I graphed the delta in weight vs nominal against a moving average of temperature.  I also threw down a best-fit linear trend line.  I can immediately see that the point cloud is much tighter than in the previous test run, which seems good.

Figure 3: Adjusted weight

The last graph is a plot of the weight reading adjusted by the trend line equation from Figure 2.  The ideal line is a straight line at 55 lb. Clearly the line is not straight but it certainly improves the data.  The weight reading varies by +/- 0.2 lb instead of +/- 0.75 lb for the unadjusted weight readings.


The adjusted weight did not seem much different from the previous test run, but it still seems useful.  I think I can now go on to implement the data logger for this project which will convert weight and temperature sensor readings to specific gravity.

There are still some unknowns here.  One is that the test runs have only been run on a constant 55 lb test weight.  It's certainly possible that the numbers will change if a different weight is used.  It might be worth re-testing with weights of 45, 65, and 75 lb weights.

Additionally, there seems to sometimes be a lag between temperature changes and weight changes.  This could be attributable to the temperature sensor and scale having different thermal mass, which is almost certainly true.  I could account for this by attaching the temperature sensor to the scale with thermal paste to ensure that they are at relatively the same temperature.