Felt_Rider said:.... I am really curious as to what the number indicates or does it give some sort of diagnosis as to why my wattage data seems to be lower than what I would expect and by comparison to similar efforts on the same course. ....
Strain gauges like those used in PT hubs generate some output signal even when they're unloaded. The 512 (or thereabouts) number is the unloaded output level of the strain gauge, in other words it's unloaded bias point. Basically Saris has sampled a bunch of 'good' PT hubs off the assembly line and for known working hubs the average unloaded bias point measurement is 512 and based on the statistics of their samples and the standard deviation of good units off the assembly line they feel confident that a number near 512 is representative of a 'good' unit.
It's a simple quick test and lot's of manufactured electronic devices use similar simple tests to show that a unit is within expected norms. It does not however definitively assure you that the loaded measurement while actually riding is correct. It's like having an old style mechanical bathroom scale with a stretched spring, you can adjust it so that the zero reading lines up but if the unit isn't functioning properly it's hard to know for certain that the loaded measurements you care about are accurate.
Static torque tests with known weights give you a loaded data point but Saris' official stance seems to be to dismiss static torque measurements and won't give credence to customer complaints based on 'stomp test' results. SRM takes a very different stance and actually provides instructions on how to perform static torque tests in their printed manual which of course is the standard 'stomp test' using known weights and several repeated measurements to reduce error.
Anyway, that 512 number is a measure of the unloaded strain gauge bias point, it shows that your unit is very similar to known good units coming off the assembly line but it doesn't guarantee that your unit reports torque (and hence power) accurately at working loads. A carefully performed static torque test can tell you that but you do have to perform the test carefully, ideally with accurate weights. Weight plates at gyms can vary by a bit, but the error in heavier weights tends to be a smaller percentage of the weight which helps. FWIW, I use a 45 pound olympic plate that's marked in both pounds and kg that I've weighed on both postal and accurate digital scales and it's a bit heavier than marked. It comes in at 20.64 kg vs. 20.4 kg expected but that's still within ~ 1% of marked weight and the PT's stated accuracy is ~+/-1.5%. Lighter weights will tend to have more error in percentage terms.
-Dave
