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Power2max temperature compensation

Bikeboard.at spent two seasons with the P2M Rotor 3D + on the road - since May 2012 including the software update of the temperature compensation text. Mahatma Photos: NoSane, Sportograf 

This year in May, I took over the system of power2max NoPain, with whom he already unwound to GC many kilometers and over here has reported. Before it was screwed on my bike, it still had the factory in Germany and got the latest software loaded, the time was not officially released yet, and only on the bike in September Euro was introduced.

Before going into any details and analysis, be said in advance: The system works without any problems and has allowed me to several thousand kilometers never fail. Before Ötzi precaution I made a battery replacement. After 5 minutes the matter was settled. So be it!
The recorded data are in line with my performance. Apparent fluctuations (= deviations of more than 10W, which would also add subjectively or stand out in the analysis) are not I've come across.

Besides the pure use in racing and training I have comparison tests with my Powertap SL + Wheel, which are said quite a low temperature drift and also made on the watt-controlled Tacx Bushido role (higher drift). While the results in the section on the entire unit were considered always constant, but have some differences in detail between the three power meters shown. 


Calibration, zeroing, temperature compensation 

Before that, however, a brief explanation of some terms that are always confused and misrepresented.

Calibration: Can be done in the factory for PowerTaps (PT) and power2max (P2M). Corresponds to the calibration of the instrument and of the used strain gauges. When the word calibration is often misused in various menus (eg Garmin Edge devices), so it is actually the zeroing meant.

Zeroing / Zero: The strain gauges and the aluminum body on which they are mounted, subject to temperature variations. Thereby, it is necessary to set the zero point, which is the value at which the current at 0 Watts abut prevailing temperature to set. The both power2max and PowerTaps make automatic (auto-zero) if they believe that power is not just performance. Usually that's the case, if you roll the wheel for a few seconds and not pedaling. The thus determined value (zero offset) is stored and used until the next zeroing as a starting value for the calculation of the performance. On the settings of the most bike computers, this process is also done manually. Here, the confirmation back to the successful power2max delivers the determined offset value to the computer / edge, which is then displayed on the screen there. 

Temperature compensation: These new for Euro Bike of power2max presented and in my software version already contained function attempted even an adjustment of the offset to changing temperatures to bring about, if not auto-zero happens (eg long uphill, time trial, indoor). To each power2max comes for 24 hours in a refrigerator, where it is slowly heated from -20 ° to +70 ° and identified an individual temperature curve. This characteristic is then stored in the software and used for the correction of values ​​between two car-Zeros. For this purpose the power2max has mounted a temperature sensor on the circuit board, which records the temperature fluctuations. Every 10 seconds, the current temperature is compared with the temperature of the last zeroing and wattage values ​​are adjusted according to the stored characteristic.

Others have no such Leistungsmessysteme temperature correction, but they will also - by a better arrangement and partly larger number of strain gauges - said to have a lower temperature drift. There is talk of about 1-2W per degree Celsius at SRM and Powertap and 2-4W at power2max. (Without temperature compensation).

Since I know neither the exact curve of my power2max and there is no way for the user to query the currently used correction values, I was beyond mere viewing and comparing the data also is not possible, a more detailed analysis for temperature compensation can be made. 


In practice 

First we went to the role. Tacx Bushido role was set to 200W and an hour, data from PT and P2M being recorded. In addition I have the software ipWatts used which can compatible with Android phones to record data from up to 4 power meters simultaneously. Thus, for example, installed Sony Xperia models ANT + sensors.

Not surprisingly, shows a strong drift of the Bushido role, while the distance between P2M and PT remains approximately constant. The P2M indicates something more than the PT, which was also to be expected: the difference should losses through chain / drive to be (about 1.5-3%).

After about 40 minutes I wanted to zero the P2M manually and the PT is not, so you can see which of the two power meter has drifted in that direction. That is unfortunately failed. When stopping the PT has completed an auto-zero, but is not correct. Thus in the next few minutes, the value of PT was increased while the P2M even after zeroing at only 4W lower performance than previously provided. An indication of a relatively small drift of P2M. Another 10 minutes later I have performed both P2M (manual calibration) and PT (proper auto-zero by rotating the rear wheel with no load), then the figures were again consistent with those of earlier.

I have made several tests on the role, also with varying power levels or stages (100W-400W). The result was always the same: Neither P2M still PT showed strong fluctuations during the Bushido role even after one hour drifted - especially higher loads and therefore have more heat it further strengthened. 

After that had P2M and PT say on the street. This soon became apparent that the temperature compensation of the "warm up" problem not beikommt. If you slide the bike out of the garage or the basement to the outside and it comes to this higher temperature differences, so the measured data of P2M and PT drifting apart neatly on the first 10-15 minutes while they again after a successful auto-zero are very close together. If you want to prevent this initial drift, you should push the bike just a few minutes before driving away into the open so that the aluminum body on which the strain gauges are mounted to adjust to the temperature. Also help deliberately administered Auto Zeros correct the divergent values ​​early.

Furthermore, I noticed that there have been a few times to wrong car Zeros - seem to be especially susceptible to the Powertap power meter. In the graph, these blocks can be easily identified, since the power values ​​over a certain time constant to be very high, or too low to most 10W. This is consistent with my subjective experience of the PT wheel on the roll. There I was with the wrong car-Zeros or not properly executed manually zero offset settings more problems than benefits. So my tip for Powertap users: Do not manually zeros! Zeros if you will, should be aware of this cause by rolling (or indoors: dismount, pick up rear wheel, turn crank and coasting).

What is striking yet that the variations between power2max Powertap and even when correctly, regular auto-zero, constant temperature and when looking at the 30s moving average, which amounted to between 10W +10 W. They are usually in the range 10W to +5 W - that is, the P2M shows, just as it did during indoor test, at a little more than the PT. A real model for the variations I can not see (eg certain cadence, watts range).
The area may seem large, but when you said that the manufacturers' accuracy of + / - 2% (P2M) and + / -1.5% (PT) so at 300W maximum deviation of + / - 6W (P2M) and + / - 4.5 is W (PT) against, they are well within the fluctuations that occurred in the test. In any case, this applies to the mean values ​​of all training sessions: These were almost identical and have always differed by only a few watts (max. 3-5W difference). 

Ultimately, had the two power meter still hold on a tour at 3200 meters altitude and 5 mountains during this time when you start to no drift was -. The wheel standing before the start about 15 minutes outside - it came at the exit again to wrong car Zeros (PT & P2M) and also some drift during the climbs. Again, it is not me again managed to find a pattern for the drift and the absolute differences in the recorded performance. On two mountains shows the P2M to 10W more, on two mountains, the values ​​are almost the same and the numbers are on a mountain by a false AZ at P2M hardly usable. 


My conclusion 

I am using the power meter power2max been very pleased. In use it is easy and the recorded data are for my purposes in any case sufficiently accurate.

One thing is certain: each measuring instrument that uses strain gauges has to "fight" with the drift to. The often-cited weakness of the higher temperature drift compared to PT and SRM power2max scored by the software update under control. In practice, no significant differences in terms of accuracy to the expensive power meters are visible.

I retain my current practice: Indoor on the role I have in front of the training, an auto-zero cause (take the rear brake, turn crank a few times, coasting), Outdoor I roll after 10-15 minutes once briefly for 2 - 3 seconds. Then I waste no more thought to it.

P2M Update: Meanwhile, the auto-zero algorithm was modified somewhat. The possibility of the "adjustment" of zero was complemented by the introduction of an additional parametersa (plausibility compared to the preceding zero). All newly supplied sensors include the most recent update, as well as the measurement of the "left-right distribution".

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