Purpose: A qualification of my PowerTap on my Tacx Fortius.  Analysis is performed in different dimensions.  The cases which are tested:

I.  constant gear, constant cadance varying power (50[Watt]-350[Watt])
II.  constant gear, constant power, varying cadance (60-100[rpm])
III.  constant cadance, constant power, varying gear

The following analysis is done, with figures and tables and conclusions in the text below.  By clicking on the image (s), the extension of the figure is achieved.
First, the total training in a graph is presented as a function of time in figure 1 and figure 2.  Comparing the measured power of the PowerTap w.r.t the Tacx in a graph, the noisy signal of the PowerTap power is filtered (from green to black line in figure 2). More analyses on the measured noise is given in table 1 (see APPENDIX below) and figure 6

The "high frequent" noise on the measured power of the powertap is, anyway, about 10-15% to it's value. To minimize this noise in analysis, extra averaging is recommended to notice the relevance of the numbers. In the numbers of table 1 (see APPENDIX below), noise qualification is listened ("pt std", "pt range"). Figure 5 presents these noise numbers.

This training in total is presented in figure 1 and figure 2. The training can be divided into 6 distinct phases.  Each stage has its own purpose:
1.  Warming up for the systems, means myself  and the Tacx system
2.  Compare the measured power  - between the Tacx and the PowerTap on a wide range of power - with a fixed bike gear (= case I) (figure 3, figure 4, figure 5, figure 6)
3.  Compare the measured power - between the Tacx and the PowerTap with a fixed set point on the power, a fixed bike gear, but a vary-ing cadance between 60-100[rpm] (= case II) (figure 7 and figure 8)
4.  Compare the measured power - between the Tacx and the PowerTap, all fixed, cadance, gear and power setpoint. (figure 9)
5.  Compare the measured power - between the Tacx and PowerTap with a fixed set point on the power, a variable gear and a fixed bicycle cadence (= case III) . (figure 10)
6.  Compare the measured power - between Tacx PowerTap with some random and sudden accelerations.  Gear and Cadence are not of interest (figure 11)

 Each objective from above is discussed in more detail below (extra tables and figures)

figure 1: whole trainings as function of time.

figure 2: whole trainings as function of time (power filtered).

 Purpose of Phase 2: (case I)
Compare the measured power  - between the Tacx and the PowerTap on a wide range of power - with a fixed bike gear (= case I)
1.  Compare in absolute numbers
2.  Comparable in relative terms
3.  Compare the numbers in the reproduction of the PowerTap

Results of Phase 2 (case I)
•  The PowerTap measured systematically lower values as the Tacx
•  The higher the capacity the greater the delta between the power of the PowerTap and Tacx into absolute perspective.  In a relative perspective, the constant delta is somewhere around 7%
•  The reproduction of the power of the PowerTap is dependant of the absolute power in a absolute sense. In a relative sense it is more or less equal. (see table 1) 

The most interesting so called AVG-numbers from table 1 (see APPENDIX below) is listened in figure 5 for better overview and oversee some trending. The ratio of the power between the tacx and the powertap is almost constant over all powers (first subplot of figure 5). The quotient is about 93%. This means the Tacx system gives systematically 7% higher values for all powers. To quantify the measured noise of the powertap (means the spread in values in the green lines in figure 1 and figure 2), it is defined in 2 ways. Both definitions are relative definitions with respect to it's mean power measured.

1) Std(signal)/mean(signal)

2) (max(signal)-min(signal))/mean(signal)

The numbers of both definitions are listened in the table above and figure 5. The relative noise (definition 1) is from above 100[Watt] somewhere constant, about 5-7% (see also the second subplot of figure 5). In definition 2 the same data the noise is from above the 100[Watt] about 20-30% (see also the third subplot of figure 5). Note: my tacx system was not calibrated in this experiment. 

Interesting is the distribution of the noise of the powertap (means the spread in values in the green lines in figure 1 and figure 2). What is it's typical distrubution curve? Figure 6 shows 7 different histograms, 1 histogram for every power setpoint (50, 100, .. , 350W). The distribution in figure 6 is not exactly (an approach of) a Gaussian (or -normal-) distribution as one can seen. The vertical red line is the setpoint, the vertical green line is the average of the histogram.

  figure 3: case I

  figure 4: case I (statistics) 

From the figure 3 and figure 4 above, the following numbers can be extracted listened in table 1

   figure 5: case I statistics (see table 1)

figure 6, histogram of the noise at a certain power level

Purpose of Phase 3 (case II)
Compare the measured power - between the Tacx and the PowerTap with a fixed set point on the power, a fixed bike gear, but a vary-ing cadance between 60-100[rpm] (= case II)

Results of Phase 3 (case II)
There seems to be some trending, the higher the cadence, the higher the PowerTap measurement.  In the measurement at 150 [Watt], this trend is clear visible.  In the measurement at 200 [Watt] this trend is not seen very clear.
figure 7: case II

figure 8: case II

Purpose of Phase 4 
Compare the measured power - between the Tacx and the PowerTap, all fixed, cadance, gear and power setpoint.

Results of Phase 4
The trend of the measured power is stable (enough!) As hoped and expected in this case.
figure 9: everything constant

Purpose of Phase 5 
Compare the measured power - between the Tacx and PowerTap with a fixed set point on the power, a variable bike gear and a fixed bicycle cadence (= case III).

Results of Phase 5
For a relatively large number of gears the measured power remains almost constant.  At very small bike gears, the power decreased.  This is not a very good result, when for example the -Keutenberg or better the Mortirollo- appears!
figure 10: case III

Purpose of Phase 6 
Compare the measured power - between Tacx PowerTap with some random and sudden accelerations.  Gear and Cadence are not of interest.

Results of Phase 6
There is some correlation between the Tacx and PowerTap measure.  But I have no faith in the Tacx results.  This means I can not explain about the PowerTap in this test.

 figure 11: random accelerations

I hope to contribute to anyone with this measurement and analysis of the results who wants to learn more about the powertap measurement w.r.t. the Tacx.  Both The PowerTap and the Tacx is not "holy" in the results (take your own opinion in this), the data show only the delta between the two in relation to different dimensions.

 Best regards,
 Marc

 =================================================================

 APPENDIX TABLE 1: (case I)

+----+-----+-------+------+------+-----+------+--------+  +--------+---------+-------+
| nr |P sp |P tacx |P pt  |pt min|ptmax|pt std|pt range|  |pt/tacx |std pt/pt|R pt/pt|
|    |Watt |Watt   |Watt  |Watt  |Watt |Watt  |Watt    |  |   -    |   -     |       |
+----+-----+-------+------+------+-----+------+--------+  +--------+---------+-------+
|  1 |  50 |  52   |  55  |  48  | 88  |  9   | 40     |  | 106    | 15.9    | 73.2  |
|  2 |  50 |  54   |  52  |  42  | 83  |  9   | 41     |  |  97    | 17.5    | 78.8  |
|  3 |  50 |  52   |  45  |  35  | 57  |  5   | 22     |  |  85    | 11.0    | 49.4  |
|  4 |  50 |  53   |  47  |  35  | 55  |  6   | 20     |  |  89    | 12.4    | 42.5  |
|  5 |  50 |  52   |  47  |  41  | 56  |  5   | 15     |  |  90    | 10.0    | 32.0  |
+----+-----+-------+------+------+-----+------+--------+  +--------+---------+-------+
|AVG |  50 |  52   |  49  |  40  | 68  |  7   | 28     |  |  94    | 13.4    | 55.2  |
+----+-----+-------+------+------+-----+------+--------+  +--------+---------+-------+

+----+-----+-------+------+------+-----+------+--------+  +--------+---------+-------+
| nr |P sp |P tacx |P pt  |pt min|ptmax|pt std|pt range|  |pt/tacx |std pt/pt|R pt/pt|
|    |Watt |Watt   |Watt  |Watt  |Watt |Watt  |Watt    |  |   -    |   -     |       |
+----+-----+-------+------+------+-----+------+--------+  +--------+---------+-------+
|  1 | 100 |  99   |  99  |  87  |149  | 13   | 62     |  | 100    | 13.3    | 62.4  |
|  2 | 100 | 100   |  93  |  88  |100  |  3   | 12     |  |  94    |  3.2    | 12.8  |
|  3 | 100 |  99   |  92  |  83  |101  |  4   | 18     |  |  92    |  4.6    | 19.6  |
|  4 | 100 |  99   |  88  |  82  | 93  |  3   | 11     |  |  89    |  3.9    | 12.5  |
|  5 | 100 |  99   |  89  |  79  | 96  |  4   | 17     |  |  89    |  4.2    | 19.2  |
+----+-----+-------+------+------+-----+------+--------+  +--------+---------+-------+
|AVG | 100 |  99   |  92  |  84  |108  |  6   | 24     |  |  93    |  5.9    | 25.3  |
+----+-----+-------+------+------+-----+------+--------+  +--------+---------+-------+

+----+-----+-------+------+------+-----+------+--------+  +--------+---------+-------+
| nr |P sp |P tacx |P pt  |pt min|ptmax|pt std|pt range|  |pt/tacx |std pt/pt|R pt/pt|
|    |Watt |Watt   |Watt  |Watt  |Watt |Watt  |Watt    |  |   -    |   -     |       |
+----+-----+-------+------+------+-----+------+--------+  +--------+---------+-------+
|  1 | 150 | 150   | 142  | 128  |151  |  7   | 23     |  |  95    |  5.2    | 16.2  |
|  2 | 150 | 149   | 138  | 127  |149  |  5   | 22     |  |  93    |  3.7    | 15.9  |
|  3 | 150 | 149   | 135  | 126  |146  |  6   | 20     |  |  91    |  4.1    | 14.8  |
|  4 | 150 | 150   | 132  | 127  |138  |  3   | 11     |  |  88    |  2.5    |  8.3  |
|  5 | 150 | 150   | 133  | 127  |141  |  4   | 14     |  |  89    |  2.7    | 10.5  |
+----+-----+-------+------+------+-----+------+--------+  +--------+---------+-------+
|AVG | 150 | 149   | 136  | 127  |145  |  5   | 18     |  |  91    |  3.6    | 13.1  |
+----+-----+-------+------+------+-----+------+--------+  +--------+---------+-------+

+----+-----+-------+------+------+-----+------+--------+  +--------+---------+-------+
| nr |P sp |P tacx |P pt  |pt min|ptmax|pt std|pt range|  |pt/tacx |std pt/pt|R pt/pt|
|    |Watt |Watt   |Watt  |Watt  |Watt |Watt  |Watt    |  |   -    |   -     |       |
+----+-----+-------+------+------+-----+------+--------+  +--------+---------+-------+
|  1 | 200 | 200   | 188  | 175  |217  | 11   | 42     |  |  94    |  6.0    | 22.3  |
|  2 | 200 | 199   | 186  | 172  |208  | 10   | 36     |  |  93    |  5.5    | 19.3  |
|  3 | 200 | 199   | 184  | 168  |194  |  9   | 26     |  |  92    |  4.7    | 14.2  |
|  4 | 200 | 199   | 179  | 166  |192  |  8   | 26     |  |  90    |  4.5    | 14.6  |
|  5 | 200 | 200   | 179  | 169  |190  |  6   | 21     |  |  89    |  3.5    | 11.7  |
+----+-----+-------+------+------+-----+------+--------+  +--------+---------+-------+
|AVG | 200 | 199   | 183  | 170  |200  |  9   | 30     |  |  92    |  4.9    | 16.4  |
+----+-----+-------+------+------+-----+------+--------+  +--------+---------+-------+

+----+-----+-------+------+------+-----+------+--------+  +--------+---------+-------+
| nr |P sp |P tacx |P pt  |pt min|ptmax|pt std|pt range|  |pt/tacx |std pt/pt|R pt/pt|
|    |Watt |Watt   |Watt  |Watt  |Watt |Watt  |Watt    |  |   -    |   -     |       |
+----+-----+-------+------+------+-----+------+--------+  +--------+---------+-------+
|  1 | 250 | 249   | 237  | 213  |257  | 13   | 44     |  |  95    |  5.6    | 18.5  |
|  2 | 250 | 249   | 234  | 212  |285  | 18   | 73     |  |  94    |  7.6    | 31.2  |
|  3 | 250 | 250   | 228  | 210  |246  | 12   | 36     |  |  91    |  5.0    | 15.8  |
|  4 | 250 | 250   | 226  | 210  |244  | 13   | 34     |  |  90    |  5.6    | 15.1  |
|  5 | 250 | 249   | 229  | 204  |244  | 13   | 40     |  |  92    |  5.5    | 17.4  |
+----+-----+-------+------+------+-----+------+--------+  +--------+---------+-------+
|AVG | 250 | 249   | 231  | 210  |255  | 14   | 45     |  |  93    |  5.9    | 19.6  |
+----+-----+-------+------+------+-----+------+--------+  +--------+---------+-------+

+----+-----+-------+------+------+-----+------+--------+  +--------+---------+-------+
| nr |P sp |P tacx |P pt  |pt min|ptmax|pt std|pt range|  |pt/tacx |std pt/pt|R pt/pt|
|    |Watt |Watt   |Watt  |Watt  |Watt |Watt  |Watt    |  |   -    |   -     |       |
+----+-----+-------+------+------+-----+------+--------+  +--------+---------+-------+
|  1 | 300 | 301   | 285  | 256  |330  | 22   | 74     |  |  95    |  7.7    | 26.0  |
|  2 | 300 | 301   | 278  | 254  |328  | 19   | 74     |  |  92    |  7.0    | 26.6  |
|  3 | 300 | 299   | 272  | 245  |298  | 18   | 53     |  |  91    |  6.4    | 19.5  |
|  4 | 300 | 300   | 276  | 250  |298  | 17   | 48     |  |  92    |  6.2    | 17.4  |
|  5 | 300 | 299   | 278  | 246  |302  | 17   | 56     |  |  93    |  6.2    | 20.2  |
+----+-----+-------+------+------+-----+------+--------+  +--------+---------+-------+
|AVG | 300 | 300   | 278  | 250  |311  | 19   | 61     |  |  93    |  6.7    | 21.9  |
+----+-----+-------+------+------+-----+------+--------+  +--------+---------+-------+

+----+-----+-------+------+------+-----+------+--------+  +--------+---------+-------+
| nr |P sp |P tacx |P pt  |pt min|ptmax|pt std|pt range|  |pt/tacx |std pt/pt|R pt/pt|
|    |Watt |Watt   |Watt  |Watt  |Watt |Watt  |Watt    |  |   -    |   -     |       |
+----+-----+-------+------+------+-----+------+--------+  +--------+---------+-------+
|  1 | 350 | 345   | 325  | 168  |365  | 43   |197     |  |  94    | 13.3    | 60.7  |
|  2 | 350 | 348   | 331  | 304  |362  | 19   | 58     |  |  95    |  5.9    | 17.5  |
|  3 | 350 | 348   | 319  | 257  |358  | 24   |101     |  |  92    |  7.6    | 31.5  |
|  4 | 350 | 346   | 328  | 301  |353  | 17   | 52     |  |  95    |  5.1    | 15.8  |
|  5 | 350 | 352   | 322  | 295  |354  | 20   | 59     |  |  91    |  6.3    | 18.3  |
+----+-----+-------+------+------+-----+------+--------+  +--------+---------+-------+
|AVG | 350 | 348   | 325  | 265  |358  | 25   | 93     |  |  93    |  7.6    | 28.8  |
+----+-----+-------+------+------+-----+------+--------+  +--------+---------+-------+