Help me figure this out - Powertap or Fluid 2 problem

Am I understanding correctly that at the beginning when GC was saying 245 watts, your PT was saying 175 watts but by the end of the workout GC was saying 245 watts but the PT was saying 260-265 watts?

If so, your effort level should tell you which was correct.  There is a huge difference in effort required to put out 175 vs 265 watts.  For me, that's about the difference between Ironman effort and Sprint effort.

The Fluid2 resistance definitely jumps up after it gets warm.  Not a smooth curve across the entire workout though.  I usually notice it suddenly get harder after about 5-10 minutes of moderate spinning.  The virtual power probably assumes the unit is already warm.

Also if you read some of the notes from the TrainerRoad guys, the Fluid2 is notoriously inconsistent when trying to determine a virtual power curve:

http://blog.trainerroad.com/an-update-on-virtualpower-for-the-cycleops-fluid-2/

I would probably trust the powertap over virtual power, unless you have reason to believe the powertap is damaged.

That is exactly what was happening, I just have never heard of anyone complaining about this problem with the Fluid 2 or any fluid trainer for that matter.  I would have thought that after the 10 minute warmup, the Fluid 2 power curve would have stabilized for the rest of the workout.

At first I figured my endurance was shot after taking a post-season break, but you're right that is a significant wattage gap.

Customer service at Cycleops was excellent the last time I called them, so I'll try them again and see if anyone else has complained of this. 

Thanks Spudone.  It definitely took longer that 5 to 10 minutes for the resistance to stabilize last night.

I'm going to try the same workout again but hold power using the readings from the Powertap this time.  The harder effort might speed up the "warm up" period.  

Yes that's absolutely true.  I usually have to shift to an easier gear after the warmup if I want to maintain the same power and cadence.

The warmup time can vary depending on the type of tire you have and how hard you're riding.

This is also my experience with fluid trainers.  It takes a while for the resistance to kick in as the fluid warms up.

This thread made me a bit curious.  My wife has a Fluid2 and a Powertap and her most recent workout had the following intervals

Power - Speed (km/hr)
175w - 32.4 (this started at the 10 min mark)
203w - 32.2 (this started at the 20 min mark)
202w - 30.3
205w - 29.9
207w - 29.5
206w - 29.2

It definitely seems that that resistance goes up and continues to go up as the workout goes on.

I did try a stomp test shortly after I bought the PM and the number wasn't too far off, about a 5% margin of error, but the PM was so much lower than the wattage on Golden Cheetah that I was convinced it was the problem.

BUT I only used a 10 pound weight and have since read that heavier is better and should be at least 50 pounds.

I'll give it a try again using my body weight and see how close it is.

He's trying to determine if his power meter (which he bought used) is giving him valid data.

Thanks Craig.  When I first bought the Powertap it was such a steal (only $200) that when the power numbers were coming back so far off from virtual power, I assumed that it was defective. I expected a drop of about 30 to 40 watts, but 70 to 80 watts seemed too low to me.

I also assumed (wrongly) that the resistance of the Fluid 2 would stabilize after my usual 5 to 10 minute warmup. 

Based on others experiences above and the two charts you provided, this does not appear to be the case and resistance seems to keep getting harder 30 to 40 minutes into a workout.  I think you're right and the Powertap is likely working fine.

The drop in my FTP is a bit of a hit to the ego, but at least I'll be working with real numbers now and not the inconsistent and unreliable wattage that it looks like I was getting from virtual power.