I have a PT and CT. It takes 10-15 minutes of CT warmup and then calibration to get the numbers really close. They will never be the same (nor should they be) because they measure power differently and from different places.
I used my Quarq paired to my Garmin 910XT while on my CT during my bike test (OS week 1). In the end I was surprised to see they were 2 watts apart on average power, CT being the lower of the two. Since the I have not bothered to use the Garmin, so I can't say. Have been doing the OS on the CT so far. RacerMate One auto saves my files and they easily upload to WKO+. Nothing wrong with using both, of course, but I am using the Polar HR strap compatible with CT just to get as much data as possible into WKO+.
Like most of the others, my CT reads lower than my Quarq. 10watts sounds about right in terms of ballpark although sometimes it is more. The Quarq also reads power spikes and surges much more accurate than my CT, probably since the wheel tends to slip some during spikes on the CT.
Also, I have the Computrainer Pro model, I'd be somewhat interested to ride the Lab model some day and see if the power tracks more closely, but long story short, I trust my Quarq power much more than the CT power. At the end of the day, it doesn't really matter which one is more accurate, the Quarq is far more consistent and is available to me both indoors and outdoors. When training and racing with power, if you had to chose between accuracy and consistency,I'd choose consistency any day.
As others have said, because they are measuring power at different locations, the numbers will be different. Since the Quarq is reading at the crank, it "should" be higher than the CT (or for that matter a PT), which is measuring power at the rear wheel (or hub for PT). You might be able to get them to read very closely by recalibrating the Quarq (maybe; I don't have one, so I don't know if this is feasible or easy).
I recalibrated my SRM slope manually because the numbers were way off when compared to the CT (after having it serviced at the SRM center). Before I had it serviced, they were very close, and, after recalibrating, they again were very close. It doesn't really matter, but I like seeing consistent numbers between the two.
I have been riding with the quarq and CT together. As others have said the CT is higher . . . for me by around 20 watts. The difference seems to decrease when wattage exceeds 250 on the CT. In my case the difference is higher since the quarq measures power at right leg, and for me the right leg is significantly weaker than the left for medical reasons. I tested for JOS on the CT cause I didn't have the quarq yet, so until I retest on the quarq I am training by the CT #s for now.
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Also, I have the Computrainer Pro model, I'd be somewhat interested to ride the Lab model some day and see if the power tracks more closely, but long story short, I trust my Quarq power much more than the CT power. At the end of the day, it doesn't really matter which one is more accurate, the Quarq is far more consistent and is available to me both indoors and outdoors. When training and racing with power, if you had to chose between accuracy and consistency,I'd choose consistency any day.
I recalibrated my SRM slope manually because the numbers were way off when compared to the CT (after having it serviced at the SRM center). Before I had it serviced, they were very close, and, after recalibrating, they again were very close. It doesn't really matter, but I like seeing consistent numbers between the two.