Power Meter VS Power Beam Readings

[GParkes]

For the coaches out there maybe... I have utilized a Power2Max for the past two years and it has been very good (reliable, durable, no issues). A few days ago I sat in on a group power session that a friend of mine runs and he utilizes the Cyclops Power Beams. I had my own head unit on as a matter of practice, and while my unit and the one he set up were reading similarly, I swear there was a 20W difference between the two based upon rate of PE. I know, it's perceived exertion. Tonight I did an easy spin at home - same watts as the other day that was reading on my head unit Saturday. Today's HR was -15 BPM, and watts same or higher by 10-15W.

So, I guess the question is who has utilized their own PM and a smart trainer/unit like the Power Beams? How wide a variance in watts, if any? How crazy am I? Thanks in advance for the insight.

[bewheels]

It is common for a smart trainer and othe power meters to be off relative to each other. Generally the smart trainers are measuring lower, as you experienced.

You can either establish base lines on both or just use you power2max to control things like Zwift.

Unfortunately you can not just do a straight +/- the amount because the difference is generally a "percentage of" rather than a straight "it is always 20 watts different".

[GParkes]

Not sure I explained properly. When used same time the other day (power beam and my PM), they read comparably. Tonight (just on trainer with my PM) watts were about 10℅ higher, and HR 10℅ lower than other day, in what would be same zone, essentially a fraction of effort.

I know there will be variances, and maybe because I felt like crap the other day, but wide swing. Would running the two units simultaneously be an issue?

[kramnnim]

No, they don't interfere with each other.

[shovelhd]

I use my one head unit (PC8) together with a power meter (SRM) and a smart trainer (Tacx Bushido TDF). The PC8 can only read from one power source, and I don't have another head unit. When I am on Zwift, the SRM drives the Tacx via ANT+ FE-C, so the power I see, and what is related to HR and RPE, is solely coming from the SRM. I have no idea if the Tacx power output is accurate, because I don't need it to be. The SRM is equally accurate indoors and outdoors, although I still have to derate my indoor power compared to outdoors.

[tigoat]

Why your HR has anything to do with the power discrepancy between the two power meter devices? I also use a P2M with a smart trainer that has a power output. If my trainer is properly calibrated, I find it pretty spot on with the power from my P2M. If I do not calibrate my trainer for about two weeks, I usually find it to start having a larger fluctuation of power, deviating between 0-20W higher than P2M. For this reason, I usually try to calibrate it on every other ride. I have ridden enough to trust the power of my trainer that I will be removing my P2M for another bike, as there is no need to waste a PM on a trainer that already has one.

[BSBD]

For the coaches out there maybe... I have utilized a Power2Max for the past two years and it has been very good (reliable, durable, no issues). A few days ago I sat in on a group power session that a friend of mine runs and he utilizes the Cyclops Power Beams. I had my own head unit on as a matter of practice, and while my unit and the one he set up were reading similarly, I swear there was a 20W difference between the two based upon rate of PE. I know, it's perceived exertion. Tonight I did an easy spin at home - same watts as the other day that was reading on my head unit Saturday. Today's HR was -15 BPM, and watts same or higher by 10-15W.

So, I guess the question is who has utilized their own PM and a smart trainer/unit like the Power Beams? How wide a variance in watts, if any? How crazy am I? Thanks in advance for the insight.

Are you graphing the data sets out to compare the PB to the P2M? Or you just going off spot checks, average power on segments/end of workout, or normalized power?