I was writing up a description of a power meter that would use air pressure to calculate applied power.

If you know;

1. Frontal area
2. Aerodynamic drag factor
3. Grade
4. Barometric pressure
5. Grade
6. Total weight
7. Speed

You could calculate the amount of power you are applying to drive the bike forward. This would be a little off since there are drivetrain losses and losses from things like rocking the bike back and forth and rolling resistance. But these are relatively minor and you could probably calculate a correction factor since these are all linear corrections.

In any event while looking things up I discovered that someone beat me to the punch.

There is the $149 WattZit and the "Starting At" $199 PowerPod.

Both of these are using this idea.

The WattZit is a start-up and the stuff is still a little crude but he seems to be moving in the right direction quite rapidly.

The PowerPod is a lot more highly developed but you would have to buy in pretty deeply to get all the real bells and whistles.

What I have been doing to this point is to use an altimeter/Speedo that gives average climb and speed then using Steve Gribbles power calculator to estimate power I'm putting out.

The numbers I've been getting from this method have been cross checked with papers on actual power outputs of different classes of racers and I have been a little on the low side of my supposed category. This matches with the way that I feel my performance is.

I have two or three hard rides a week in good weather and I tend to be the last one up every hill. Though now that I'm using Propel in my water bottle I'm not exhausted after every ride.

Three years ago I could give some really hard bursts of performance but that slowed up over the last three years until I'm down to slow and steady. I got a copy of Joe Friel's "Fast Over 50" but it appears to me that he lives on flat terrain and his training methods aren't easily converted to hilly terrain. But doing the best conversions I can I've seen some improvement if not what I'd like to think I remember.

So since the WattZit is cheap I got one to try.

His instructions on how to use it are pretty bad but perhaps that's the result of my concussion. They DO give you the information you need if you have to study it a bit. I finally discovered that the empty box of SmartLink included was from the component that had already been installed in the WattZit.

Then the App just would not install. I tried a dozen times on Friday and nothing would work. I got up Saturday and tried again and it downloaded from the email he had sent me and installed with no problem. I couldn't have done it wrong since all you had to do was click on the App and then hit "INSTALL" when it took over the whole screen.

In the meantime he upgraded the APP from 1.1 to 2.0. The bike unit measures speed by a wheel magnet but on the updated APP you can use GPS instead. I will try both but my suspicion is that the GPS is slightly better.

WattZit claims a +/-5% and PowerPod a +/-2%. My guess is that this difference is in the initial set-up procedure. The WattZit appears to make an approximation of Frontal Area and estimates aerodynamic drag with handlebar type.. PowerPod uses the same hanglebar type but has you do some "set-up" runs that give a better estimate of frontal area.

As I'm trying this WattZit I'll keep you informed of what I think of it.