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Lets remember what we are trying to accomplish here. We want our " weather station" to help measure the factors in the atmosphere that affect our engines performance. What are those primarily?
1) Available oxygen
2) Amount of water vapor

How do we increase horsepower? We increase th amount of available oxygen to burn, which in turn allows us to burn more fuel. More fuel, more power. More power, faster we go.

Density altitude was originally, ( and still) used in the aviation industry. You will notice that Kestrel units are sold in places like SPortys Pilot Shop and so forth. A simple device for pilots to use. Pilots needed to know this to determine the amount of thrust they need, or could develop to lift the aircraft.

Density altitude takes into account temperature, humidity,and pressure. Systems like I use assume a 20.9% available oxygen.( per volume) . I sue actual oxygen as measured by the air sampler.

Water vapor is a biggie in my opinion. It affects the rate of fuel burn, the dilution of the fuel ( water,and we all know engines dont like to burn water)and it actually changes the density of the air. If you know how this affects your engines performance, you'll be miles ahead.

Again, what do we want our wetaher station to tell us? Its the ability to accurately tell us what changes in the weather are occuring.

There are some rule of thumbs:

200' rise in DA = .01 slow down ( gas)
400' rise in DA = .01 slow down ( alky)

While these are only guidelines, you must know YOUR combination, and start watching trends. DONT just look at DA, it is not the end all do all to performance. Watch those grains of water, vapor pressure and so forth, then start determining how that is affecting your ET, ALONG WITH, what you are seeing on DA. DO NOT depend on someone elses weather station to predict your ET. Although they may show "X" amount of DA change and so forth, you must stay consistent and use your own data from your own machine.

If you took a 1 degree change in temperature, a .1 Hg change in pressure, and a 10% change in humidity,or even a .1% change in oxygen, any of those could equate to a 100' or better change in DA.

Also trhink of those nights when you are racing, and the car keeps pickup up, picking up, picking up, and your thin king " holy cow. we are going to run a new best " by the end of the night. Then the car just stops climbing, the 60" stops getting better, and you may even slow down. Heck, the air is cooler, " i can feel it"...cold air equals fast right? Ooops, better check the humidity and water vapor....We just started to try and burn dew.

Look at ALL the data, get a feel for what each change ( especially minor changes) is doing to your ET, and just keep using it, using it and logging data. Dont get hung up on DA alone.

If you ever notice, what do the Pro fuel teams watch the most? Grains of water. ( it does affect them heavily beacuse of the comression factors involved with blowers and such, but effects ALL engines none the less.)

Good luck Bill. I will call you about that Junior deal soon too.

Todd