MoTeC Global Forum

I'm trying to create a Math channel in I2 PRO that will display a boost value taking the altitude into consideration.

The formula for this is:

boost = manifold pressure - atmospheric pressure

To estimate atmospheric pressure at a given altitude you can use this formula:
(kPa at sea level * e ^ -(altitude/7000)*100)

which at Wakefield would be:
(1.01 * 2.7182818285 ^ -(661/7000)*100) = 91.925 kPa

So I attemted to create a Maths channel in I2 PRO like this:
power(1.01*2.7182818285,-1*('GPS Altitude' [m])/7000)*100

and it tells me that the expression is invalid. So I attempted this:
power(1.01*2.7182818285,-661/7000)*100

which I2 will happily resolve to the wrong value. Assuming there was a problem with negative exponents with the power function I tried to create a simple unitless expression to 4 decimal places like this:
power(10,-2)

The result I get is 0 not 0.01. So getting desperate I tried:
1/power(10,2)

and I still got 0 as the result. Getting even more desperate I tried:
1/10/10

and I still got 0 as a result.

Does anyone know what is going on here? There must be some sort of rounding issue I'm not seeing as I2 thinks 3/2 = 1.0000

Thanks Matt,

That explains a few things and I can now get this to work: 1.01*power(2.7182818285,-(661/7000.0))*100

However this: 1.01*power(2.7182818285,-('GPS Altitude' [m]/7000.0))*100 gives me an expression invalid error.

Can you please point me in the right direction again?

Regards,

Mark