I got a slightly different result, and used a slightly different method. Please correct me if I am mistaken.

Here's what I did:

rise/run = (35.3 - (-14.7)) / (4.5-0.5) = 50/4
point slope form is: y - y1 = m(x - x1)

using the known point (0.5, -14.7) you get

y - (-14.7) = (50/4) (x - 0.5)
y + 14.7 = (50/4) x - 6.25
y = (50/4)x - 6.25 - 14.7
y = 12.5x - 20.95

or you could use the other point (4.5, 35.3) :

y - 35.3 = (50/4) (x-4.5)
y - 35.3 = (50/4)x - 56.25
y = (50/4)x - 56.25 + 35.3
y = 12.5x - 20.95

So at 0 volt signal, you would be logging almost 21 inches of mercury in vacuum.

Now I use that psi-per-voltage formula to determine the theoretically loggable range if the sensor were completely linear:

y = 12.5(5) - 20.95 = 41.55
41.55 + 20.95 = 62.5

Now with that theoretcal range scaled from 0-255, I get:
y = 62.5/255x - 20.95

This is very close in actual value to acamus's formula, but my Y intercept value is lower, so can anyone tell me where my error is -if I made one?

 

^delta, I get the same answer as you, worked out on a psi/step basis.

 

apologies for necromancing but if anyone would be so kind

i am trying to hijack the signal from my Apexi 2.5Bar boost gauge. I used a hand vacuum pump and got these voltages

-14.7psi = 0.25v
0psi = 0.5v
14.7psi = 0.75v
29.5psi = 1v
36.9psi = 1.125v

The algebra is owning me at the moment.

i get

y = 58.3x - 29.34

Just need to work out the 0-255 and figure out where to plug that into my xml

0-255 vs 0.25-1.125

y = 291.43x - 72.86

 

0-255 is equivalent to 0-5V. So substitute for x, i.e. x=5*z/255

 

my sensor's range is not 0-5v. do you know where in the xml or ecu flash i go to change that?