math channel why wrong subtracted product?

[Howard Coleman]

really simple stuff here but wrong calculated product.

GP Temp 1 (air thermocouple out of turbo) minus GP Temp 3 (air thermocouple into motor)

a - b

a (320 F) minus b (138.6) produces 100.8 as Math Block Output 1 in Parameters

320 minus 138.6 is actually 181.4

taking 4 more readings i find that the error is always the same %

181.4 is 1.799 X of 100.8. all the other data points are 1.799 

for instance

a is 219.9 and b is 157.1  difference is 62.8

Math Block shows 34.9

62.8 is 1.799 times 34.9.

both GPTemp 1 and GP Temp 3 use the same Linear Calibration Table 2.

a = 0 V

b = 5 V

output units Temperature

a = minus 15 F

b = 2248 F

what is the fix?

 

thanks

 

 

[Vaughan]

160 C (320F) - 59.2 C (138.6F) - 100.8

All calculations in the ECU are done in SI units, the imperial units you see in PCLink are purely a visual conversion. 

[Adamw]

I think if you wanted the result in deg F you could change your formula to something like (a-b)*1.8+32

[Howard Coleman]

thank you Adam.

[Vaughan]

8 hours ago, Adamw said:

I think if you wanted the result in deg F you could change your formula to something like (a-b)*1.8+32

You would actually want to do:

(a*1.8 +32) - (b*1.8 +32) = a*1.8 - b*1.8 = (a-b)*1.8

[Adamw]

Oh yeah, we want the difference not absolute.  Brain fart.