You need to multiply by unity i.e. multiply by 1.
TL;DR
For the nominal calibration, a rise of 100PSI, from 0PSI to 100PSI, is equivalent to a rising of 16mA, from 4mA to 20mA.
So
(100PSI - 0PSI) ≡ (20mA - 4mA)
100PSI ≡ 16mA
Divide both sides by 16mA:
100PSI ÷ 16mA ≡ 16mA ÷ 16mA
(100/16) PSI/mA ≡ 1
6.25 PSI/mA ≡ 1
That 6.25 PSI/mA is the slope of the nominal calibration line, and it is equivalent to unity (one), as shown above.
If the minimum calibration values (mA and PSI) change, and we can't measure new calibration values around the maximum (20mA ≈ 100PSI, then we assume the slope of the mA vs. PSI characteristic calibration line from the minimum reading [
0PSI,
3.997mA] to the "normal" reading [
51PSI,
normal mA] must be same as it is from the minimum reading [
0PSI,
3.997mA] to the maximum reading [
max_PSI,
20mA]:
(normal_PSI - min_PSI) (max_PSI - min_PSI)
Slope = ---------------------- = -------------------
(normal_mA - min_mA) (max_mA - min_mA)
Except for
max_PSI, we presumably know everything:
- normal_PSI = 51PSI;
- normal_mA = mA reading taken at 51PSI;
- min_PSI = 0PSI;
- min_mA = 3.997mA;
- max_mA = 20mA.
Solve for
max_PSI.