I want to add a new module(funktion) in a set of existing modules, compiled
together as excel add-in.
The formula is called "PET_GAS_COMPRESS_Cg" and is as follow:

Cg = 1/P -((1/Z)*(deltaZ/deltaP)
whe deltaZ = Zi - Zi-1
and deltaP = Pi - Pi-1

The values of "P" will be as column somewhere in worksheet (I dont know
where; I want to click and choose it)
The values of "Z" will be as column somewhere in worksheet (I dont know
where; I want to click and choose it)

What I have written so far is:

Function Pet_GAS_COMPRESS_Cg(P, Z)
Rem File Sait Petroleum Functions (.xls & .xla)
Rem Gas Compressibility (Not to be confussed with "Z" factor)
Rem P = Pressure, kPa
Rem Z = Gas Deviation Factor
Rem Written by: Patrioti

Rem Test for Errors
If IsNumeric(P) = False Or IsMissing(P) = True Then GoTo perr:
If IsNumeric(Z) = False Or IsMissing(Z) = True Then GoTo zerr:
If P < 0 Then GoTo perr:
If Z <= 0 Then GoTo zerr:
GoTo starthe
perr: Pet_GAS_COMPRESS_Cg = "**Problem: P Outside Range": GoTo hereend:
zerr: Pet_GAS_COMPRESS_Cg = "**Problem: Z Outside Range": GoTo hereend:

starthe
If Z(i - 1) < 0 Then Cg = 1 / P(i)
deltaZ = Z(i) - Z(i - 1)
deltaP = P(i) - P(i - 1)
Cg = 1 / P - ((1 / Z) * deltaZ / deltaP)
hereend: End Function


The result is always zero where it should be a number. What I am missing?

TIA

Tim