Creating a custom function with VBA Excel, error #VALUE - vba

Question

Public Function CopperThermalConductivity(T As Double, RRR As Double)
Dim P1, P2, P3, P4, P5, P6, P7 As Double
Dim W0, W1, W10 As Double
Dim Beta, BetaR As Double
Beta = 0.634 / RRR
BetaR = Beta / 0.0003
P1 = 0.00000001754
P2 = 2.763
P3 = 1102
P4 = -0.165
P5 = 70
P6 = 1.756
P7 = 0.838 / (BetaR ^ 0.1661)
W0 = Beta / T
W1 = (P1 * (T ^ P2)) / (1 + (P1 * P3 * (T ^ (P2 + P4)) * WorksheetFunction.Exp(-(P5 / T) ^ P6)) + W0)
W10 = (P7 * W1 * W0) / (W1 + W0)
CopperThermalConductivity = (1 / (W0 + W1 + W10))
End Function
Sub DescribeFunctionCopper()
Dim FuncName As String
Dim FuncDesc As String
Dim ArgDesc(1 To 2) As String
FuncName = "CopperThermalConductivity"
FuncDesc = "Returns the Thermal Conductivity in W/m-K"
ArgDesc(1) = "Temperature in Kelvin"
ArgDesc(2) = "Residual-resistivity ratio"
Application.MacroOptions _
Macro:=FuncName, _
Description:=FuncDesc, _
ArgumentDescriptions:=ArgDesc, _
Category:="Cryogenics"
End Sub
I wrote some code to create an accessible function for Excel. Somehow when I use this function on Excel I get a #VALUE error, any idea? I don't get any error when I build the macro. Help me please.

Answer 1

This line is the source of error:
W1 = (P1 * (T ^ P2)) / (1 + (P1 * P3 * (T ^ (P2 + P4)) * WorksheetFunction.Exp(-(P5 / T) ^ P6)) + W0)
You cannot use worksheet function EXP in VBA code. There is VBA equivalent Exp() and it should be used instead of worksheet function.
By the way, note that when you declare variables like that:
Dim P1, P2, P3, P4, P5, P6, P7 As Double
Dim W0, W1, W10 As Double
Dim Beta, BetaR As Double
only P7, W10 and BetaR are declared as Double, other are declared as Variant. If you want to declare all those variables as Double you should do it like that:
Dim P1 As Double, P2 As Double, P3 As Double, P4 As Double, P5 As Double, P6 As Double, P7 As Double
Dim W0 As Double, W1 As Double, W10 As Double
Dim Beta As Double, BetaR As Double