Here is the code:

P=8e7
T=300

Public Function SpecVolLiq(ByVal P As Double, ByVal T As Double) As Double

Dim Pstar As Double
Dim Tstar As Double
Dim Pi As Double
Dim Tau As Double
Dim n(1 To 34) As Double
Dim I(1 To 34) As Integer
Dim J(1 To 34) As Integer
Dim Sum(1 To 34) As Double
Dim x As Integer
Dim temp As Double
Dim R As Single 'Specific Gas Constant of Water (J/kg K)

I(1) = 0
I(2) = 0
I(3) = 0
I(4) = 0
I(5) = 0
I(6) = 0
I(7) = 0
I(8) = 0
I(9) = 1
I(10) = 1
I(11) = 1
I(12) = 1
I(13) = 1
I(14) = 1
I(15) = 2
I(16) = 2
I(17) = 2
I(18) = 2
I(19) = 2
I(20) = 3
I(21) = 3
I(22) = 3
I(23) = 4
I(24) = 4
I(25) = 4
I(26) = 5
I(27) = 8
I(28) = 8
I(29) = 21
I(30) = 23
I(31) = 29
I(32) = 30
I(33) = 31
I(34) = 32

J(1) = -2
J(2) = -1
J(3) = 0
J(4) = 1
J(5) = 2
J(6) = 3
J(7) = 4
J(8) = 5
J(9) = -9
J(10) = -7
J(11) = -1
J(12) = 0
J(13) = 1
J(14) = 3
J(15) = -3
J(16) = 0
J(17) = 1
J(18) = 3
J(19) = 17
J(20) = -4
J(21) = 0
J(22) = 6
J(23) = -5
J(24) = -2
J(25) = 10
J(26) = -8
J(27) = -11
J(28) = -6
J(29) = -29
J(30) = -31
J(31) = -38
J(32) = -39
J(33) = -40
J(34) = -41

n(1) = 0.14632971213167
n(2) = -0.84548187169114
n(3) = -3.756360367204
n(4) = 3.3855169168385
n(5) = -0.95791963387872
n(6) = 0.15772038513228
n(7) = -0.016616417199501
n(8) = 8.1214629983568E-04
n(9) = 2.8319080123804E-04
n(10) = -6.0706301565874E-04
n(11) = -0.018990068218419
n(12) = -0.032529748770505
n(13) = -0.021841717175414
n(14) = -5.283835796993E-05
n(15) = -4.7184321073267E-04
n(16) = -3.0001780793026E-04
n(17) = 4.7661393906987E-05
n(18) = -4.4141845330846E-06
n(19) = -7.2694996297594E-16
n(20) = -3.1679644845054E-05
n(21) = -2.8270797985312E-06
n(22) = -8.5205128120103E-10
n(23) = -2.2425281908E-06
n(24) = -6.5171222895601E-07
n(25) = -1.4341729937925E-13
n(26) = -4.0516996860117E-07
n(27) = -1.2734301741641E-09
n(28) = -1.7424871230634E-10
n(29) = -6.8762131295531E-19
n(30) = 1.4478307828521E-20
n(31) = 2.6335781662795E-23
n(32) = -1.1947622640071E-23
n(33) = 1.8228094581404E-24
n(34) = -9.3537087292458E-26

R = 461.526
Tstar = 1386
Pstar = 16530000#
Pi = P / Pstar
Tau = Tstar / T

temp = 0
For x = 1 To 34
Sum(x) = -n(x) * I(x) * (7.1 - Pi) ^ (I(x) - 1) * (Tau - 1.222) ^ J(x)
'Debug.Print "Sum( " & x & ") = " & Sum(x)
temp = temp + Sum(x)
Next x
'Debug.Print "Total = " & temp
'Debug.Print Pi
'Debug.Print Tau
SpecVolLiq = Pi * temp * (R * T / P)
End Function

Ouput from Spreadsheet = 0.00097118089402163
Output from VBA Function = 0.000971180896076593

Let me know if you have any ideas. Thanks!!

"Jerry W. Lewis" wrote:

The most likely explanation for discrpancies in that range is that you have
declared some variables "As Single". If the problem persists after changing
"Single" to "Double", then post your code.

Jerry

"Clint Schwartz" wrote:

I am writing some functions in vba after performing the calculations on a
spreadsheet. When I compare the results, it consistently appears that the
output from the vba function is not as accurate. It seems to lose accuracy
once you get to the 7th or 8th significant digit. Does anyone know why this
happens? Thanks!!