Hi. If I'm not mistaken, I believe your equation results are slightly off
because of display error.
For example, your x^3 term in System Rate should be about 8.34 *10^-12 (vs
your -5*10^-21).
When Solving equations, you may find it better to extract the coefficients.
For Example, select 4 horizontal cells and Array enter the following with X
& Y referring to your data.

=LINEST(Y ,X^{1,2,3}) (Ctrl+Shift+Enter)

These coefficients appear correct when compared to the output from another
program.
(System):
8.34352E-12 ,0.000241208 ,1.19407E-05, 123.0624551
(Pump):
-2.02024E-09 ,2.01471E-05, -0.089731972, 2279.045625

Subtract the coef. to get something like...
-2.0285*10^-9*x^3-0.000221*x^2-0.08974*x+2155.98317

Solve the cubic equation (equation =0) to get three real solutions...
-108474.58402
-3389.100225
2890.94350

Only 2890 appears as the feasible solution.
HTH
--
Dana DeLouis
Win XP & Office 2003


"Cadelima" wrote in message
...
Polynomial equations are the best fit. As I mentioned, the system pressure
and pump output pressure varies by fluid properties. An example of a data
set
from one fluid is as follows:

GPM Pump Head System Head
0 2279 123
300 2254 145
800 2219 277
1300 2192 531
1800 2171 905
2300 2155 1399
2800 2141 2014
3300 2130 2750
3800 2118 3607
4300 2105 4584
4800 2089 5681
5300 2069 6900
5800 2042 8239

Graphing the data and selecting a 3rd order polynomial for the trendline
gives the following equations:

System rate f(x): y=-5E-21x^3=0.002x^2-1E-13x+123.08
Pump rate g(x): y=-2E-09x^3=2E-05x^2-0.0896x+2279.4

The solution for flow rate is where the pump and system pressures are
equivalent, or where the two equations are equal.

Hope this helps.

"B. R.Ramachandran" wrote:

Hi,

What kind curves are they (i.e., what kind of functions are those
trendline
equations) - linear, polynomial, logarithmic, ....? Can you post sample
trendline equations for the pump curve and the system curve?

Regards,
B. R. Ramachandran

"Cadelima" wrote:

I am trying to otain a solution for the number where two curves meet.
Specifically trying to determine the expected flow rate given a pump
curve
and system curve. The curves shift depending on variables that are read
from
a spreadsheet. I can get the curve formulas by graphing them and
requesting
the trendline feature, but then I have to manually enter the curve
formulas,
set them equal to each other, and solve the equation. Can Excel do this
automatically from two data sets? Any help would be appreciated.