We all know that Microsoft Excel® is a great tool for automating engineering and scientific calculations. Although Excel includes many powerful features, its table lookup functions are rather limited. They might be adequate for the average user, but the technical user needs something more powerful. To the technical user, data in a table often represents points on a curve rather than just a collection of information. We use curves to represent all types of information such as thermodynamic properties, transport properties, material properties, equipment performance, correction curves, efficiencies; the list is endless. If your table represents points on a curve, Excel’s built-in lookup function will only return the value from a specific point in the table. What we really need are functions that can find the value of any point along the curve, even if it falls between two points in the table

Techware’sThe **XLInterp** functions work with two-dimensional
tables so that they can be used to analyze data with two variables.
In addition, **XLInterp** includes both forward and inverse
functions. With the forward functions, you provide the variable(s)
and it finds a value on the curve (or surface for two-dimensional
problems.) With the inverse functions, you provide a point on the curve
(or a variable and a point on the surface for two-dimensional problems)
and it finds the unknown variable.

**XLInterp** has a subtle feature that greatly
enhances its power; its functions can work with non-numeric column and row
labels. When non-numeric data is used for either the row or column
values, the functions do not interpolate in that direction. Instead, they
search for a label that matches the input variable and then interpolate
along that row or column. This feature enables you to assemble
tables which represent collections of one-dimensional curves that you can
access by name.

**XLInterp** also includes advanced functions, which
calculate partial derivatives of the curves represented by the data in
your table. You can calculate derivatives with respect to row or column
variables from either forward or inverse functions.

- Added 64-bit add-in to work with 64-bit versions of Excel.
- Increased allowable table size to full worksheet size in Excel 2007 and later.
- Converted all help files to compiled HTML format.

- Removed Help file link from Help Menu for Excel 2007 and later.

** XLInterp 1.2** is available as a
free
upgrade to licensed users of **XLInterp1.1**

- Functions can be used as if they were built in to Excel
- Treats data in your spreadsheet tables as points on curves or surfaces.
- Can find any point on the curves using non-linear or linear interpolation.
- Works with data tables of one or two dimensions.
- A two-dimension table can represent a surface function or a family of one-dimensional curves.
- Accepts variable grid spacing to allow greater accuracy in non-linear areas while minimizing data required for other areas.
- Accepts data in ascending, descending, or changing order.
- Performs table lookup in tables using labels for row or column headings.
- Includes both forward and reverse functions. This allows a single table of data to be used for all calculations.
- Includes functions for calculating partial derivatives.
- Supports 32-bit and 64-bit versions of Excel.
- Check pricing for attractive quantity discounts.

The following table lists all of the **@Air**
functions currently available.

Function(Input Arguments) |
Type |
Output Value |

InterpRCT(table,row value,column value) |
Forward | Value from table |

InterpRTC(table,row value,table value) |
Inverse | Column value |

InterpCTR(table,column value,table value) |
Inverse | Row value |

InterpRCdTdR(table,row value,column value) |
Forward | Partial derivative dT/dR)_{C} |

InterpRCdTdC(table,row value,column value) |
Forward | Partial derivative dT/dC)_{R} |

InterpRTdTdR(table,row value,table value) | Inverse | Partial derivative dT/dR)_{C} |

InterpRTdTdC(table,row value,table value) |
Inverse | Partial derivative dT/dC)_{R} |

InterpCTdTdR(table,column value,table value) |
Inverse | Partial derivative dT/dR)_{C} |

InterpCTdTdC(table,column value,table value) |
Inverse | Partial derivative dT/dR)_{C} |

InterpVer() | Version and serial number |

**A Simple Example Illustrating the Ease and Power of
XLInterp**

The spreadsheet table below named ** TABLE1, **defined
as cells (A1..E6), represents a function of two variables. To make this
example meaningful, the table actually represents the enthalpies of steam
as a function of pressure and temperature. The numbers in row 1 represent
temperatures in degrees F, while the numbers in column A represent
pressures in psia. The cells in the area (B2..E6) hold the enthalpy values
of steam in Btu/lb.

A | B | C | D | E | F | G | |

1 | 400 | 500 | 600 | 700 | |||

2 |
10 | 1240.58 | 1287.78 | 1335.55 | 1384.05 | ||

3 |
50 | 1234.95 | 1284.11 | 1332.92 | 1382.02 | ||

4 |
100 | 1227.36 | 1279.33 | 1329.57 | 1379.46 | ||

5 |
150 | 1219.10 | 1274.32 | 1326.14 | 1375.88 | ||

6 |
200 | 1210.13 | 1269.04 | 1322.61 | 1374.25 |

Suppose we are interested in determining the enthalpy of
steam at a pressure of 60 psia and a temperature of 440 deg F. The formula
** =InterpRCT(TABLE1,60,440,0)** entered in any cell finds the
value of the function described in the table using linear interpolation.
In this example, the value returned is 1253.32, rounded to the nearest
hundredth. If the last argument in the formula is changed from a 0 to
either a 1 or an "NL", the function uses non-linear
interpolation and returns a value of 1253.48. It is interesting to note
that the ASME steam tables lists the enthalpy of steam at 60 psia and 440
deg F as 1253.5 Btu/lb, rounded to the nearest tenth, and Techware’s

If you wish to try **XLInterp**, you may download
a fully functioning copy for a 30 day evaluation period. If you
decide to purchase **XLInterp** you may continue to use the download
and we will provide you with a serial number that allows permanent
use. Go to our Order page when you are ready to purchase **XLInterp
1.1**.