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Manually Calculating PA (C0) and PM (C1) Coefficients: Slope Intercept/Line Fit/Linear Regression Calculation


Most customers use our Compass for Pressure or CalTool for RPTs programs to run calibrations of several of our products (Ex. 6270A, 8270A, 8370A, PPC4, RPM4, EDWT, etc.) and to calculate and/or apply calibration coefficients. If you don't have this software or want to calculate the adder (PA = Pressure Adder) and the multiplier (PM  = Pressure Multiplier) manually and collect the data by other means this is fairly easy. PA is the same as C0 in some products. PM is the same as C1 in some products.

The adder/multiplier is a standard slope intercept calculation of y = m(x) + b, where m is the slope and b is the adder. Easiest way to calculate this is Excel with LINEST formula. Here are some instructions for this:

See the above example of data collected and how the Excel formula was applied. If you then look in the Function Arguments Box you will see it has calculated the multiplier (PM) as 0.936 and adder (PA) as 138.592 . This calculation yields the same results as COMPASS or similar calculation, and can be manually entered into our products that use this type of calibration coefficients.

Note that the above example does not note that the Reference Pressure and DUT Pressure values are raw values (without existing calibration coefficients applied). The data values used to calculate new calibration coefficients must be raw values. If your data has existing calibration coefficients applied, reduce the data to raw values by the same y = m(x) + b formula. A column "DUT Raw Pressure" can be added to the table and used with the Reference Pressure to calculate new coefficients. 

One more note, a number of our products also support an Autozero (zOffset) value. When this is used, the formula becomes y = m(x) + b + z, where z is the zOffset value. This must also be removed from data that has existing coefficients applied to it before calculating new coefficients.

Other Excel functions that are similar to the LINEST function are SLOPE and INTERCEPT. See the attached Excel file for coefficient calculations using these functions. The attached workbook also allows for entry of existing slope, intercept and AutoZero coefficients.  

We also still offer the CalTool for RPTs software for free which can also gather the data and apply the coefficients to some of the older products (PPC, RPM, etc.). It can be downloaded here: However it does not operate on 64-bit operating systems (most Windows 10 computers). 

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