Using the Bestselling dataTaker Data Loggers
Many measurement instruments do not have a linear measurement scale, and require a more complex equation to properly scale them to engineering units in a dataTaker data logger. However, you can use Microsoft Excel to create the correct polynomial equation using the calibration sheet included with these devices. All that’s required is Microsoft Excel (or Openoffice.org Calc), and the calibration sheet for the measurement device, which in this case will be for a thermistor.
The calibration sheet that you receive will have a table of physical measurements and actual signal measurements that will look something like this:
| Degs F | -50 | -25 | 0 | 25 | 50 | 77 | 100 | 125 | 150 |
| Ohms | 61200 | 27575 | 13260 | 6805 | 3678 | 2001 | 1241 | 765.5 | 489.8 |
Check to make sure that the data has been entered in the correct columns or the transfer function will be inverse. Enter these into Excel in this same format, with the physical measurement on top and the signal measurement on the bottom. Now create a new line chart in Excel and select this information as the data.
Right-click on the trend line and choose ‘Add Trendline’. In the box that pops up, choose ‘Polynomial’, set ‘Order’ to anything over 2 (5 is the highest order the dataTaker can accommodate) and check ‘Display Equation on chart’. Note that using the highest order is not always optimal for your particular application. You may need to click and drag the equation into a more visible area of the chart.
After you have entered the correct type of signal into the DEX interface of the dataTaker, click on the ‘Scaling’ tab and choose ‘Spans and Polynomials’. Click on ‘Add’ and select ‘Polynomial’. Now enter the coefficients from the equation.
After this polynomial is properly entered, go back to the ‘General’ tab and make sure that ‘Display Units’ contains the correct engineering units. It’s recommended to increase the number of decimal places for the engineering units. Now you’ve finished creating your calibration polynomial for use in scaling.
For a more detailed version of this article, containing additional images and examples, download the .pdf file at the top of the page.