Offset and Span Calibration for the DHR/AR

Introduction

The temperature control devices on the rheometer can be independently calibrated by calculating a span and offset. The two approaches are described below:

Offset and Span Calibration Method #1

Set a low and high temperature [ST₁ & ST₂] and measure the actual temperatures by another [reference] technique [RT₁ & RT₂]. If this is done at just two temperatures covering the temperature range of interest, the calculation of span and offset can simply be done as follows:

Span = [ST₂ – ST₁]/[RT₂– RT₁]

Offset = ST₂ – [RT₂*Span]

For a more accurate calibration, you will need to measure at more than two temperatures. In this case, it is easier to use a spreadsheet (such as Microsoft^® Excel). Enter data into a table, then use the graph Wizard (or similar) to create an XY scatter graph. Insert the "best fit" line by right-clicking on a data point. An example is shown below.

From the equation of the best fit line [y = Slope*x + Intercept], calculate the span and offset as follows:

Span = 1/Slope = 1/1.02 = 0.98

Offset = –Intercept/Slope = 2/1.02 = –1.96

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Offset and Span Calibration Method #2

The second way to calibrate the span and offset is to use a 'standard' oil which hopefully covers the temperature range of interest. Measuring viscosity at the set temperature is not useful in this matter. You must adjust the set temperature until you get the certified viscosity value; some trial and error required. Note this temperature as the Adjusted Set Temperature [AST]. For a two-point calibration, calculate the span and offset as follows:

Span = [AST₂– AST₁]/[ST₂–ST₁]

Offset = AST₂ – [ST₂*Span]

If you have more than two points, use a spreadsheet as shown in this example.

In this case there is no need for more calculation, as:

Span = Slope = 0.98

Offset = Intercept = –3.86

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