NIR Transmitter Operation: Absolute vs. Relative for Multiple Product Applications

Near Infrared (NIR) Transmitters are secondary measurements that are calibrated to established laboratory methods to provide immediate results on-line, at-line and in the lab.  The benefit of the NIR measurement is that it is non-contact, non-destructive, accurate, and quick. 

Different family of products can be set up to be measured accurately with different calibration recipes.  For instance, snack foods may have different additives or compositional changes requiring different product or calibration codes to be selected for accurate measurements.  The same when a cereal line processes different types of cereal.  The operator simply selects the appropriate product code or recipe for that product.

In some industries, such as converting, multiple recipes are required as a result of multiple variables.  The calibration for moisture or coat weight may vary for different products, as well as the web width or required cross direction scanning speed.  Ideally, the operator selects the correct product calibration code at the operator interface, or this information is sent to the transmitter via an Ethernet connection from the HMI (Human Machine Interface) Central Operating Console.  This allows the NIR transmitter to provide an absolute measurement for operation control and data archiving.

Sometimes, a company has a great many products and limited laboratory resources for calibration.  Skilled labor may also be a limitation for some plant sites so that making sure that the correct product code is entered becomes another variable and there is no HMI Ethernet interface available.  In these cases the NIR transmitter can be used as a relative measurement. 

Different papers may share the same sensitivity (span/gain) setting but have different offset (zero) settings.  The transmitter may be calibrated for one type paper, but there is a zero shift for another type of paper.  Instead of changing to a different product code, the operator simply relies on a different set point target.  For example, a transmitter is properly calibrated for Product A at a target of 4.5.  Product B may have a different chemical composition or color that causes an offset in the reading so that it reads 7.0 when measured using the calibration for Product A.

The operator recognizes that this value is what the analyzer reads when making good product, and does not change the product code.  This is using the transmitter as a relative measurement.