The chemical curve is the theoretical concentration curve based on nD and TEMP. It is defined by a set of 16 parameters.
A chemical curve is specific to the given process medium, for example, sucrose or sodium hydroxide. The set of parameters is given by Vaisala and should not be altered, except in case of changing to another process medium.
Vaisala provides a field calibration service that adapts the calibration to the factory laboratory determinations based on the data supplied. The field calibration procedure should be made under normal process conditions using standard laboratory determinations of sample concentration.
Record the calibrating data on the Refractometer field calibration form, also available by emailing a request to firstname.lastname@example.org. Email the completed field calibration form either to email@example.com or your local representative. A computer analysis of the data is made at Vaisala and optimal calibration parameters is sent to be entered in the system.
For a complete report, 10 to 15 valid data points are needed. A data point is of use for calibration only when the diagnostic message is Normal operation. Each data point consists of:
- LAB%: Sample concentration determined by the user
- CALC: Calculated concentration value
- T: Process temperature measurement in Centigrade
- nD: Actual refractive index nD
- CONC: Measurement in concentration units, the large size number
In addition to the calibration data, write down the refractometer serial number.
Accurate calibration is only achieved if the sample is taken correctly. Pay special attention to following details:
- The sampling valve and the refractometer should be installed close to each other in the
WARNING Wear protective clothing appropriate to your process when operating the sampling valve and handling the sample.
- Run the sample before starting to collect data points to avoid sampling old process liquid that has remained in the sampling valve.
- Read the values CALC, T(emp), nD and CONC at exactly the same time with sampling.
- Use a tight container for the sample to avoid evaporation.
Offline calibration using process liquid very seldom gives reliable results, as problems are caused by:
Always make calibration using the process liquid in‑line.