Vaisala provides a field calibration service that adapts the calibration to the factory laboratory determinations based on the data supplied. This field calibration corrects the instrument reading to show the same values as the laboratory measurements.
Perform the field calibration procedure under normal process conditions only, using standard laboratory determinations of sample concentration.
Measure the calibrating data by using the Field point functionality on the on the Sanitary OEM Refractometer homepage. For more information, see Measuring field samples. Record the data either on the Refractometer field calibration form, or print out the field points on the web page.
If you have a constant offset, you can perform a bias adjustment, see Direct BIAS adjustment. For a more detailed field calibration, send the completed calibration form to either email@example.com or your local representative. A computer analysis of the data will be made at Vaisala and optimal calibration parameters will be sent to be entered in the system.
For a complete report, 10 ‑ 15 valid data points (see below) 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
CAUTION Offline calibration using process liquid very seldom gives reliable results, as problems are caused by:
- Low flow which makes sample to form an unrepresentative film on the prism
- Sample evaporation at high temperature or undissolved solids at low temperature giving deviations from laboratory determinations
- An ageing sample which is not representative
- Outside light reaching the prism
Calibration using the process liquid must always be made inline.