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 field calibration form found in Field calibration form, also available by emailing a request to helpdesk@vaisala.com. Email the completed field calibration form to your local Vaisala representative. Vaisala makes a computer analysis of the data and sends optimal calibration parameters to be entered in the indicating transmitter DTR.
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. If prism wash is employed, do not take samples during the wash. Each data point consists of
- LAB%: Sample concentration determined by the user.
- From DTR display: See Figure 1.
- 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 indicating transmitter serial number, the sensor serial number and the sensor position, meaning whether it is installed as Sensor A or as Sensor B.
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
process.
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, nD and CONC in the
DTR’s display at exactly the same time with sampling.
The easiest way of doing this is to use the FIELD SAMPLE soft key available through the Sensor status display (DTR program version 2.0 or newer). The value of each sample is the average of 10 consequent measurements to increase accuracy and reduce possible process noise.
Figure 1. Using FIELD SAMPLE soft key 
- 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:
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Calibration using the process liquid must always be made in‑line.