A major reason is the fluids are in fact different under different conditions.
This is demonstrated by using calibration fluids. The calibration fluids Cambridge uses have known characteristics that are documented and tied to traceable standards. All Cambridge instruments are factory calibrated with these traceable standards, and the calibration is certified. We note the particular fluids used, their traceability and the results of the calibration. In all cases the factory calibration sheets supplied with each instrument shows the instrument maintains its specified accuracy (1% of the full scale value for the VISCOpro 2000, for instance).
Calibration fluids can be used to evaluate process measurements. Specifically, by removing the process instrument from the line and testing it with calibration fluid, it can be compared to the result from a lab instrument using the same fluid. In such a test, both instruments should yield the same result.
Unfortunately, process fluids are typically not calibration fluids and are affected by the characteristics of the process itself. Here are some of the characteristics that can affect the readings between in-lab and in-line measurements.
Table: Comparison of characteristics that affect in-line and in-lab fluid measurements
| Characteristic |
In-Line |
In Lab |
| Shear |
Uncontrolled Shear from pumps and pipe bends | Controlled shear |
| Flow conditions |
Varying | Fluid is static in a vessel |
| Closed/Open System |
Processes are frequently closed systems after the addition of the basic ingredients, water/solvents, surfactants, etc. | Fluid is exposed to air which changes its composition through off-gassing or other interactions |
| Temperature |
Frequently process temperatures are elevated, and often fluctuating | Measurement is made at a different temperature than the process is running, and it is usually constant,
|
| Measurement |
Measurement conditions are often consistent (experiencing the same levels of fluctuations in the process, measurement to measurement) | Measurement conditions may vary from lab technician, especially with cups and capillary tubes. Calibration may not be recent |
| Timing of Sample |
In a flowing line it is often difficult to be sure that sample passing over the sensor is exactly the same as sample drawn from a sample port. Any difference may cause the in-line measurement to differ from the sample taken to the lab | Sample can be tested and retested |
Therefore the in-line instrument and the lab obtain different results because the fluid is in fact different.
In order to maintain consistency with historical data, we are sometimes asked if the in-line value could be calibrated to match the lab values adjusting for all of the process and temperature differences. By using the ASTM temperature compensation techniques incorporated in many of our viscometers, the effects of temperature fluctuations and differences can be mitigated.
