Rheonics sensor are shipped factory calibrated. They do not need re-calibration in field, however for various regulatory (ISO, FDA, others) fullfillment of requirements, it may be necessary to verify or re-calibrate the sensors periodically.


This article uses various third party companies and organizations, there use here is by no means an endorsement of any product or company mentioned in this article. Users are responsible for making their own decisions regarding the selection and should review information from the respective supplier to assess the safety risks and proper management of those risks.


SRV measures a product of dynamic viscosity and density. 


To compare the measured values against reference viscosity values from viscosity standard, you can follow one of the following procedure.


Preparation:


1. Getting suitable reference viscosity standards, we recommend the following companies and standards bodies that provide traceable viscosity standards

  • Cannon Instruments is the supplier of NIST traceable viscosity standards, widely available in USA, Europe, Australia, Asia.
  • Nippon Grease standards, generally available in Japan
  • PTB (Physikalisch-Technische Bundesanstalt) is the national metrological body in Germany. Newtonian calibration liquids for viscosity are supplied by calibration laboratories accredited by Deutsche Akkreditierungsstelle GmbH (DAkkS). These liquids are traceable to the National Standard of Viscosity according to DIN EN ISO/IEC 17025. 

2. Sensor handling and preparation. To calibrate or verify the sensor calibration, it is important to ensure the operator follows proper procedure for handling the sensor, calibration fluids and maintain suitable operational conditions. 

  • Sensor should be placed ideally in a clamp or sensor holder
  • Fluid should be poured in a calibration cylinder around the sensor or in a beaker in which the sensor is dipped
  • Calibration fluid should not have bubbles 
  • Temperature should be stable when making measurements


Getting Calibration fluid data:


Reference fluids are generally provided with measured data by the calibration labs which follow ASTM, ISO, DIN, EN standards. However there is a wide variety of calibration fluids available in the market. Here is a short analysis of the various available fluids and our recommendations.


  • Rotation viscometer calibration fluids (silicones): these are typically silicone oils which are provided with a single kinematic value at a single temperature. Like the rotation viscometers (eg Brookfield), these are very inaccurate test fluids and useless for any calibration since the fluid and rotational viscometers have both quite high inaccuracy. Further silicone oils are viscoelastic so their viscosity depends on the frequency of oscillation of instrument. This will give very different results for the silicone oils when measured with a vibrational instrument like the SRV, SRD, DVP.


  • Mineral oils: these are Newtonian and ideal for viscometer calibration. However often they only have single measurements. For calibration or verification, it is better to get these oils with both density and viscosity values measured at various temperatures. You will need both density and dynamic viscosity of the oil to perform proper calibration check on the SRV.


  • RHEONICS RECOMMENDED - NIST traceable viscosity references: These can be sourced from Cannon Instruments in USA and worldwide. There are other suppliers. We will refer to calibration fluids we source from Cannon Instruments in this document. 


            NIST Traceable viscosity standards from Cannon Instrument

        

            Viscosity reference selection based on target range



SRV sensor:

  • SRV measures a product of dynamic viscosity and density
  • SRV has a density output, which is by default set to 1.000
  • For SRV, calibration check or re-calibration we recommend using any two of the following: S20, S60, S200, S600
  • For higher viscosity ranges, N4000 can be used
  • It is important to ensure the higher viscosity fluids (S200, S600, N4000) are poured carefully in the measurement container to avoid entrapping air. Also, heating the fluid to 80 °C and then cooling it down to the required temperature helps remove any entrapped air.
  • Rheonics provides sensor holder, calibration cylinders which can be used for calibration, check accessories page
  • Rheonics provides a temperature control system, STCM for doing multi-point temperature ramps useful in calibration and testing. Learn more here: STCM info page


Calibration/verification procedure:


1. Get viscosity reference fluid
2. Use a beaker, hold the sensor in a clamp (not hand) or in the rheonics sensor holder
3. Ensure the sensor is not touching the side or the bottom of the beaker, if you are using rheonics calibration cylinder then ensure it is inserted fully with an o-ring at the bottom.
4. Fill the beaker with calibration fluid, ensure as little bubble as possible. Getting rid of bubbles is important to achieve good calibration.

4. Let the sensor stabilize, when the sensor and fluid temperature has stabilized --> you will see temperature output from sensor will start varying by less than 0.1 °C, this takes time, it can take between 30 to 60 minutes unless you are using an oven or STCM.


SRV density set to 1.0 (default setting), if you have the density set to a different value, see the section below.


5. Check the SRV density shows 1.0, this is default setting
6. Record SRV viscosity output, take 5 readings - your variance should be less than 1% in between these values
7. Compare the SRV reading against the reference fluid value
8. Generally you have to use a software like Viscdisk from Cannon to get the exact viscosity at the temperature of measurement. You can generally ask Rheonics support to give you a spreadsheet which gives the values at various temperature for NIST fluids. For this you need to provide the label that comes with the cannon bottle. For other brands, please contact the supplier directly.

9. Estimate calibration accuracy = measured viscosity by SRV / (dynamic viscosity * density)
10. Remember to compare the measured viscosity of SRV with the product of dynamic viscosity and density on the bottle. If you do not multiply dynamic viscosity of the fluid with density, then the measured viscosity will appear much lower than the reference viscosity, that is due to not using multiplying the reference value with density.

SRV density set to user value:


11. Enter the correct density from the calibration reference data

12. Now SRV will output dynamic viscosity

13. Compare reference fluid dynamic viscosity against dynamic viscosity output from SRV

14. Estimate calibration accuracy = measured viscosity by SRV/reference dynamic viscosity of calibration fluid


A. If you see a difference higher than expected, redo the measurement and be careful about ensuring constant temperature and no bubbles in the calibration fluid.
B. If difference persists and higher than expected/ordered - send the data to rheonics support, detailing the fluid, fluid data, steps taken, results.

Troubleshooting:


What are the common issues?

1. Sensor is not in the fluid completely.

Answer: Make sure sensor sensing element is completely submerged in the fluid. The picture below shows a potential issue since the sensing element is not completely in the fluid.


2. Viscosity value is still changing. Temperature is not stable.

Answer: Wait long enough to stabilize temperature or use an oven or STCM.


3. SRV viscosity output is compared against only dynamic viscosity of the calibration fluids.

Answer: SRV viscosity output should be compared against dynamic viscosity only if the calibration fluid density is entered in the SRV electronics using RCP (see RCP manual for help). If SRV shows a default density output of 1.000 g/cc, then you should compare the viscosity output of SRV against dynamic viscosity multiplied by density of the calibration fluid.


4. How to get density from only dynamic viscosity and kinematic viscosity on the calibration fluid datasheets?

Answer: If you divide dynamic viscosity by kinematic viscosity, you will get density in g/cc.


Tutorial:


[video link]


References:


Cannon Instruments NIST Newtonian viscosity standards >> weblink >> pdf in attachment

PTB Newtonian calibration liquids for viscosity >> weblink >> pdf in attachment