TABLE OF CONTENTS


1. Overview

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


Rheonics inline viscometer: SRV measures a product of dynamic viscosity and density, called kinematic viscosity. To compare the measured values against reference viscosity values from viscosity standard, you can follow the procedure from this article.


Rheonics SRV inline viscometer designed for precise viscosity and density measurement in industrial applications

Figure 1. Rheonics SRV Inline Viscometer

 


This article mentions various third-party companies and organizations, their use here is by no means an endorsement of any product or company mentioned.


Users are responsible for making their own decisions regarding the selection and should review information from the respective suppliers to assess safety risks and ensure proper risk management


 

2. Preparations for Viscometer Calibration

2.1 Choosing the Right Reference Viscosity Standard for Calibration

For obtaining suitable reference viscosity standards, we recommend the following companies and standard 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. 


2.2 Proper Handling and Preparation for Accurate Viscometer Calibration

To calibrate or verify the inline viscometer calibration, it is important to ensure the operator follows proper procedure for handling the sensor and calibration fluids while ensuring appropriate operational conditions. 

  • Sensor should be fixed ideally using a clamp or sensor holder
  • Fluid should be poured in a calibration cylinder around the viscometer or in a beaker in which the sensor is dipped. For minimizing calibration fluid volume, Rheonics recommends using calibration stands and cylinders. Take a look at our article: Using Rheonics Process Viscometer and Density Meter as Tabletop Instruments.
  • Calibration fluid should not have bubbles.
  • Let the temperature stabilize to ensure accurate measurements.



This method only uses a beaker, which is fine for normal temperature calibration. For different temperature setpoints, Rheonics recommends the STCM to ensure thermal equilibrium,


 

The following images illustrates the main considerations for SRV and SRD calibration.


Image showing considerations for using the Rheonics SRV viscometer, including proper sensor clamping, fully immersing the sensing area, and optional fluid mixing or heating for temperature testing

Figure 2. Considerations for Rheonics Viscometer: SRV

 


Diagram of Rheonics SRD sensor setup, highlighting proper clamping, fully immersing the sensing area, fluid recommendations, and optional heating for temperature testing

Figure 3. Considerations for Rheonics Viscosity-density sensor: SRD


Learn more about SRD's thermal equilibrium in this article: Maintaining the SRD's temperature balance for high density accuracy.


3. Reference Calibration Fluids and Standards

Calibration fluids are essential for accurate viscometer and density meter calibration. These 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. Below is a short analysis of the various available fluids and our recommendations.


3.1 Rotation Viscometer Calibration Fluids (Silicones)

Overview

Silicone oils are typically provided with a single kinematic value at a single temperature.


Limitations

  • Like the rotation viscometers (e.g. Brookfield), these are very inaccurate test fluids and useless for any calibration since the fluid and rotational viscometers have both quite high inaccuracy
  • Viscoelastic nature causes viscosity to vary depending on the frequency of oscillation.
  • Results differ significantly when measured with vibrational instruments like the SRV, SRD, or DVP.


Recommendation

Avoid using silicone oils for calibration due to their inherent inaccuracies and inconsistencies.


3.2 Mineral Oils


Overview

Newtonian fluids, ideal for viscometer calibration.


Key Considerations

  • Often provided with single measurements at specific conditions.
  • For proper calibration or verification, obtain oils with both density and viscosity values measured at various temperatures.
  • Both density and dynamic viscosity of the oil are necessary to perform an accurate calibration check on the SRV.


3.3 Rheonics Recommendation: NIST Traceable Viscosity References


Overview

These viscosity references can be sourced from Cannon Instruments in the USA and worldwide. Other suppliers may also provide similar references. In this article, we primarily refer to calibration fluids sourced from Cannon Instruments.


Key Features

  • NIST Traceable Standards: These viscosity references adhere to national standards, ensuring high accuracy and consistency.
  • Wide Applicability: Suitable for calibration across various viscosity ranges.


Additional Resources


 


Bottles of CANON General Purpose Viscosity Standards used for calibrating viscosity measurement devices

Figure 4. CANON General Purpose Viscosity Standards

 


Certificate of Analysis for CANON certified viscosity reference standard, including kinematic viscosity, dynamic viscosity, density, and Saybolt viscosity dataFigure 5. CANON Certificate of Analysis



4. Rheonics SRV Viscometer: Calibration Overview

  • Measurement Principle

    • The SRV sensor measures the product of dynamic viscosity and density: kinematic viscosity.

  • Default Density Setting

    • The SRV sensor has a default density value of 1.000.
    • For typical viscosity ranges: Use any two of the following:
      • S20, S60, S200, S600
    • For higher viscosity ranges:
      • N4000

  • Handling High Viscosity Fluids

    • Pour fluids like S200, S600, N4000 carefully into the measurement container to avoid air entrapment.
    • To remove bubbles, heat the fluid to 80°C, then cool it to the target temperature before calibration.

  • Required Accessories

    • Sensor Holders & Calibration Cylinders: Rheonics provides tools specifically designed for secure and precise calibration setups. Review our Accessories Product Page more information.
    • Temperature Control System (STCM): For performing multi-point temperature ramps, ideal for both calibration and testing. Learn more at STCM page.


5. Calibration/Verification Procedure for SRV Sensor


5.1 Prepare the Calibration Setup

  • Get the appropriate viscosity reference fluid.
  • Use a beaker to hold the fluid and secure the sensor in a clamp or Rheonics sensor holder. Avoid holding the sensor by hand.
  • Ensure the sensor is not touching the side or bottom of the beaker. If using a Rheonics calibration cylinder, insert it fully with an O-ring at the bottom.


5.2 Fill the Calibration Fluid

  • Pour the fluid into the beaker, ensuring minimal bubble formation. Getting rid of bubbles is critical to achieve a good calibration.


5.3 Stabilize Temperature

  • Allow the sensor and fluid to stabilize until the temperature variation is less than 0.1°C (sensor output). This may take between 30–60 minutes unless an oven or STCM is being used.


5.4 Verify the Density Setting

  • Check that the SRV density is set to 1.0 (default setting).


5.5 Record and Compare Data

  • Record five viscosity readings from the SRV sensor. Ensure that the variance between these readings is less than 1%.
  • Compare the SRV readings to the reference fluid value. Usually, for precise viscosity values at specific temperatures, a software such as Viscdisk from Cannon Instruments needs to be used. For other suppliers, contact them directly.


5.6 Calculate Calibration Accuracy

  • Use the formula to calculate accuracy:
Calibration Accuracy (%) = Measured Viscosity (SRV) / (Dynamic Viscosity * Density)


5.7 Adjust for Non-Default Density

  • If the density is set to a different value, input the correct density from the calibration reference data into the SRV.
  • The SRV will now output dynamic viscosity, which can then be compared against the reference fluid's dynamic viscosity.


5.8 Recheck Calibration Accuracy

  • Compare the measured viscosity by SRV with the product of dynamic viscosity and density from the calibration fluid bottle. If the product is not used, the measured viscosity will appear much lower than expected.



6. Calibration/Verification Procedure for SRD Sensor

The calibration procedure for the SRD sensor follows the same steps as outlined for the SRV, with the following additional points:


  1. The SRD sensor measures both kinematic viscosity and dynamic viscosity, so the measurements must be compared to the corresponding reference values on the fluid bottle.
  2. Once the sensor readings have stabilized:
    • Take five readings of viscosity and density, ensuring variance between readings is less than 1%.
    • Compare the values to the reference fluid data.
  3. Estimate calibration accuracy using these formulas:


Dynamic Viscosity Accuracy (%) = Measured Viscosity (SRD) / (Reference Viscosity × 100)


Density Accuracy (%) = Measured Density (SRD) / (Reference Density × 100)


7. Troubleshooting


7.1 Inaccurate Readings

  • Issue: The sensor's sensing element is not completely submerged, leading to inaccurate readings.
  • Solution: 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.


7.2 Unstable Viscosity Readings

  • Issue: The viscosity values are inconsistent because the fluid and sensor temperatures have not stabilized.
  • Solution: Wait long enough to stabilize temperature or use an oven or STCM.


7.3 SRV Viscosity doesn’t match with Calibration Fluid

  • Issue: The SRV viscosity output is only being compared to the dynamic viscosity of the calibration fluids, ignoring the density value.
  • Solution: Compare SRV viscosity output to the product of dynamic viscosity and density if the calibration fluid density has not been entered in the SRV electronics.



If the default density output of 1.000 g/cc is shown, ensure dynamic viscosity is multiplied by density for accurate comparison. (Refer to the RCP manual for detailed instructions.)


 

8. ISO 17025 Certification Compliance


For factory calibration and calibration verification Rheonics uses NIST traceable viscosity and density reference fluids from Cannon Instruments. Cannon Instrument holds ISO 17025 and 17034 certifications for calibration and production of references. These certificates can be accessed from Cannon Instrument website. 


When a calibration certificate is required, ensure it is ordered prior to shipment. CUS option in the calibration is required for the calibration certificate that contains information on the Reference fluid and Lot number.



9. References

  1. Cannon Instruments NIST Newtonian Viscosity Standards

  2. PTB Newtonian Calibration Liquids for Viscosity

  3. Cannon Instrument ISO 17025 and ISO 17034 Certificates