Topics covered in this article:

What is calibration?

Calibration is the comparison and required adjustment of a certain result with a “pattern” or certified value known to be correct. The adjusting or tuning can be done mechanically, through electronics or software, while the “pattern” value can be extracted from a certification sheet, a previously calibrated sensor exposed to the same input, or a controlled equipment.


When using a sensor, we need to know that the values obtained are accurate. This is possible by relying on calibration.


Why is calibration of measuring instruments important?

Ensure high accuracy - Industry automation, tracking, and controlling systems rely on sensors. Then, the sensors’ accuracy needs to be ensured, and this is only possible with calibration. Calibrated sensors allow users to maintain confidence in the measurements, keep correct traceability, achieve scalability, and make decisions based on the sensor’s readings. 


Certification compliance - For many industries calibration is needed to ensure compliance with regulations or standards governed by law, such as ISO 17025 or FDA, for food and farmaceutical industries. These regulations look for a standardized verification of the sensor’s operation to ensure safety, quality, and correct production.

Rheonics inline viscometer SRV

The Rheonics SRV is an inline process viscometer that measures simultaneously the real-time viscosity and temperature of a fluid, in a wide range of applications. Review all information about the SRV in the product webpage and Rheonics Support Articles.

The Rheonics SRV outputs multiple parameters, including calculated values such as kinematic viscosity, concentration, etc. based on dynamic viscosity, temperature, and an input of density. See all parameters here.

Density values can be constant, temperature-dependent, or coming from an external sensor. See more in this article: Set density input to SRV inline viscometer for dynamic viscosity and kinematic viscosity outputs.


For the inline process viscometer SRV, both dynamic viscosity and temperature can be calibrated.


How is the viscometer SRV calibrated in factory?


All Rheonics sensors are factory calibrated using NIST traceable fluids. The calibration procedure is the following:

  1. The sensor probe is placed in a thermal chamber, property of Rheonics, correctly immersed in a NIST traceable reference Newtonian fluid.
  2. Temperature is closely monitored and controlled over a given operating range to review the viscosity and temperature readings of the sensor.
  3. Sensor readings are compared against the NIST traceable fluid’s certified values of dynamic viscosity and temperature.
  4. The sensor is calibrated and fine-tuned by changing coefficients in its configuration until the readings obtained are within the accepted accuracy.
  5. The process is repeated for different NIST traceable fluids of different viscosity values - low or high viscosity ranges.


The calibration process is done with a proprietary software of Rheonics that calculates the right coefficients, monitors, and ensures tight temperature control for an even distribution along the probe’s body.

The viscosity range, temperature range, and accepted accuracy are defined during sensor configuration by the user, these can be standard (STD) or custom (CUS) values.  


The temperature accuracy is defined by the type of PT1000 used. By default the SRV sensor uses a PT1000 Class B. Its accuracy according to IEC 751 / EN 60751 is defined by ±(0,30 °C + 0,005|t|) where “t” is the measured temperature.


PT1000 class A and AA for the SRV sensor are also available on request.


What calibration certificates are available for SRV from factory?


While completing the RFQ form - Request for Quotation - of the sensor, the user defines the kind of calibration required, which can be any of the following:


STD Calibration -  Standard Calibration: 

The standard calibrations means that:

  • A standard calibration certification is included in its delivery, which doesn’t specify the NIST traceable fluid used in factory.
  • Rheonics will use standard traceable fluids for calibration at default setpoints.
  • Calibrated accuracy is standard, 5% of actual readings.
  • The viscosity range is defined by the user between standard (V1), extended (V3, V4) or custom (V4) ranges.

Figure 1: Standard calibration certificate example


CUS Calibration -  Custom Calibration: 

The custom calibrations means that:

  • A sensor calibration certification is included in the delivery, which details the reference fluid and lot number used.
  • The user defines the viscosity setpoints at which the sensor should be calibrated. If these are not defined then default values are used.
  • The user defines the accepted accuracy of viscosity.

The CUS calibration is recommended when the user needs a calibration certificate that specifies the reference fluid and lot number used. This is useful for later recalibration on site to comply with regulatory norms that may apply or internal QC (Quality Control) procedures from the client.

Rheonics use Cannon Instruments NIST traceable reference fluids for sensor calibrations. The calibration certificate, included with the sensor, specifies the reference fluid and lot number. Cannon Instrument, which is manufacturer of density and viscosity references,holds ISO 17025 and ISO 17034 (manufacturer of references) certifications.

The certificates from Cannon Instrument are available on their website:CANNON Quality Certifications. 

Figure 2: Custom calibration certificate example


Factory verification and re-calibration

Rheonics sensors can be returned to factory for a calibration verification or re-calibration. The need of a factory verification should be discussed with Rheonics Support Team through support@rheonics.com. The user may be asked to perform a local calibration verification following the steps explained in the next section.

If the sensor is returned to factory for calibration verification, the following is needed:

  • Order the calibration verification or re-calibration by filling out the FCV (Form for Calibration Verification). Request form to Rheonics Support Team at support@rheonics.com 
  • Ship back the Sensor Probe
  • Ship back the SME Electronics

With the above information, Rheonics will send an offer and timeline for the request.

SRV calibration in field - options and equipment

Rheonics recommends clients to use the following verification and calibration procedures with the SRV inline viscometer. 


Quick verification of SRV calibration in air

  1. Remove the SRV from the process
  2. Clean the SRV if the probe is dirty
  3. Leave the SRV in the air, and verify that nothing is in contact with the sensing element
  4. Verify the viscosity reading is zero. This can be checked through the SME-TRD display, RCP software, or any external integration system (e.g. PLC).

This “air check” is useful as a quick verification of the SRV calibration. The sensor does not use moving parts, so if readings in the air are zero, there is no way that the factory calibration has been affected, or any long-term drift has occurred.


Calibration verification to NIST traceable viscosity reference in field 

This method follows the Rheonics factory calibration procedure. The process is to measure with the SRV a Newtonian fluid with a known certified viscosity at given temperatures. Then compare the SRV readings to the expected values and verify the accuracy is within the expected range.


1. Prepare setup for calibration. User should:

  1. Clamp the sensor probe firmly on its body to avoid oscillations. It is not recommended to do a calibration by holding the probe by hand or fixing the sensor from its cable.
  2. Have enough space and fluid to surround the sensor’s sensing element (red zone in next figure). Have the fluid higher than the minimum level required.
  3. Control the temperature. To register the viscosity readings, wait for the sensor to output stable temperature values.
  4. Mixing allows uniform temperature of the fluid. However, mixing can also cause swirls in the fluid, make sure this doesn’t affect the immersion of the sensing element in the fluid.

2. Use a calibration reference fluid with known viscosity and temperature. Rheonics recommends using the same or similar NIST traceable fluid used for factory calibration and specified in the Custom Calibration Certificate.

3. Check the readings from the SRV, and wait for stable readings of temperature and viscosity.

4. Compare the SRV viscosity readings with the calibration reference fluid values. Calculate the error %.

Figure 3: SRV calibration verification setup example

Scaling SRV Readings

If a systematic error is noticed during the calibration verification of the SRV with the steps explained before, the user can scale the SRV readings to correct the error. 

The scaling function is available through the RCP Software. Review more information here: Scaling SME Parameters  


Rheonics calibrator

Rheonics offers an in-field Calibrator, the Rheonics Calibrator, for calibration checks and recalibration of the SRV. This compact Calibrator device has a footprint smaller than a typical desktop coffee machine. It uses an internal heating and cooling system to control the temperature of the fluid under test and maintain a thermal equilibrium along the Rheonics SRV probe.

The Rheonics Calibrator Software is used along with the Rheonics Calibrator to calibrate the SRV at given configurable temperature points.

Contact Rheonics Support Team at support@rheonics.com for more information.

Figure 4: Rheonics In-field Calibrator Setup

Related Articles

[1]: Introduction to Correlation Scale-up

[2]: Scaling: How to create correlation between lab and inline measurements?

[3]: Scaling SME Parameters