1. What is the temperature measurement on Rheonics Sensor?

A temperature sensor is an instrument used to measure process temperature. It works by detecting changes in the temperature of its environment and transmitting this information back to the sensor electronics. Temperature sensors are used in many industries and applications, ranging from food processing and manufacturing to aerospace and power generation.

Rheonics viscosity and density meters SRV, SRD, DVP, and DVM provide accurate and reliable readings of various materials, including liquids, slurries, and solids. To ensure the accuracy of a Rheonics meter, calibration is necessary to ensure that the viscosity, density, and temperature readings are as precise as possible.

2. What is temperature calibration?

Calibrating a temperature sensor is a way of ensuring that temperature readings from the sensor are accurate and trustworthy. Temperature sensors are typically calibrated in a water bath at a known temperature. After that, you adjust the calibration values on the sensor until it accurately matches the temperature reading from the water bath or dry wells.

3. Calibration reference standard

A reference standard is a physical object that represents the quantity of interest in such a way that it can be used as a unit of measurement. In this support article, the measurement parameter is temperature. Examples of these instruments that could be used as a reference standard could be a water bath, dry wells, temperature simulators, or another calibrated instrument. 

4. Calibration Methods

4.1. Calibrating with a water bath

The temperature calibration bath is a calibrator that is enclosed by a uniform fluid and is capable of adjusting the temperature for the test points. Temperature sensors of different sizes and shapes can be calibrated with their large volume and flexibility.

This method involves calibrating the Rheonics sensor against the temperature set in the water bath or dry well. It is imperative to wait for the sensor to stabilize, this is when the sensor's temperature output starts to vary by less than 0.1°C, this can take between 30 and 60 minutes unless you use an oven or STCM (Stabilization time is often quite long for high viscosity fluids or low thermal conductivity fluids).

Figure 1. Calibration water bath[1]

Tests are performed against a reference gauge to calibrate a probe under test. The following steps are the most commonly used in calibration.

  • The Rheonics sensor must be handled carefully

    Rheonics sensors are robust and are unlikely to fail under normal circumstances, but always start with a visual inspection of the probe. However, it should still be handled with caution to ensure accuracy and reliability as they are high-precision devices.

  • Prepare the water bath

  1. Ensure that instrument is clean and suitable for use.

  2. Ensure that the bath is powered off and that the water bath is up to the allowed mark.

  3. Power on the system.

  4. Set the desired temperature of the water bath. In general, it is better to start at the highest temperature and proceed down to the lowest temperature when calibrating over a wide temperature range.

  5. After heating the water, be careful.

  6. Power off the instrument when finished

  • Prepare the reference temperature

  1. The technician can use the internal thermometer for calibration, for this follow the recommendation of section 5. It is best if the reference thermometer is an externally calibrated probe.

  • Measuring the temperature sensor output signal

Open the RCP software and check the temperature that is shown in the measurement tab. For checking temperature outputs over 4-20mA channels, go to the service tab and verify that the output from channel 3-Temperature matches the expected value based on the temperature and the scaling limits.

Figure 2. RCP software-verification of the analog output.
  • Insert the Rheonics sensor in the water bath

Insert the Rheonics sensor inside the water bath, the immersion depth is going to affect the accuracy of the readings. Technicians often recommend following these rules[5]:

1. 1% accuracy - immerse 5 diameters + length of the sensing element

2. 0.01% accuracy - immerse 10 diameters + length of the sensing element

3. 0.0001% accuracy - immerse 15 diameters + length of the sensing element

  • Wait for Stabilization

Wait for the sensor to stabilize, this is when the sensor's temperature output starts to vary by less than 0.1°C, this can take between 30 and 60 minutes.

  • Calibration points

In order to verify that the sensor is linear, you need to pick enough calibration points. It is usually sufficient to calibrate three to five points throughout the range. Remember to always calibrate in the temperature range.

4.2. External calibration probe

To calibrate using an external calibration probe instead of the internal probe from the water bath, it is important to verify that the reference probe has a traceable certificate(NIST traceable certificate or equivalent). The steps for calibration are the same as above but at the moment of inserting the Rheonics sensor in the water bath follow these recommendations:

  1. The reference probe should be placed in the working area of the bath.

  2. Place the probe to be calibrated as close to the reference probe as possible, without touching the bath tank surfaces.

Figure 3. 5615 Secondary Reference Temperature Standard Probe[7]


Allow sufficient time for the probes to settle and for the bath temperature to stabilize after inserting the probes to be calibrated.

It is important to insert the probes at the same depth in the bath liquid. It is recommended for immersion depth to reduce the stem effect to a minimum: 20 x the diameter of the unit under test + the sensor length.

It is not recommended to submerge probe handles or Rheonics sensor cable (M12 connector) in water.

Reference probes and calibrators need to be themselves calibrated and should have a calibration certificate.

5. What can cause the difference between the calibration bath, reference probe, and a Rheonics sensor?

  • Ensure that the temperature reading is uniform between the external probe and the SR sensor. Reaching this uniformity can take 30 minutes to 1 hour(even more with high-viscosity fluid).

  • In order to use an external reference sensor, you need to insert it into a suitable hole. By inserting the external reference sensor, it can measure the same temperature as the calibrated sensors more precisely.

  • It would be ideal if the reference sensor shared the same thermal characteristics as the calibration sensors (the same size and thermal conductance). The external reference sensor and the sensor to be calibrated will more accurately follow temperature changes as the insert temperature changes.

  • During the calibration, you should ensure that your reference sensor is inserted to the same depth as the sensor(s) to be calibrated. If you know the lengths and the locations of the sensing elements, try to align the centers horizontally. In Figure 4, The first picture shows the best calibration results when both sensors reach the bottom of the insert.

    As you can see in the second picture, the reference sensor and the sensor to be calibrated are at different depths. Temperature differences between the two sensors will result in calibration errors.

    Similarly, the third picture shows an example where the sensor to be calibrated is short and the reference sensor is positioned correctly at the same depth as the sensor to be calibrated. The insert can be calibrated using this method, although it lacks homogeneity at the upper part of the insert.

Figure 4. Sensor and external reference at different depths. [2]

  • Procure axial temperature homogeneity, The difference in temperature along the vertical length of the boring in the insert is known as axial homogeneity (or axial uniformity). There may be a slight temperature difference at the bottom of the boring compared to the temperature at the top of the boring.

    Usually, if the block's temperature is very different from the ambient temperature, then the temperature will be different at the very top of the insert.

Figure 5. Axial temperature homogeneity[2]

  • Radial uniformity, "Radial uniformity" refers to the temperature difference between each boring (hole) in the insert. There can still be a small difference between two borings, especially if the opposite boring is made of metal compounds and has good thermal conductivity.

Figure 6. The temperature difference between the borings. [2]

  • Influence of loading,  When the block's temperature differs from the ambient temperature, some heat is conducted through the sensors to the environment (stem conductance).

    In the case of multiple sensors installed in the insert, more temperature will be "leaked" to the environment. Temperature leakage will also increase with thicker sensors.

    The greater the temperature difference between the insert and the environment, the greater the leakage.

Figure 7. Influence of the load.[2]
  •  The temperature sensor only measures its own temperature. Consequently, it won't measure the temperature where it's installed, but it will monitor its own. As the temperature changes slowly from the surrounding to the temperature sensor itself, it takes some time for all parts of the system to stabilize to the same temperature, i.e. reach equilibrium.


[1] Fluke Calibration 6102/7102/7103 Micro-Bath 

[2]Uncertainty components of a temperature calibration using a dry block 

[3]3 methods for calibrating temperature sensors: pros & cons 

[4]How to Calibrate Temperature Sensors & Electronics -3 Methods 

[5]How to calibrate temperature sensors 

[6]Temperature Measurement Standards - Standards Products - Standards & Publications - Products & Services 

[7]5615 Secondary Reference Temperature Standards 


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