This article explains the importance of maintaining thermal balance of the SRD density and viscosity meter. The accuracy of its density measurements is strongly influenced by the temperature distribution in its resonator. Here we explain the significance of thermal imbalance and present solutions that improve the thermal balance of the sensor, and therefore the accuracy of the SRD’s density readings.


Introduction

The Rheonics SRD sensor is a density and viscosity inline meter. Its sensing element is a balanced torsional resonator (BTR technology) whose resonant frequency and damping change when immersed in a fluid.  These changes are interpreted by the associated electronics to produce values for density and viscosity of the fluid. 


The SRD’s resonator is the same across all SRD variants (short, flush, long, slimline, and reactor probes). These variants along with the customizable process connection allow full flexibility in their installation and application.

Rheonics density viscosity meter variants design

Figure 1: SRD Long (-X5), Short (-X1,-X2 and -X3) and Flush (-X4) variants.

Visit specific articles on each SRD variant:


Density and Resonator's Thermal Balance

The main installation requirement for the SRD is that its sensing element is immersed in the fluid without dead zones that can lead to deposits in the sensing area.

Rheonics SRD sensing area requirementFigure 2: SRD Installation Criteria


Although total immersion of the sensitive element is necessary for all installations, additional attention must be given to maintaining a uniform temperature of the probe.

The highest accuracy of the SRD will be achieved if the entire portion of the probe containing the resonator (about 100 mm) is immersed in the fluid. In the case of short versions of the SRD, in which only the shorter portion of the sensor is immersed, there will be a a systematic offset in the measured density if the process fluid temperature differs by more than 15 °C from the ambient temperature.


Effect of thermal imbalance on the SRD’s accuracy

The SRD’s sensing element is a resonator that has a resonance frequency that depends both on the fluid’s density and the temperature distribution in the resonator.  As shown in Fig. 3 below, only one part of the resonator is in contact with the fluid. The rest of the resonator extends into the body of the probe. The resonator’s frequency depends on the temperature of both the immersed part and the part that is encased in the SRD’s body. If the body is at a different temperature from that of the fluid, the density calculated from the resonator’s frequency will have an offset whose value depends on this temperature difference. 

Rheonics short SRD probe exposed to temperature differencesFigure 3: SRD short probe exposed to different temperatures.


The short probe shown in Figure 3 shows how the process fluid temperature and ambient temperature difference produces a temperature gradient in the resonator. This affects the SRD’s accuracy because:

  • The resonance frequency of the sensor depends on the temperature distribution in its resonator
  • Density is calculated based on the SRD’s resonance frequency
  • Since the resonator is not at a constant temperature, its resonance frequency is determined by both the fluid’s density and temperature, and the temperature gradient in the resonator.

Since the temperature gradient depends on many factors that are not under the user’s control, Rheonics offers solutions that lead to improved accuracy by reducing thermal imbalance in the SRD’s resonator. 


Deeper immersion of the SRD improves thermal balance in its resonator

Rheonics long density meter SRD in high temperaturesFigure 4: Long insertion SRD with resonator exposed to the same temperature.


Rheonics offers several versions of the SRD that greatly improve its density accuracy by reducing temperature imbalance of its resonator. The long insertion, slimline, and reactor probe variants ensure that a longer portion of the sensor is immersed in the fluid. 

  • The part of the probe that contains the back end of the resonator is submerged in the fluid
  • This keeps the entire resonator at the temperature of the fluid
  • The resonance frequency of the resonator is now only dependent on density, so accuracy is maintained

Selecting an SRD for particular applications

For inline installations:

  • Best solution is the use of the Long insertion, Slimline, or Reactor probe variants that can protrude further in the fluid to avoid affecting the accuracy due to thermal differences between fluid and ambient temperatures.

For laboratory measurements:

  • Make sure that the entire probe is at the fluid’s temperature. This can best be achieved by placing the entire probe, along with the fluid, in a temperature-controlled oven or climate chamber.
  • Avoid benchtop measurements, unless it can be ensured that the fluid being measured is at the same temperature as the ambient (room) temperature
  • It is impossible to verify the accuracy of the probe’s density measurement unless one of these criteria is met.