Enter fluid density to get dynamic and kinematic viscosity from SRV

1. Introduction to density input for SRV

The SRV measures viscous damping, which is directly proportional to any change in the process fluid viscosity and density. The sensor output is a product of dynamic viscosity and density. Fluid dynamic viscosity is obtained by dividing that product by density. By default, density is set to 1.0 g/cc (1000 kg/m³).

If fluid density is known and different from 1.0 g/cc, the sensor supports input of a constant or temperature‑dependent density so dynamic and kinematic viscosity values can be estimated and output on the viscosity channel. With density input, the SME can output dynamic and kinematic viscosity directly over digital and analog channels, with no need for a PC.

The Density SRV output can be described as a polynomial of the form:

With this model, either a constant density or a temperature-dependent density can be described.

a. Constant density input
To describe a constant density, set coefficient D_o0 equal to the constant density value, and set all other coefficients to zero.

b. Temperature-dependent density input
To describe a temperature-dependent density, assign values to the coefficients that reflect the density behavior of the fluid with temperature. If a fluid reference table is available (for example Cannon fluids, Nippon grease), fit the density data to the polynomial model to obtain the coefficients.

Once the coefficients (D_o0, D_o1, …) are known, use any of the methods described below to upload them to the sensor electronics (SME).

2. Setting density for SRV sensor

Using RCP

a. From RCP, navigate to the "Service" tab.

b. Look for the "Density (SRV Only)" section — this is only active when an SRV sensor is connected to RCP.

c. Select between Constant Density or Temperature Dependent Density (Polynomial).

d. For constant density, edit the value and click "Write".

e. For temperature-dependent density, input the coefficients and click "Write". Ensure temperature (T) was in °C when the polynomial coefficients were estimated.

Over Modbus RTU/TCP

Over Modbus (RTU or TCP), a set of holding registers is associated with the density input for SRV sensors. Two holding registers are associated with each D_o coefficient of the polynomial, forming a Float32 value in Big Endian format.

These registers are password protected: write the user password to holding register 0 before writing to the coefficients. Use Modbus function 16 to write multiple registers, and function 03 to read holding registers.

Table 1. SRV density holding registers

Example using ModbusPoll software to set a constant density:

a. Write the password to holding register 0.

b. Write the constant density value to coefficient D_o0, and set all other coefficients to 0.

For temperature dependence, write the full set of coefficients to get the behavior expected for the fluid and application.

Using a configuration file from Rheonics Support

The Rheonics support team can help configure the density input on an SRV for a specific application. This produces a configuration file that is valid only for the specific sensor it was created for.

a. Open RCP.
b. Go to the "Settings" tab and click "Expert Mode".

c. In the Expert Mode window, enter the password and click "Yes". The Expert Mode password can be found in the RCP manual.

d. A new set of buttons will appear or become enabled in the software.

e. Click "Import" to load the file provided by the Rheonics support team into the sensor.
f. The "Import" button turns green briefly if the file loaded correctly, or red if it did not. If it turns red, click "Refresh" and re-attempt loading the file.
g. The "Export" button is triggered automatically, generating a configuration file to send back to the Rheonics team for verification.
h. Click "Refresh" to populate and review the new settings in the software.

These steps are described in detail in the support article Updating sensor configuration.

3. Dynamic viscosity output from SRV

Using RCP

Since the SRV measures a product of density and dynamic viscosity, density must be input using one of the methods above to get a correct dynamic viscosity reading.

To display and log viscosity in RCP, go to the settings page and, in the "Graph Settings" section, select "Viscosity" from the dropdown list. This displays the dynamic viscosity value in the Measurements tab and saves it when logging records (assuming density has been input as described above).

Over Modbus

Dynamic viscosity can be read over Modbus at the following parameters:

a. Median Dynamic Viscosity (5 points by default; can be modified with filters) — Parameter 1
b. Raw Dynamic Viscosity — Parameter 6
c. Median and Last Good Dynamic Viscosity — Parameter 13

"Last Good" holds the last valid value of the parameter until a new valid value is generated.

Each parameter uses two input registers to build a Float32 value.

Table 2. SRV dynamic viscosity input registers

For more detail on the Modbus protocol and register maps, see Modbus Input Registers (RTU) and Modbus TCP – Input Registers.

4. Kinematic viscosity output from SRV

Using RCP

In RCP, go to the settings page and, in the "Graph Settings" section, select "Kin. Visc." from the dropdown list. This displays the kinematic viscosity value in the Measurements tab and saves it when logging with RCP.

Over Modbus

Kinematic viscosity is available by reading the input registers associated with Parameter 4.

Table 3. SRV kinematic viscosity input register

For more detail on the Modbus protocol and register maps, see Modbus Input Registers (RTU) and Modbus TCP – Input Registers.

5. Manually estimating dynamic and kinematic viscosity from SRV outputs

The SRV measures the product of dynamic viscosity and density:

SRV Viscosity Output = Fluid Dynamic Viscosity × Density

By default, SRV density is set to 1 g/cc. If measurements were made at the default density and kinematic viscosity is needed for those readings, follow the steps below.

For a reference fluid where kinematic viscosity and dynamic viscosity at a given temperature are known from a fluid reference table (for example Nippon Grease), fluid density at that temperature can be calculated:

To calculate fluid dynamic viscosity from the SRV measurement:

And to calculate kinematic viscosity from the SRV measurement:

Example

a. At 26.5 °C, SRV measures a viscosity value of 21026.7 cP with density set to the default value of 1 g/cc.
b. According to the Nippon Grease reference table:

At 26.5 °C, the reference kinematic viscosity is 26730 mm²/s and dynamic viscosity is 23590 cP.

c. The reference density at that temperature is 23590 / 26730 = 0.8825 g/cc.
d. Using the kinematic viscosity equation above:

e. The reference table gives an expected kinematic viscosity of 26730 mm²/s at that temperature, a difference of 0.992%, within Rheonics sensor accuracy.

References/Further information

SRV Whitepaper
SRV Product Information
RCP Manual — Rheonics Control Panel software, used to acquire data from Rheonics sensors and to configure the sensor.
Modbus Input Registers (RTU)
Modbus TCP – Input Registers
Updating Sensor Configuration
Rheonics SensorView app: Android  |  iOS