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

If fluid density is known and different to 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 dynamic and kinematic viscosity can be directly output by the sensor over digital and analog channels without a need for any PC. 

The steps below show how the user can set the density on the sensor using the RCP software and Modbus RTU/TCP to get dynamic or kinematic viscosity directly from the sensor electronics as well as on the RCP software.

Enter fluid density to get dynamic and kinematic viscosity from SRV

1. Introduction to density input for SRV

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


With this model we can describe a constant density or a temperature dependent density.

a. Constant density input

To describe a constant density, the coefficient Do0 should be set equal to the constant density value, and all the other coefficients should be set to zero. 

b. Temperature dependent density input

To describe a temperature dependent density, assign values to the coefficients, that reflects the density behavior of your fluid with temperature. If you have a fluid reference table (for example Cannon fluids, Nippon grease), you can fit the density to a polynomial model to get the coefficients. 

The equation above expects temperature in °C. If you have data in °F or °K, then convert temperature to corresponding °C before estimating the coefficients.

Once you have the coefficients (Do0, Do1, ...), you can use any of the methods described below to upload these into the sensor electronics (SMET). 

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 you have a SRV sensor 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. Input the coefficients and click "Write".  Ensure Temperature (T) was in °C when estimating the polynomial coefficients.

Over Modbus RTU/TCP

Over Modbus (either RTU or TCP) there are a set of registers associated with the density input for the SRV sensors. 

There are two holding registers associated with each of the Do coefficients of the polynomial. 

The  two holding registers of each coefficient should form a float 32 data type in Big Endian format. 

The registers are password protected. This is, you should input the user password in the holding register 0 to be able to write to the coefficients. 

You can use the software of your preference to write to the holding register. The write function is 16 to write to multiple registers, and 03 to read the holding registers. 

RegisterHolding Register AddressData Type
PasswordAddress = 0;      User Password = 0x5312Uint16

An example using ModbusPoll software to get constant density:

a. Write the password into the holding register 0. 

b. Write constant value to coefficients Do0, and set all other coefficients to 0. 

For temperature dependence, Write to the set of coefficients to get the expected behavior, according to the fluid and application.

Using configuration file from Rheonics Support

Rheonics support team can help you configure the Density input to SRV for your application. For that, you will get a configuration file that will be valid ONLY for the specific sensor it was created for. 

a. Open RCP

b. Go to "Settings" tab and click on "Expert Mode"

c. In the expert mode window input the password and click "yes": Password for Expert mode can be found in the RCP manual.

 d. Now a new set of buttons will appear or be enabled in the software: 

e. Click "Import" button to load the file provided by rheonics support team into the sensor. 

f. "Import" button should turn GREEN for a moment if the file was properly loaded. Otherwise it will turn red. If that's the case please, click "Refresh" button and re-attempt loading the file. 

g. The "Export" button will automatically triggered. This will generate a configuration file that should be sent back to rheonics team for verification. 

h. Click "Refresh" to populate and review the new settings in software. 

Steps are described in detail in the support article Updating sensor configuration

3. Dynamic viscosity output from SRV

Using RCP

Since SRV measures a product of Density and Dynamic Viscosity of the fluid, to get the correct Dynamic Viscosity measurement from the SRV, the density should be input following any of the steps described above. 

To display and log the viscosity, in RCP Software, go to settings page, and in "Graph Settings" section select "Viscosity" from the dropdown list. 

This will display the Dynamic Viscosity value in the Measurements tab and also will be saved when logging the records. (Assuming that the actual density has been input as described in the points above). 

Over Modbus

We can access Dynamic Viscosity over Modbus, these are: 

    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 onto the last valid value of the parameter till a new valid value is generated. 

Each parameter is assigned two input registers to build a float 32 data type format. 

You can access the registers according to the table below. 

ParameterInput register addressData Type
Median Dynamic Viscosity - Parameter 1
Float 32
Raw Dynamic Viscosity - Parameter 6
Float 32
Median and Last Good Dynamic Viscosity - Parameter 13
Float 32

For more details about Modbus protocol, and registers, go to: Helpdesk : Rheonics - Modbus Input Registers

4. Kinematic viscosity output from SRV

Using RCP

In RCP Software, go to settings page, and in "Graph Settings" section select "Kin. Visc." from the dropdown list. 

This will display the Kinematic Viscosity value in the Measurements tab and also will be saved when logging using RCP.

Over Modbus

You can access Kinematic Viscosity by reading the input registers associated with parameter 4. 

ParameterInput register addressData Type
Kinematic Viscosity - Parameter 4
Float 32

For more details about Modbus protocol, and registers, go to: Helpdesk : Rheonics - Modbus Input Registers

5. Manually estimating dynamic and kinematic viscosity from SRV outputs

SRV measures the product of Dynamic Viscosity and Density, this is: SRV Viscosity output = Fluid Dynamic Viscosity * Density. 

By default SRV Density is set to 1 g/cc. However, as described in steps above, you can change the SRV density. If your measurements are made with the default density, and you want to obtain the kinematic viscosity for those measurements, follow the instructions below. 

For a reference fluid in which, the Kinematic Viscosity and Dynamic Viscosity at a certain temperature is known from the fluid reference table, (for example Nippon Grease); we can calculate the Density of the fluid at that temperature.

to calculate the Fluid Dynamic Viscosity from the SRV measurement, we can estimate using: 

and, to calculate the Kinematic Viscosity from the SRV measurement, we can make:

For example:

a. At 26.5 °C, SRV measures a Viscosity value of 21026.7 cP and density was set as the default value of 1 g/cc in the sensor.

b. According to the Nippon Grease reference table:

at 26.5 °C the kinematic Viscosity value is 26730 mm2/s and dynamic Viscosity is 23590 cP

c. If we calculate the reference Density at that temperature, it is  23590/26730 = 0.8825 g/cc

d. Then we can calculate the SRV Kinematic Viscosity by using the Kin. Visc. equation above:

e. According to the table, the expected kinematic viscosity at that temperature, is  26730 mm2/s, that leads to a percentage error of 0.992%. Which is within the rheonics sensors accuracy. 

NOTE: if the fluid density at that temperature (0.8825 g/cc) is input, as described in the first section of the document, then SRV will directly output the Dynamic Viscosity value (as well as kinematic viscosity value when selected in either RCP, on the TRD display or over digital channels (Modbus RTU/TCP, Serial over USB, Serial over Ethernet, EthernetIP, etc). 

References/Further information

SRV Whitepaper  

SRV Product Information 

RCP - Manual  RCP manual - Rheonics Control Panel, software to acquire data from rheonics sensors as well as to configure the sensor

Rheonics SensorView app: download for     Android         iOS