This article explains how the ASTM-D341 standard can be used in the inline density and viscosity meter SRD, and process viscometer SRV to calculate the kinematic viscosity of petroleum oil or liquid hydrocarbon in certain conditions and ranges.

## 1. Introduction

The SRV and SRD sensors are Rheonics inline process viscometer and density meter respectively. These sensors output the real-time dynamic viscosity of a fluid based on a torsional resonator and temperature by an inbuilt PT1000.

The SRV supports manual input of a constant, temperature-dependent or external sensor density value to calculate the kinematic viscosity of the fluid, dividing the dynamic viscosity by the density. Review more information here. However, the RCP software also allows users to get kinematic viscosity through the Calculation Tab using any formula.

 Rheonics SRD is an inline sensor to measure density and viscosity of a fluid. Due to the inline density readings, the fluid’s kinematic viscosity is automatically calculated.

All Rheonics sensors have a powerful electronic unit (SME) that allows different equations to be loaded into the unit to calculate specific parameters. The equations will need inputs such as real-time viscosity and temperature, which can be measured by the SRV, or constant values as coefficients that should be calculated by the user and input in the Rheonics Control Panel (RCP) software.

## 2. ASTM-D341 model

The ASTM-D341 standard provides Kinematic Viscosity vs Temperature charts, which can help determine the Kinematic Viscosity of a petroleum product or liquid hydrocarbon at any temperature within a limited range, by providing just two data points which are the kinematic viscosities of the fluid at two known temperatures.

The standard defines the equation to relate kinematic viscosity and temperature:

Equation 1: ASTM-D341 standard equation

Where ν = Kinematic Viscosity in cSt and = is the temperature in Kelvin

And constants A and B are:

Equation 2: Coefficients A and B

The two data points known for kinematic viscosity and temperature are defined by subscripts 1 and 2 in ν and T in the equations above for A and B.

With parameters A and B calculated, kinematic viscosity can be estimated at any temperature within the range.

 The kinematic viscosity obtained by the ASTM-D341 standard, should be used only in the range in which hydrocarbon or petroleum fluids are homogeneous liquids.

## Example

We can take the specifications of a fluid given by the vendor to obtain the kinematic viscosity at two different temperatures. For example, an hydraulic oil from BELCO.  Kinematic viscosity for this fluid is given at 40°C (31.4 mm2/s or cSt) and 100°C (5.4 mm2/s). Calculating values for A and B, we can plot the estimated kinematic viscosity within that temperature range as shown below.

Figure 1: Kinematic Viscosity plot for hydraulic oil following ASTM-D341 standard

From the plot, it can be noticed that the kinematic viscosity has a strong dependency in the temperature.

## 3. SRV and SRD sensor ASTM-D341 calculation:

Through configuration in the Calculation Tab in the RCP software, the SRD and SRV sensors can calculate the kinematic viscosity using the ASTM-D341 model and apply temperature compensations (i.e. removing temperature dependency) in the results. This allows to monitor any changes in kinematic viscosity that are not created by temperature.

From the previous example, user can configured the sensor to compensate the kinematic viscosity at 60°C (Tref), then the output would look like the red line (compensated viscosity) in the graph below.

Figure 2: Temperature compensated Kinematic Viscosity for hydraulic oil at 60°C

## 4. RCP configuration of ASTM-D341

 The ASTM-D341 standard model is available for RCP version 5.4.0.6 onwards.

With the SRV or SRD connected and recognized by the RCP, enable the Expert Mode from the Settings Tab using the password provided in the RCP manual.

Go to Calculation Tab and follow next steps:

1. Select Viscosity and the Model ASTM-D341 for Kinematic Viscosity

2. Input the known Kinematic Viscosity and Temperature of the fluid at two different points. Specify the reference temperature for compensation.