What products are involved?
SRD - Inline density and viscosity meter
What is the purpose of this article?
To give an overview of the SRD with Flange/Custom adapter connection (SRD X2), installation guidelines, instructions and special considerations. Flange norm, size, pressure rating, etc. should be defined by the customer with the order.

1. Introduction

SRD is Rheonics sensor for inline density and viscosity measurement. Read more on SRD here.

This sensor configuration (SRD-X2) allows the client to determine the type of flange/adapter needed in case none of the other standard connections are suitable. There are very few limitations to what kind of process connection can be used, but it must follow the next considerations:

  • Flange material must be 316L or 304.
  • Flange/adapter thickness must be less than 1”.
  • Flange/adapter diamenter should at least have a 33mm size.
  • The flange connection on the line (customer interface) must have an inner diameter bigger than 26mm for the SRD sensor to fit.

When ordering this option, the customer needs to provide the norm, size and pressure rating of the connection. Either state this via text or attach a drawing of the flange. Because this is a custom build, the lead times are typically around 6-8 weeks.

Consider that even though large flange sizes are possible to mount, these rarely make sense from a process integration perspective. Large flanges tend to have large standpipes on the process side. This means that there are potential risks of a stagnation zones and wall effects, that highly affect the SRD readings.. With the SRD flange, the extension into the fluid is limited to about around 80 mm. In case longer extensions are needed, you will need an SRV FPC - Long insertion sensor (SRV-X5). 

Figure 1: SRV custome flange dimensions.


Rheonics SRD

Order code
Connection type
Customizable flange/adapter
Drawing and CAD file


2. General installation guidelines

SR-sensors can be placed at any point in a process and at any orientation but is helpful to consider some terms that are usually involved in the sensor measurements.

2.1 Submersion

The SRD sensor has a sensing element shown inside the red section in Figure 2. This section of the sensor should always be fully submerged in the fluid since the SRD measures what is in contact with its sensing element. Incomplete submersion can be a problem when the flow rate is low and the pipe isn’t full. A possible solution for low flow rates is placing the sensor horizontally and parallel to the fluid (e.g. in an elbow) instead of a vertical and perpendicular installation.

Figure 2: SRD sensing area

2.2 Stagnation/dead zone

Also is not good practice to have dead or stagnations zones around the sensing area (Figure 2). A stagnation or dead zone is where a fluid transfer is not good and older fluid may not be fully displaced by newer fluid. This leads to incorrect measurements due to part or all of the measurement being influenced by stationary fluid that is no longer representative of the actual process fluid. This is a clear consequence of long standpipes, once again, if the standpipe can’t be short enough, the client should use an FPC version of the sensor so the insertion depth can be modified.

A clearance of at least 12mm is also recommended between the sensor tip and a pipe wall or any other obstruction.

2.3 Recommended orientation

SRD has a sensing tip that is recommended to be aligned with the fluid flow direction to avoid creating recirculation zones around it. Read more on this topic, here.

Figure 3: SRD sensing tip.

2.4 Flow

For Newtonian fluids flow rate does not affect the viscosity, so SRD should measure the same values in static and moving states of a fluid. For non-Newtonian fluids, the flow rate does matter and viscosity readings may differ between static and moving conditions. The recommendation for processes with non-constant flow rates is to install the SRD in a section in the pipeline with the most consistent flow rate to have a steady viscosity value. Density measurements in the SRD are unaffected by the fluid’s flow rate and state.

Flow rate is also relevant to ensure the full submersion of the SRD sensing element into the fluid. For a process with a low flow rate, pipes may not be full at all points, so the SRD should then be placed in a section where the pipe is usually full of fluid (i.e. after a pump).

In all cases, the SRD’s sensing element should be completely submerged in the fluid. It’s best to avoid installations with long standpipes, since that may lead to a bad fluid transfer resulting in measurements that do not reflect the true state of fluid or worse high noise measurements.

When installing in a standpipe, choose the appropriate insertion length of the sensor by selecting long insertion probes. This allows the sensing element to clear the stagnation zone and be in the fluid that is of interest for measurement, as in the next figure.

Figure 5: Long insertion probes for long standpipes.

2.5 Fluid types

SRD has good performance with the following fluid types and scenarios:

  • Static and moving Newtonian fluids
  • Moving non-Newtonian fluids: Viscosity can vary at different flow rates, SRD needs to be fully submerged and without stagnation zones.
  • Slurries: Typical use case for the SRD. Density in slurries vary with time, as slurry ages, and as it is mixed.
  • Fluids with solids of micrometers scale: Solids of the size of rice corn shouldn’t affect measurements.
  • Fluids with greater solids: Solids with a bigger size will most likely create spikes in the SRD readings when these hit the sensor. Depending on the percentage of solids in the fluid and after some data analysis, Rheonics may be able to apply some filters to handle these fluids better. However, it is hepful to avoid this kind of solids or install the sensor axially.

3. Process connection installation instructions

Since this sensor configuration (SRD-X2) has a custom flange, installation instructions can vary but the next steps are for a standard flange (eg. ANSI 150lb 1” RF).

3.1 Connection steps

A standard flange installation will require two flanges, a gasket in between, a bolt, two washers and insulating sleeves (for the bolt) for each hole on the flange.

Figure 6: Standard flange connection components.

i. Place the isolating sleeves, washers and bolts in each hole of one of the flanges (e.g. the process flange).

ii. Place the gasket, aligning it with the flange. Gasket type changes with flange norm and design.

iii. Insert the sensor with flange connection.

iv. Place the isolating sleeves, washers and nuts in the missing flange.

v. Ensure the gasket is still well placed and start bolting down the flanges. Consider tightening the nuts in a cross sequence.

 3.2 Specific Process Connection installation instructions

  • In line: For inline installations consider the sensing area (Figure 2) and a minimum clearance of the sensor tip to the pipe inner wall (12 mm for the SRD). For most 1”, 1.5” or 2” pipes (for 2” it depends on the pipe norm) a perpendicular installation with the SRD correctly submerged is not possible. In those cases, the installation is recommended to be parallel as in figure 7.a.For lines bigger than 2” (figure 7.b), the sensor can be mounted perpendicular to the flow as long as the standpipe allows the sensing element to be submerged in flowing fluid.If the standpipe can’t be reduced (figure 7.c), the SRD Long Insertion Probe (FPC) is recommended with a customizable insertion depth. The SRD Hygienic FPC is an alternative that creates a smoother variation in cross-section than the standard probe.
  • In tank: Place the sensor at the bottom, wall or lid of the tank as long as the sensing element can be fully submerged in the fluid and dead zones are avoided. Usually, it is useful if the sensor is placed at a tank’s height that has always fluid.
  • Others: Larger pipelines or tanks often need a longer probe (i.e. jacketed tanks or pipes), in those scenarios, the client should also use a long insertion sensor.

Figure 7: SRD Flange installation.

3.3 When should you use a Long Insertion Sensor (FPC) instead of a short flange sensor?

  • Installation with long standpipes.
  • Installation at a tank’s roof and short sensor can’t be fully submerged.
  • Jacketed walls in tanks or reactors.
  • Elbow and cross-pipe flange installations