What Products are Involved?
SRD Inline density and viscosity meter
What is the purpose of this article?
To give an overview of the flush version of the SRD sensor (SRD-X4) uses and installation options for hygienic and sanitary applications.

1. Overview

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

The flush version of the SRD inline density and viscosity meter as shown in Figure 1 and 2, is used to mount the viscometer flush with the inside surface of the process line or tank. The main advantage of this flush sensor is its hygienic design, which means that it is easier to clean and disinfect with standard procedures (e.g. Clean in Place - CIP). This design is also useful for small pipes and process connections smaller than those available with the standard SRD. 

Following the specified hygienic considerations, a flush installation can considerably reduce or eliminate the possibility of plugging, deposits, and dead zones that can cause systematic errors in the density and viscosity readings of the SRD.

Not all process connections are suitable for the SRD flush sensor. Some available adapters are:

  • DIN 11853-1/2/3 min DN20

  • DIN11864-1/2/3 min DN20

  • DIN 11851 min DN20

  • ISO 2852, DIN 32676, BS 4825 Part 3 in combination with Tri-Clamp seals

  • ISO 2853, BS 48525 Part 4 in combination with T-seals

  • Varinline® type T and P with EPDM O-Ring in size B,F,N,G

All process connection mentioned above can be ordered with an EHEDG certification.

Rheonics inline density meter and viscometer hygienic EHEDG

Figure 1: SRD-X4-15T Flush Density Meter and Viscometer

Figure 2: SRD-X4 Flush probe dimensions

SensorSRD inline density and viscosity meter
Order code


Connection typeCustomizable
Sizes availableCustomizable
Drawing and CAD file (e.g. Tri-Clamp 1.5”)
Drawing and CAD file (e.g. Tri-Clamp D34”)
Other sizes

Contact support team

2. Rheonics EHEDG certification and installation manual

SRD EHEDG documentation

3. Installation considerations

Key recommendations and concepts about the installation of the SRD sensor are detailed in other articles and these apply to the flush sensors. It is important to verify that the sensing area, shown in Figure 3, is always immersed in fluid and any stagnant zones should be avoided. These are defined as any space, inside the sensing area, where fluid transport is insufficient leading to immobile fluid, as well as possible accumulation of solids and deposits, or unwanted material like air.

The installation main considerations are labeled in the next drawing. Flush sensors have the sensing element right after the connection, so dead zones must be avoided at all cost (i.e. long standpipes are clearly ruled out).

Figure 3: SRD-Flush installation principles

4. Installation examples

Some suitable adapters for the SRD flush sensor are mentioned in section 1. Installation will depend on the type of adapter used. Rheonics offers a collection of accessories for installation with flush sensors. These accessories and some other well-known installation cases are shown below.

4.1 Rheonics adapters

a. HAW Accessories

The HAW line of Rheonics weldolets enable the installation of the SRD-X1-12G in tanks and pipes (also suitable for the SRV). This sensor is not configured as flush (X4) but with the HAW accessories a flush installation can be ensured.

These weldolets have two versions, for thin walls (around 1.5mm) and thick walls (max. 10mm), and each one has two further variants for Ø16.1x1.6 mm O-ring seal and metal-metal seal. 

These accessories are suitable for flush hygienic installations.

  • Fig a. HAW-12G-OTN: Weldolet with a Ø16.1x1.6 mm O-ring seal for thin walls

  • Fig b. HAW-12G-MTN: Weldolet with metal-metal seal for thin walls

  • Fig c. HAW-12G-OTK: Weldolet with a Ø16.1x1.6 mm O-ring seal for thick walls

  • Fig d. HAW-12G-MTK: Weldolet with metal-metal seal for thick walls

HAW Accessories
Fig a. HAW-12G-OTNFig b. HAW-12G-MTN
Fig c. HAW-12G-OTK
Fig d. HAW-12G-MTK

SRD - G1/2” Flush Hygienic Installation
DescriptionSupplierOrder code
Inline density and viscosity meter - SRDRheonicsSRD-X1-12G
Hygienic weldolet G1/2”RheonicsHAW-12G-OTK

Figure 4: Flush installation example with HAW accessory

b. Flush Tri-Clamp adapter

This Tri-Clamp connection is a common solution for wall or bottom tank installations. It uses a flush weld-on adapter that Rheonics can provide.

Flush SR-sensors need special attention at installation to ensure good performance. It’s short insertion depth is beneficial for small spaces but a good exposure to fluid is needed.

SRD - Tri-Clamp Flush Hygienic Installation
DescriptionSupplierOrder code
Inline density and viscosity meter - SRDRheonicsSRD-X4-TC-1.5”
Flush Tri-Clamp flangeBurkert
Aseptconn AG

BBS-07-ZT.Z40.64- A.025.35.0


Order codes from third parties in table above are indicated for the Rheonics SRD 1.5” Tri-Clamp as examples for next image. Other flush flanges can be used, varying in length and size if sensor Tri-Clamp size changes.

Figure 5. Flush installation example with standard TC flush flange

Figure 6. Exploded view of assembly example with TC flush flange

4.2 Other installation options

a. Varinline:

The SRD can be installed in a Varinline housing equal to or bigger than DN50. 

The design of the Varinline housing ensures no dead zones around the sensor, leading to good flow surrounding the sensing element.

SRD - Varinline Hygienic Installation
DescriptionSupplierOrder code
Inline density and viscosity meter - SRDRheonicsSRD-X4-Varinline N
Varinline housing DN 50GEATDN50
Figure 7: Flush installation drawing of SRD in DN50 Varinline Housing


The SRD has a flush, short and long insertion variants. All have a compatible hygienic design and are EHEDG certified. In this article, focus is on the flush version even if an example has been given in Figure 4 with the short variant. The long insertion pobe design has a unique article here

Pictures, photos, and images used are for illustration purposes and do not constitute any warranty or assertions on the suitability of use and should not be construed as an explicit or implicit recommendation or endorsement. All illustrations are given proper credit from our source of access and by their use here we do not specify or establish any copyright which belongs and stays unaffected with the existing copyright holder of that material.


[1]: https://rheonics.com/resources/certificates/

[2]: https://www.buerkert.ch/de/produkte/733905?utm_source=DataSheet-BBS07&utm_medium=Text-Link&utm_campaign=Article-Link&utm_content=733905

[3]: https://www.aseptconn.ch/en/aseptic-connections/novaseptic-tank-connections-na-connect/welding-flange-na-connect-sms-3008

[4]: https://www.gea.com/en/products/pumps-valves-flow-components/valves-components-hygienic-liquid-processing/inline-technology/varinline-pocket-free-instruments.jsp