What products are involved?
SRD - Inline Density and Viscosity Meter
What is the purpose of this article?
To give an overview of the SRV NPT 3/4”-14 variant, installation guidelines, instructions, special considerations and available accessories.

1. Introduction

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

The NPT 3/4”-14 connection is Rheonics most commonly used design for SRD. In most cases, it is the most price-efficient option with the shortest lead time and the largest selection of accessories.

NPT threaded connections are very common in industrial applications. It is used both in low and high-pressure environments, up to 500 bar / 7500 psi. This type of connection allows for an easy process integration using some of Rheonics accessories or a thread in a pipe or vessel.

This threaded connection requires the use of a Teflon band or compound. This helps to ensure a sealed connection and avoids cold welds (the process connection adaptor may get stuck with the sensor, leading to severe damage).

Figure 1: SRD NPT dimensions.

Order code
Connection type
Thread size
Threads per inch (TPI)
Wrench for installation
32mm (1-1/4")
CAD files

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 and recommendations to ensure the best possible performance of the sensors.

2.1 Submersion

The SRD has a sensing element shown inside the red section in Figure 2. The sensor measures what is in contact with its sensing element, so the red section should always be fully submerged in the fluid. This can be a problem when on a pipe, the flow rate is low and the pipe isn’t usually full, or when the sensor is installed too far out of the main pipe (like in a long tee section) where fluid is not flowing but is almost static.

Usual solutions are placing the sensor horizontally and parallel to the fluid (e.g. in an elbow) and not vertically and perpendicular. Also, the use of flow cell accessories can be considered, explained later in this article.

A special consideration for the SRD is that obstructions in the dark red section from Figure 2 are likely to influence the accuracy of the measurements. If they can’t be avoided, an inline calibration may be required. 

Figure 2: SRD sensing zone.

 Figure 2: SRD sensing zone.

2.2 Stagnation/dead zone

Also related to the sensing zone of the sensor, stagnations and dead zones are important to avoid. These are defined as any space, inside the red zone, where solids can accumulate and deposit, or where there isn’t fluid, but air or other unwanted materials that have negative effects on the measurement.

A clearance of at least 12mm is recommended between the sensor tip and a pipe wall or any other obstruction. In case this can’t be ensured, or an obstruction is present in the darker red zone of Figure 2, then in-situ calibration may be needed.

2.3 Orientation dependency

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 orientation.

2.4 Flow

For Newtonian fluids, the flow rate does not affect the measurements, so SRD should measure the same viscosity and density values in static and moving states of a fluid at the same temperature. For non-Newtonian fluids, the flow rate does matter and viscosity readings may differ between static and moving conditions. Density reading won't be affected as much as the viscosity reading, so, for pure density applications, Newtonian and non-Newtonian fluids can be treated similarly.

For processes with non-constant flow rates, the recommendation is to install the SRD in a section in the pipeline with the most consistent flow rate to have steady reading values.

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 of fluid at all points, so SRD should then be placed in a section where the pipe is usually full of fluid (i.e. after a pump). Installing the sensor upwards can also solve the issue but should not be considered for fluids with solids or that can create deposits around the sensing element. 

In all cases, the SRD’s sensing element should be submerged in the fluid. It’s best to avoid installations with long standpipes (i.e. using long weldolet or tee), since that may lead to not good fluid transfer resulting in measurements that do not reflect the true state of fluid or worse high noise measurements. 

Figure 4: Flow conditions in installation.

2.5 Fluid type

The SRD in general works well with almost all fluids as long as the installation is correct. Some common fluid types and scenarios will have special recommendations (i.e. fluids at different flow rates, bubbly fluids, fluids with solids, etc.), review this article about it.

2.6 Consideration for fluid versus ambient temperature

It's important to consider that the short version of the SRD (e.g. SRD-X1-34N) is not good to maintain accuracy on its density readings over a large temperature mismatch between ambient and fluid. It is suitable for high accuracy application only when ambient conditions (where probe back is exposed) and fluid are at similar temp (within 5 °C). This is due to the large temperature mismatch between front (fluid) and back (body) of resonator which causes a frequency shift that is not due to density. For applications with large differences in ambient and fluid temperature, you need the long insertion probe version of SRD so both ends of the resonator are at the same temperature.

3. Installation common issues

For accurate, repeatable and reproducible readings of the SRD it is important to maintain the sensing element immersed and without stagnation zones that can lead to deposits around the sensing area.

Below some examples are listed where these concepts are not achieved leading to potential issues for the SRD density and viscosity readings.

3.1 Potential issues- NPT Tee connection

Potential issue explanation: A common mistake with installations of the SRD-X1-34N is using an off-the-shelf NPT Tee shown in Figure 5. An installation of the SRD mounted vertically or horizontally in a Tee port can create stagnant and dead zones. Also, the SRD tip may pass through the tee and the sensing area (shown in Figure 2) can be obstructed and exposed to uneven inner walls that can create changes in the fluid’s flow lines (increasing turbulence) as in Figure 6.

Figure 5: Standard NPT Tee [6]

Figure 6: Installation in standard NPT Tee with potential issues

Recommendation: In these cases, the Rheonics IFC-34N-SRD is recommended to ensure the flow surrounds the sensing element properly and stagnant zones are removed as in Figure 7.

Figure 7: Correct installation in Rheonics NPT flow cell 

3.2 Potential issues - NPT connection adapter

Potential issue explanation: At times, users may want to adapt the 3/4” NPT connection of the SRD-X1 to a different flange, like Tri-Clamp. They would use an adapter similar to the one shown in Figure 8. Mounting the SRD probe in these adapters can lead to both incomplete immersion and stagnation zones prone to fouling or deposits at the base of the sensing element which will affect the measurements (see Figure 9). This scenario may not affect applications for low-viscosity fluids like ink but could be a significant problem for high-viscosity fluids like dough, adhesives, glue, and others.

Figure 8: Standard NPT to Tri-Clamp adapter [7]

Recommendation: The client should order the SRD with the required process connection to directly mount the probe inline without the need for additional adapters (Figure 9, installation to the right). If Tri-Clamp is required visit the SRD-X3. If custom flanges are required visit the SRD-X2.

Figure 9: Installation recommendation with NPT adapters

3. Process connection installation instructions

3.1 Connection steps

Optional: Wrap a sealing compound (e.g. Teflon tape) around the sensor NPT thread 1.5 to 3 times in a clockwise direction, to ensure a leak-free seal for some fluids.

i. Place the sensor in the female threaded accessory.

ii. Turn by hand the sensor NPT thread until hand tightened.

iii. Use a wrench (32mm size or 1-1/4") to give the sensor from 1 up to 3 more full turns.

In total, according to ASME Standards, at least 3.5 and at maximum 6 full turns need to be ensured for a correct NPT installation. The sensor shouldn't be completely tightened.

3.2 Specific Process Connection installation instructions

  • In line: Ensure the sensing element is exposed to fluid, and avoid stagnation and dead zones. Consider placing the sensor upwards or parallel to the fluid for small flow rates that can’t fully surround the sensing element.

  • 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. Rheonics TMA-34N accessory is a tank mount adapter used for NPT SR-sensors, that should be considered in tank installations. This is detailed sections below.

  • Others - Thread flange: SRD NPT sensor can be mounted on a flange thread of the same size (3/4"-14). The flange needs to have an inner diameter larger than 26mm on the process side, the material should be either 304 or 316L and the height can't exceed 25mm. If the flange has a bigger height, an FPC-type (long insertion) sensor is needed.
    Once again, in these and any installation cases, stagnation zones around the sensing element should be avoided. For thread flanges, the height of the standpipe of the connection is relevant as shown in the next figure.

Figure 10: Threaded flange.

4. Accessories available

4.1 Weldolets

a. WOL-34NS

Weldolet for 2” pipes ANSI standard (60.3 OD) that ensures maximum exposition of the sensing element to the fluid. Also suitable for SRV NPT 3/4” -14. Refer to the WOL-34NS page for more information.


Figure 11: WOL-34NS

b. WOL-34NL

Weldolet for 2.5” and larger pipes, also suitable for tank installations, that ensures maximum exposition of the sensing element to the fluid. Also suitable for SRV NPT 3/4” -14. Refer to the WOL-34NL page for more information.

Figure 12: WOL-34NL

4.2 Flow cell - IFC-34N-SRD

Suitable only for SRD NPT 3/4”-14 and recommended for inline installation with nominal diameters of DN5 to DN25. Connecting threads for this accessory are NPT 3/4” -14. (Another variant is available for the SRV NPT).

For a diameter larger than DN25 (1”), we recommend using a bypass. Then, the flow cell can be connected via flexible hoses or fixed piping. Installing the sensor in a bypass is a common solution for process lines that require intense cleaning through means of pigging.

Read more about this on its accessories page.

Figure 13: IFC-34N-SRD

4.3 Tank mount adapter - TMA-34N

Suitable for SRV and SRD only in their NPT 3/4"-14 variant, this accessory is recommended for open vessels or tanks with a lid. It is composed of a small cell, wherein the sensor is installed, and a 3/4" pipe fixed at the top and with a variable length so the sensor can go as far as needed.

Read more about this on the accessories page or in its support article here.

Figure 14: TMA-34N with SRD.


[1]: https://es.rheonics.com/products/inline-density-meter-srd/

[2]: https://rheonics.com/product-accessories/

[3]: https://rheonics.com/product-accessories/wol-34ns/

[4]: https://rheonics.com/product-accessories/wol-34nl-accessory-page/

[5]: https://www.asme.org/codes-standards/find-codes-standards/b1-20-1-pipe-threads-general-purpose-inch

[6]: https://www.mcmaster.com/products/tees/low-pressure-stainless-steel-threaded-pipe-fittings-9/

[7]: https://www.dernord.com/products/sanitary-female-threaded-pipe-fitting-to-tri-clamp?variant=27282774949952