What products are involved?
SRV - Inline 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

SRV is Rheonics sensor for inline viscosity measurement (viscometer). Read more on SRV here.

The NPT 3/4”-14 connection is Rheonics most commonly used design for SRV. 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.

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

Figure 1: SRV NPT dimensions.


Rheonics SRV 

Order code
Connection type
Thread size
Threads per inch (TPI)
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 SRV 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 SRV measures what is in contact with its sensing element. This can be a problem when in a process, the flow rate is low and the pipe isn’t usually full.

A possible solution is 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.

 Figure 2: SRV sensing zone.

2.2 Stagnation/dead zone

Also related to the sensing zone of the sensor, the 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 clearence of at least 5mm is recommended between the sensor tip and a pipe wall or any other obstruction. In case this can’t be ensured, then insitu calibration may be needed.

2.3 Orientation independence

SRV has a symmetrical tip, making it possible to face a flowing fluid in any direction. Its shape enables the fluid to be in contact with the tip without creating recirculation zones. 

Figure 3: SRV sensing tip.


2.4 Flow

For Newtonian fluids flow rate does not affect the viscosity, so SRV 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. For processes with non-constant flow rates, the recommendation is to install the SRV in a section in the pipeline with the most consistent flow rate to have a steady viscosity value.

Flow rate is also relevant to ensure the full submersion of the SRV sensing element into the fluid. For a process with a low flow rate, pipes may not be full of fluid at all points, so SRV 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 solve the issue but should not be considered for fluids with solids or characteristics that can create deposits around the sensing element.

In all cases, the SRV’s sensing element should be submerged into the fluid. It’s best to avoid installations with long standpipes (i.e. using a 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

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

  • Static Newtonian fluids.

  • Moving Newtonian fluids regardless of flow rate changes (for non-Newtonian fluids viscosity changes with flow rate, but SRV ensures reproducibility at constant flow rates).

  • Bubbly fluids under moving conditions.

  • Inks.

3. Process connection installation instructions

3.1 Connection steps

i. 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.

ii. Place the sensor in the female threaded accessory.

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

iv. Use a wrench 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: SRV 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 figures.

Figure 5: Threaded flange.

4. Accessories available

4.1 Weldolets

a. WOL-34NS 

Weldolet for 2” pipes that ensures maximum exposition of the sensing element to the fluid. Also suitable for SRD NPT 3/4” -14. Refer to the WOL-34NS page for more information.

Figure 6: 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 SRD NPT 3/4” -14. Refer to the WOL-34NL page for more information.

Figure 7: WOL-34NL

4.2 Flow cells

a. 1905-A HPTH SRV

Suitable only for SRV NPT 3/4”-14 and recommended for lines with a small nominal diameter (around DN3 or 1/8”) and high pressures (up to 500 bar, 7500 PSI). Connecting threads for this accessory are 9/16-18 UNF.

Read more about this on the Accessories page.

Figure 8: Flow cell SRV-HPHT-Cell

b. IFC-34N-SRV

Suitable only for SRV 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.

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 the Accessories page and find its drawing here.

Figure 9: IFC-34N.

4.3 Tank mount adapter - TMA-34N

Suitable for SRV and SRD only in its 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 on its support article here.

Figure 10: TMA-34N.


[1]: https://rheonics.com/products/inline-viscometer-srv/

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

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

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

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