What products are involved?
SRV - Inline Viscosity Meter
What is the purpose of this article?
To give installation options and instructions for the viscometer SRV with NPT 3/4” process connection.


1. Overview

Three important requirements are needed for any installation of the SRV. These are submersion of the sensing element, avoiding stagnation zones, and considering a tip’s clearance to obstructions (i.e. pipe inner wall). Read more on this article: SRV NPT 3/4”.

Figure 1: SRV main installation considerations.


NPT (National Pipe Taper) is a tapered thread connection standard based on ANSI/ASME B1.20.1. It requires the use of a sealant (Teflon tape or sealing compound) to create a proper installation without leaks or damage in the threads.


2. Installation options


2.1 In tank:

When mounting the sensor in a tank, the user should decide whether to install the sensor vertically from the top, horizontally from the tank’s wall, or at the bottom with the sensor vertically or upwards. This decision should be based on the characteristics of the fluid and tank, i.e. if tank has moving parts that can hit the sensor like mixer impellers, if the tank is double jacketed that complicates a side installation, if there are existing ports at the tank’s lid, wall or bottom that can be reused, etc.


Rheonics SRV is unaffected by the installation as long as the 3 previous points in Figure 1 are followed.


a. In tank lid or open vessel:

When installing the sensor from the top of a tank or in an open vessel Rheonics recommends to use:


Figure 2: SRV mounted in the TMA-34N in a tank’s lid and open vessel.


b. In tank wall or bottom

Here, as in any other installation, the sensor should be mounted in a way that the complete sensing element is submerged in the fluid. Coating depth has to be sufficient. Potential issues could be that the fluid is static and moving which can change the apparent viscosity of the coating itself.

With a standard short NPT sensor, Rheonics recommends the use of this accessory:

Figure 3: SRV NPT in WOL-34NL in tank’s wall.


Some fluids in tanks can create a zone with non-moving layers. This is only present if the fluid’s reactions create a deposit on the wall. In these cases, the sensing element is ideally placed right after this zone, so near the wall where the fluid is constantly moving. It is good practice to also insert the 16 mm neck (sensor’s section just above the sensing area) into this zone. If the sensing element can’t sufficiently protrude into the tank with a standard NPT sensor, consider using an FPC sensor as in the figure below. FPC will use a 1 1/4” NPT thread.

Figure 4: SRV FPC NPT 1 1/4”

2.2. In line

a. For lines smaller than or equal to 1”: The SR-Sensors can’t fit into nominal diameters lower than DN20.  For these cases Rheonics recommends the use of the next flow cell, this also helps with low flow rates ensuring the sensing element is completely in contact with the fluid. 

  • IFC-34N-SRV: Flow cell with 3/4” NPT sensor, inlet and outlet connections.


Figure 5: Inline flowcell IFC-34N-SRV


Problems with standard off-the-shelf NPT tee connections is that these don’t ensure that the sensing element is well submerged in fluid. Also there is a clear dead zone around the sensor, the fluid in contact with the sensing element will not correctly represent the main flowing fluid of interest. 


Figure 6: Comparison Rheonics flow cell and standard tee piece.


b. For 1.5” and 2” lines: Unfortunately, there are limited installation options for the SRV 3/4” NPT in these pipe sizes. For most pipe norms perpendicular installation will not comply with the requirements of avoiding stagnation zones, having the complete sensing element in the flowing fluid and the required clearance for the sensor’s tip. Installation in elbows with elbolets will most likely not have the sensing element sufficiently immersed in the fluid, leading to deposits around the sensing element, as in next figure.

Figure 7: Potential issue with standard SRV in elbolet.

In ANSI standards, a 2” pipe (60.3 OD) will be enough for the sensor to be installed vertically. For this and other pipes with enough ID, Rheonics recommends using the next accessory:


  • WOL-34NS: Weldolet with 3/4” NPT connection for 2” lines (60.3 OD). With this pipe size the sensor can be installed perpendicular to the flow. 


Figure 8: SRV NPT in WOL-34NS for 2” Pipes (63.6 OD).


If the 2” pipe has an OD<63.6 mm, Rheonics recommends:

  • Use a long insertion (-X5) sensor in an elbow. This variant will have a 1 1/4” NPT connection because of the sensor diameter of 30mm. Hygienic version of the -X5 is also an option to reduce the cross-section decrement in the line when the sensor is installed.


Figure 9: SRV FPC 1 1/4” NPT in an elbow.


  • Consider other process connections. Tri-Clamp is another connection type available for the SRV that has installation accessories for pipes with 1.5” and 2” sizes. 
  • A tilted installation  (i.e. an installation at an angle) is usually not recommended, since for non-newtonian fluids the sensing element’s base never gets full in channel which makes it susceptible to plugging. 


c. For lines bigger than 2”: For these pipes, a perpendicular installation of the sensor can fulfill the 3 the basic needs of installation, sizewise, mentioned in section 1 of this article. One must be ensure that the entire sensing element is fully submerged and exposed to flow.

  • WOL-34NL weldolet. Rheonics recommends using the WOL-34NL weldolet to avoid stagnation zones and maximize the immersion depth of the sensor into the pipe for optimal measurement conditions. This weldolet is used for 3/4” NPT connection in lines equal to or bigger than 2.5” and tanks. 

Figure 10: SRV NPT in WOL-34NL for pipes equal to or bigger than 2.5”


3. Path to correct installation and reliable data collection

Rheonics recommends the user to ask the Support Team for recommendations before installation. The client should point out what are the possible installation points, fluid characteristics, photos and any other useful information (Tank’s drawing for a tank installation).


To get feedback from Rheonics Support Team, the client should explain the intended fluid to measure and share pictures or drawings of possible installation points.


A correct installation of the sensor is a prerequisite for a reliable data collection. Incorrect installation of the sensor can lead to systematic measurement errors that cannot be corrected by data processing.