Choosing A Compliance Flow Monitor (Updated 2020)


Choosing A Compliance Flow Monitor (Updated 2020)

With the current installed base of compliance flow monitors getting older and the availability of parts and support getting scarce, many users are looking at replacement in the next few years.  Replacing your existing flow monitor with a different type, or upgrading the existing system becomes a decision worth weighing.

There are three major Continuous Flow Monitoring Systems measurement technologies worth exploring – S-type pitot tube, infrared and ultrasonic.  Which one is chosen can depend on a lot of factors including cost, exhaust gas conditions (dust, temperature, stratification), placement challenges, and more. 

Below is a rundown on the three technologies used for compliance applications along with their pros and cons.  Sometimes the application will simply dictate which method will work and which won’t.  Often multiple technologies are appropriate so the decision might rest on cost, maintenance requirements and installation issues.

S-Type Pitot Continuous Flow Monitoring System (typ. Price - $17-$20,000)

We admit to a bias towards S-type pitot tubes – and with good reason: they’re simple, easy to use, inexpensive to maintain, and very reliable.  When measuring any parameter, the simplest method that works is usually the best.

S-type pitot tube systems utilize a pitot tube probe assembly like the one shown.  Pressure differential (DP) is measured by subtracting the low-pressure reading from the high-pressure reading.  Temperature and this DP are used in the Bernoulli equation to calculate velocity.  Flow is calculated by multiplying velocity by the cross-sectional area of the stack or duct.

The probe at the stack needs no utilities and is completely self-contained.  With no electronics in it, it’s not susceptible to damage from lightning strikes.  Just a pitot and an RTD to measure temperature make up the probe.  From the probe, two hoses go down the stack along with the RTD wire.  They run into a controller that measures the DP and temperature and calculates the flow value.  The controller also performs the daily zero and span calibration checks and outputs calibration statuses.

Advantages: There are many advantages to this method of flow measurement:

  • It is based upon a very simple concept – the fundamental laws of mass and momentum.
  • It is a cost-effective solution - less costly than other flow technologies.
  • Any competent plant technician can easily maintain it as there is very little maintenance required.
  • Only one port is required on a stack or duct.
  • The components used to measure the differential pressure and the temperature are standard off-the-shelf type components (not special components that lock a user into a specific supplier).
  • The system allows for calibration checks against a primary standard – an inclined manometer.
  • It requires no calibration fudge factors, such as a curve fit to provide accurate readings.

Disadvantages: There are a few things that make S-Type Pitot Continuous Flow Monitoring Systems a poor fit in some applications:

  • They require a laminar flow.  Since they measure a single “point” in the exhaust gas, turbulence can affect their readings.  A representative sampling point is needed for accurate results.  In some cases, multiple pitot tubes can be paralleled together and averaged for a more accurate reading in a stratified exhaust stream
  • Because they require a laminar flow a user needs a longer run of straight stack or duct (ideally 4-8 diameters of a straight run).  This doesn’t lend itself well to short stacks where there may only be 2-4 diameters of straight length.
  • High dust will clog the pitot.  A constant purge of the tubes can help, and all pitot systems have timed purge systems to blast the probes with clean air on automated intervals (once a day to once an hour). 
  • Excessively corrosive exhaust streams can shorten the life of the pitot significantly with replacement becoming expensive.  There are different materials of construction for Pitot tubes besides the standard Stainless Steel like Hastelloy that can help.  Ceramic pitot’s can also be manufactured but are very expensive.
  • Excessive moisture can cause premature plugging.  Pitot’s work well in non-condensing exhaust streams where the gas is above its dewpoint.

As anyone familiar with a RATA knows, the S-type pitot tube is the Reference Method used and accepted by the EPA.  It has long been the established method due to its high degree of accuracy, simplicity of design, and ease of calibration and maintenance.

A single pitot tube will usually comply with the EPA recommendations for a flow measurement location.  If there is stratification, then two or more pitot’s can be installed and connected to provide an average velocity to compensate. 

On new or existing installation, or where flow is required due to a Title V Permit or participation in a NOx trading program, a S-type pitot tube flow system is simple to install and maintain and will not become obsolete anytime in the near future.

Infrared Continuous Flow Monitoring Systems (Typ. price - $13,500)

Another Continuous Flow Monitoring System measures the velocity of stack gases using a highly accurate time of flight procedure that is derived from a cross-correlation analysis of the infra-red emissions of turbulent gas (i.e. it measures the heat signature of a traveling gas).

Two robust infrared detectors are used for the prime sensing, mounted on the stack or duct typically one meter apart in the direction of flow. 

The total release is calculated as follows:
Mass flow = Mass concentration x Gas velocity x Area of Duct

The method used is similar in principle to the technique of flow measurement by the injection of chemical dye or radioactive tracers, where the velocity is derived from the transport time of the tracer between two measuring points a known distance apart.  Instead of an artificial tracer being added, the naturally occurring turbulence of the gas stream is used as the tracer.

This device offers a solution where pitot systems could get clogged (high dust) or there is too much stratification present.  Because it is not an in-situ measurement technology the Transducers never come in contact with the exhaust gas stream which means corrosive environments, temperature and other hinderances to measurement are not a problem.  Since they use time-of-flight and an infrared signal, high dust loading is not a problem – in fact particles help to create the IR signal.

Advantages: There are many advantages to this type of technology:

  • Low cost is a huge advantage.
  • IR Flow units stand up to high heat.  With the transmitter and receiver mounted on the outside of a stack or duct, the high exhaust gas temperature limit is in excess of 1832°F. 
  • Because it uses the IR temperature of moving particles, and measures on a straight plane (versus cross stack or duct), IR Flow units have no problem working in high dust applications where other instruments could clog or have signals blocked.
  • These units are EPA certifiable and are equipped with full high and low span automated calibration checks.
  • Since these units are not in direct contact with the exhaust gas, maintenance is minimized to simple window cleaning once a year.  Purge air connections allow the lenses to be kept clean between maintenance intervals.  No moving components increases reliability even more.

Disadvantages: Like any technology there are applications where they are not a fit:

  • Because the technology measures IR heat, a minimum exhaust gas temperature of 158°F is required to create enough signal for detection.
  • These instruments require two ports a meter apart in the same direction of the flow of the exhaust gas.  This can be a limiting factor where access is not available and more costly with regard to installation (adding a second port).  Typically though both ports can be accessed from the same platform.
  • The technology can’t be used for high flow rates.  A typical range is 0-50 m/s (vs. 0-275 m/sec for pitot)

Ultrasonic Continuous Flow Monitoring Systems (Typ. price - $30,000)

Ultrasonic Continuous Flow Monitoring Systems have been used in compliance applications for decades.  They’re reliable, accurate and simple to maintain.

An ultrasonic system uses two transducers at a 45-degree angle.  It measures the transit times T1 and T2 of tone bursts between the pair of transducers. Velocity is then calculated using:

Vel. = [PL/(2cosØ)][(T2-T1)/T2*T1]

Where PL is the pathlength and Ø is the angle between the line-of-sight of the transducers and the direction of the flow.

Ultrasonic flow monitors have the capability of measuring flow with two sets of transducers - this is typically referred to as an “X” Pattern.  An “X” Pattern allows for enhanced measurement in an application where there is turbulence presence (similar to averaging a pitot system)

Ultrasonic flow monitors are often installed because of fear of stratification or cyclonic flow – users feel they are more forgiving.  These sites should have ample RATA data to see if this is the case and if there is a true indication of flow problems. 


  • Like all the flow monitors, the technology is accurate, reliable and relatively simple and inexpensive to maintain
  • Often there is no purge air required (with gas temperatures up to 260 °C)
  • Measurement takes place continuously across the entire duct diameter allowing for representative results where a single point measurement may not.
  • Uses non-intrusive technology that does not come in contact with the stack gas.


  • Highest cost of all the technologies.  Doesn’t necessarily buy better accuracy or results.  The “X” pattern is double the equipment and significantly higher upfront expense.
  • Expensive electronics up on the stack – potential for expensive lightning damage.
  • Often a dual trace oscilloscope or computer is required on the stack to set up and calibrate.
  • The technology can’t be used for high flow rates.  A typical range is 0-60 m/s (vs. 0-275 m/sec for pitot).
  • Lowest temperature range of the technologies – can be used with exhaust gas temperature up to 650°F -850°F.
  • Requires two ports which can be an added expense especially if an additional platform is required to reach 2nd transducer.


Simply happy to help.
Monitoring Solutions is here to help! Call 908-500-4010 or email for a competitive quote.

Would you like our  Newsletter Electronically? 
Signup for our E-Newsletter at the bottom of this page. 
Stay connected and up to date with us join us on Social Media.

This website requires javascript. Please enable it or visit to find a modern browser.