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SA thermal flow sensors

Technology overview

Thermal flow sensing technology relies on the cooling effect of flowing liquids. A higher flow velocity results in a greater cooling effect and this can be reliably measured. The images below depict heat being conducted from the sensor tip by a flowing liquid.

Diagram showing the heat transfer away from the sensing element as media flows by it


Two platinum RTD elements and a heater are in the sensor tip. The difference in temperture between the two RTDs is held constant (4 °C) by varying the amount of power applied to the heater. The power required to maintain the temperature difference is directly proportional to the flow rate.


Pump run dry protection

What if you could easily monitor all your pumps for run dry protection?

The SA insertion-style design easily installs in pipes and provides a simple switching output of flow / no flow.

HVAC equipment

What if you could monitor flow rate of water/glycol solutions in chiller pipes up to 16” in diameter?

The SA can be programmed for different media, including glycol, to display flow rate in gpm.

Unlock sensor potential with IO-Link

Via IO-Link, the SA family can provide:

  • Current flow, up to 120% of rated values
  • Current temperature
  • Switching status
  • Maximum flow velocity (acyclic parameter)
  • Current flow rate (acyclic parameter)


Q. What is the difference between absolute and relative modes?

A. In absolute mode, the SA shows the actual flow rate, either in velocity (ft/sec or m/sec) or in volumetric flow (gal/min or l/min). In order to accurately display the volumetric flow, the inside pipe diameter of the pipe must be entered in the programming menu. In relative mode, the display shows a % flow based on the high and low teach performed in the application. Relative mode is preferred, especially in pipes under 2”, since it is the most accurate setting. For more accurate flow measurement in pipes under 2”, consider the SM, SV, SB or SU families.

Q. How do I convert from flow velocity to flow rate?

A. For round pipes, the following is true:
                        V = v x A
           Where V = volumetric flow rate
                        v = flow velocity
                       A = cross sectional area of the pipe

Take care to ensure all the units of measure are consistent.

The following charts can be used in lieu of the calculation. 

  1. Find the volumetric flow rate on the y-axis. (Example: 10 GPM)
  2. Follow the line horizontally until it intersects the line for pipe diameter. (Example: 3/4” pipe diameter).
  3. From the intersection point, drop straight down to read the x-axis to find the given flow velocity. (Example: 6 ft/sec)

Q. What media curves are available for the SA?

A. Media curves are integrated for oil, water glycol (all when MODE is set to LIQU) and air (when MODE is set to GAS). Expected response times for each medium are:
                    Water = 0.5 sec
                    Glycol = 0.8 sec
                          Oil = 1.8 sec
                          Air = 7 sec

Q. Why do you only specify accuracy for water in relative mode?

A. The SA measures flow velocity at one single point. Many factors can influence the accuracy of a thermal flow sensor, including turbulence, insertion depth, medium variation, etc. Therefore, we only specify the accuracy in relative mode, where the installation conditions and flow rate are taught. We estimate the accuracy in absolute mode to be about 10 – 15%.

Q. How does changing glycol concentration affect the SA?

A. The glycol medium setting on the SA sensor is designed for a 35% glycol/water solution.  Increasing the glycol concentration decreases the evaluation signal of the SA. Likewise, decreasing the concentration increases the measuring signal. For a concentration of 50% glycol, we estimate a decrease in measured value of about -25%. For a concentration of 15% glycol, we estimate an increase in the measured value of about +25%. 

Q. Is the SA affected by media temperature changes?

A. It can compensate for slow temperature changes, assuming the reference element is fully inserted in the flow, but it will be influenced for a minute or two by rapid temperature changes. As the medium temperature goes from hot to cold, the SA will drift to “UL” on the display. As the medium temperature drifts from cold to hot, the SA will drift to “OL” on the display. Assuming the reference is fully inserted in the flow to measure the medium temperature properly, the SA will return to proper reading of flow after a few seconds..

Q. Why are there 3 IODD files for the SA sensor?  Can I use LR Device for programming?

A. Because there are differences in the signal range between liquids and gases in absolute mode and all media in relative mode, each mode requires a specific IODD file. Once the operating mode has been selected using the pushbuttons on the sensor, LR Device can be used for further programming.

Q. Is a frequency output the same thing as a pulse output for totalizing?

A. No. For totalizing applications, a pulse output is required.  A 24 VDC signal is sent every time a specified volume (e.g., 0.1 gallons, 1 ml, etc.) flows through the meter regardless of the flow rate.  A frequency output provides a digital signal directly relatable to the flow rate.  This digital signal can be further evaluated with counters, PLCs, etc. Frequency outputs are often used for mechanical flow meters like turbine meters. The rotational motion of the turbine, which turns according to the flow rate, is detected and converted into frequency signal. 

For meters with totalizing function, please refer to the SM and SU families.