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I need a "Flow Switch"

I need a "Flow Switch"

I need a "Flow Switch"

Hello all,
I am looking for a method to determine whether fluid (most probably water) is flowing/being pumped from a remote location. Perhaps a switch or sensor that activates a current once flow is detected.
It needs to be
-withstand about 2000psi

Perhaps someone can point me in the right direction, I have heard the term "flow switch" tossed around, but is it viable, and are there any other options?

Thank you,

RE: I need a "Flow Switch"


A flow switch, by itself, isn't going to do the job.  Think about the project a little and determine if you need to know if fluid is actually flowing or if it's sufficient to know that a pump has started or a valve has opened.  If it's the former, you need something that actually measures flow not just voltage.  If it's the latter, than why the 2000 psi?  Look at Crane's Technical Paper 210 "Flow of Fluid" - at the very beginning they have a discussion on flow measurement.

Patricia Lougheed

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RE: I need a "Flow Switch"

This is for a groundwater monitoring application. It will not suffice to know whether the pump is working. The chamber will be filled with fluid and not always pumping. For controls sake, it is necessary to have a sensor to determine whether the fluid is being pumped, regardless of whether the pump is working or not. The fluid could be as high as 2000psi, however, there is no need to know the flow rate, only whether there is flow or not.

Thank you

RE: I need a "Flow Switch"

rcontractor, If all you want is go/nogo indication for flow, a switch is the way to go (no pun intended). You can get them in either flow condition, switch on with flow or switch off with flow.

Check with Kobold for their PSR paddle type flow switch. the ss units are rated at 3600 psig @ 230F.

Hope this helps.

RE: I need a "Flow Switch"

Another way that is likely cheaper, is to measure the power being used by the pump motor. For a smaller HP pump(say less than 3HP), the power should be monitored as it is more linear with flow. If it is a larger pump, then you could just monitor the current (less expensive). The data you need is the current consumed with adequate flow and ideally, current consumed without adequate flow. Then you can purchase a current relay with an adjustable trip point to activate a relay or similar contact.
All of the above is assuming a reasonable sized pump/motor for the application (not grossly oversized). If it is oversized it will be more difficult to differentiate between adequate flow and not adequate. It can still be done.
I would definately handle it with monitoring the power. All of the actual flow meters/switches I have seen are a lot more expensive and a lot harder to install. Good luck.

RE: I need a "Flow Switch"

buzzp - Current draw is a great way to get a "flow switch" type input.  Has been done before, but is difficult in a mass produced product because of all the different types of pumps out there some pumps draw less power at reduced flow, some pumps draw more power at reduced flows, and everywhere in between).  Requires knoweledgeable person to purchase and use such a product, so they are not common except when controls people do one-offs.
But on a one-off situation this is a great way.

We have had a serious problem with flow switches for many years.  In the last year we negotiated a custom made flow switch using thermal dispersion.  No moving parts, highly accurate, highly repeatable, and our tests reveal what we thought, high reliability because we adapted it from the steel mill industry where electric is nasty.

Not many companies make thermal dispersion flow switches, therefore hard to get.  Let me know if you are interested and I will give you contacts or sell you one of ours.

Reason we left the paddle types is that they are unreliable, hard to install properly, do not last very long.


RE: I need a "Flow Switch"

I would like to have a look at some literature on theses thermal dispersion flow switches.

I am working within a budget, as well as space restrictions(as I am sure we all are at most times), and this will play a big part in determining the feasability of these switches.

Thank you very much for your help.


RE: I need a "Flow Switch"

I always recommended a flow test if it could be done to come up with the trip points. Granted, sometimes this is not practical so the complete system would have to be known to come up with the theorhetical trip point.

Are there any web sites that talk about motor power as it relates to different pumps under dry running conditions?

RE: I need a "Flow Switch"

buzzp - There are no sites that I know of, and probably there could be little to learn in a big general sense because the relationship of pump output to current is highly specific to each brand, type, motor, application etc.

Franklin has their own little dohicky that attempts to determine that the pump is either dry or running at dead head based on current, but they explain nothing about the dohicky except to say that it is current based and only applies to their submersible motors.  That dohicky I'm sure is a big problem to them if someone couples the Franklin motor to a high Ns value pump where the current relationship is reverse from a low value Ns pump.


RE: I need a "Flow Switch"

I am no pump guru. I am a little familiar with Franklins device (only fits in their control boxes doesn't it?).
What sorts of applications (or pump types) would the relationship be reverse?

RE: I need a "Flow Switch"

buzzp - Ns (Specific Speed) is a value describing how energy is imparted to the water.  Low Ns value (500-2000) pumps impart energy to the water primarily by centrifugal force.  High NS value (10,000+) pumps impart energy through "propelling" or lifting the water.

Propeller pumps draw more energy as flow is reduced. Centrifugal pumps draw less energy as flow is reduced.
Mixed Flow and Fancis Vane pumps (mid range Ns) are as you might expect, everywhere between the two extremes.

Hard to tell by looking at exterior of many pumps, actual Ns calculation has to be done.  But suffice to say that there are many, many pumps on the market that look just like a centrifugal that are high Ns value pumps with current/flow relationship reverse of what you might think.

Pumps used to supply potable water to a home, or run a small sprinkler system are low Ns centrifugal pumps.  Many pumps working on chillers, water features, or cooling towers have High Ns values.


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