Detecting flow through obstructed pipe and out spray nozzle


Thread Starter


I have an array of spray guns with liquid supplied by one pump. I need to detect whether the guns are actually spraying or if any are clogged, so I need a sensor at or just before each nozzle, the question is which type of sensor do I use?

First some requirements, it has to be: compact(~1/4" pipe leading up to nozzle), unobtrusive to the spray, compliant with medical and FDA standards if in contact with the solution, relatively cheap, rugged with no moving parts that can survive in harsh environment, mounted in one piece either right next to the nozzle or just before, and preferably not inline. There isn't much access to the lead pipe going to the nozzle heads, so nothing that clamps onto a pipe will work.

Technologies I've considered:

Too big and bulky, wayyyy too expensive

Differential Pressure gauges-
Induces a pressure drop and they are usually fairly big or inline.

Optical: Diffuse sensors, through beams, fork sensors and fiber optics-
Viable option, but it is a fairly fine spray that might not be detected. Would have to have air spraying on the lens so material doesn't accumulate on it but that's not a problem. The optics would look at the spray coming out and if it ever doesn't detect a spray it would trigger an alarm. I am considering this option.

Capacitive Proximity Sensors -
Another viable option if it can actually sense the spray, I am going to do testing on that matter though.

RTD's and Thermistors using heat dispersion sensing-
Very viable option and one i'm leaning towards. There are many thermistor and RTD anemometers for gas and air flow but I haven't found any specifically for liquids. I was thinking of just poking a little self-heating thermistor into the pipe and creating a circuit that detects the temperature change induced from flowing liquid. The lower the temperature, the higher the flow. I would rather not have to create my own if there is one commercially available that will do this for me. Again though, I am not looking for just a temp sensor; it has to produce heat and detect the relative change in temp around it.

I've been looking at website after website of different sensors for a full week now and I'm going crazy, so if anyone could tell me something they've done in the past that would work here I would greatly appreciate it.


Bob Peterson

I have used thermal dispersion flow switches in liquids several times. They have some serious limitations and I suspect they would not be a good option for you. They tend to be good sized.

off the top of my head I cannot think of any good options for sanitary flowmeters in this size.

A small venturi could be made that would have low pressure drop. There are a number of PCB level DP sensors that could be combined to make a rather inexpensive, no moving parts meter. Contact me off line if you are interested in such a meter.

John Catch
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