Choosing an air flow sensor to use with our laser cutters' safety cutoff

I’d like help choosing a good air flow sensor for that purpose.

I see plenty of different types online-- any advice?

I would put a tee in and check the pressure while running. Then you have your range to choose from for a switch. There are many to choose from.

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I have several sizes of barbed tees and I think some poly to check the pressure.

Those look like a good solution for this.

We could MAP sensors that most closely match our laser cutters’ hardware.

Keep in mind that Map sensors sense vacuum not pressure.

How simple do you want to make it?
Simple differential switch will send a contact closure.

Pressure transducers & a micro controller to make it more technical.

However they have their caveats, if the nozzle should become clogged, it would still thing there is flow.

Something like a hotwire anemometer would certainly indicate flow better.

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The MCU in the timer box on Donner has an available 10 bit ADC. In the next phase, we are likely to have these on all three Thunder lasers. An LED could flash a warning, the display could show a message and require a button to be pressed to confirm the setting.

mouser.com has air flow sensors with a wide variety of rates:

https://www.mouser.com/Sensors/Flow-Sensors/_/N-zqi2?P=1yp690u&Ns=Pricing|0

Any of those wouldn’t even need an MCU to flip the safety switch, so we’d wire that directly so it could work even if the MCU died. But an MCU would be n ice to do what bpamplin said.

What is the air flow of our Thunder lasers? I’ll choose some parts based on that.

Very clever-- simple is good. I’ll be at DMS Monday evening to see this laser suction inlet. Could you describe it or post a photo of it so I can find it?

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No idea what the expected flow is. The transistor anemometer seems a good fit in that it’s similar to hot wire systems, but nowhere near as fragile.

Of course, a hinged flap with a microswitch or optical beam break device might work just as well. Depends on whether you need a flow measurement, or a go/nogo signal.

To clarify, are we talking as to proof that the blowers are running or the proof that the air assist is on?
I was thinking of the latter.

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That would be a bit different. I was thinking blowers. Hopefully OP will enlighten us. :slight_smile:

As for choosing the sensor parts, we should use zmetzing’s idea to guide the design, for the best safety:

Yep, I’d put the sensor right at the laser suction inlet and watch that vacuum.

The sensors we mentioned so far are:

  • MAP sensor [edit: earlier I typed CAM]
  • Series-Connected Transistors Use Differential Heating To Sense Airflow
  • various air sensors from mouser.com

See that does change things.
You could calculate flow pretty easily. 2 pitot tubes & 2 transducers could give you an actual flow rate.

Or a simple vacuum pressure switch.

If we’re talking about the exhaust blower duct, then we don’t even need pitot tubes. Just static pressure taps in the walls and a differential pressure sensor.

Or a computer fan blade ( no motor, only bearings ) and a bit of reflective tape on one or more blades. Put that in the duct, then use an LED / phototransistor pair to watch for reflected pulses as the fan spins.

Do we have available a hand held anemometer ? A simple measurement with that at the duct inlet would be a good start.

I was meaning to check actual flow in CFM. Static is just static which would work for a standard switch or dp. But if you want actual air flow in CFM it would need to be Total pressure & static pressure readings to get the velocity pressure.

The point is to make sure there is negative pressure in the laser cabinet itself.
Measuring flow in the hose itself isn’t really necessary, and I think there’s too much potential for the sensor to get gunked up and it would be a maintenance nightmare.

If the pressure is measured in the hose just outside the exit of the cabinet, there’s a possibility that the grate inside the laser is actually blocked and there is no flow at all, but still negative pressure at that point.

To measure pressure inside the cabinet, you could use a differential pressure sensor outside the cabinet with a small (like 1/8") tube leading from it to inside the cabinet. Or, put a barometric pressure sensor inside the cabinet (I’d put it inside a tiny plastic project box with a hole drilled in it and a piece of cloth over the hole as a filter).

Here’s an option that should work inside the cabinet.

Or, a diff sensor for outside:
https://www.mouser.com/ProductDetail/Omron-Electronics/2SMPP-02?qs=3P%2bcriiv584EfFf%2bLos22A%3D%3D&gclid=EAIaIQobChMIzJr_-fr73AIVBRBpCh2WEwz8EAQYASABEgLV5_D_BwE

We can then experiment to determine what pressure is “normal” and what isn’t when the blower is running.

I can do it with two static pressures, but I forgot the calibrated nozzle in the duct in between.

Probably out of our budget. I think I can dig up the specs though if someone wants to get REALLY carried away and machine one. :slight_smile:

Do the cabinets have interlocks ? If not, this will fail as soon as someone opens the lid to check on their work.

I think a simple flow sensor ( flap switch, fan style sensor, or the transistor anemometer ) in the pipe is the best bet.