Thread: Sensitive vacuum switch for DC ducting.

Do a search for GAS FURNACE FIELD ADJUSTABLE AIR PRESSURE SENSING SWITCH on eBay and you will see they go for about $20.

They have adjustable sensitivity ranging from 0.1 to 10" WC.
The range you need switching for is obtained by inserting a spring from a range supplied.

I have only seen then for sale in the US so shipping varies from $14 to $60.

I have one of these on my mains gas powered forge that senses if air is being supplied to the furnace otherwise it switches the gas off.

If you look closely (Green arrows) you will see I have used drip retic fittings to connect to the sensor at P1 and P2

AirflowSwitch.jpg

Wouldn't fitting a position sensing switch on the blast gate achieve the same result far more reliably and significantly cheaper?

A burglar alarm type magnetic reed switch would be easy to fit and it can be wired to be effective if the DC is not running.

Originally Posted by Bohdan

Wouldn't fitting a position sensing switch on the blast gate achieve the same result far more reliably and significantly cheaper?
A burglar alarm type magnetic reed switch would be easy to fit and it can be wired to be effective if the DC is not running.

The BS gate could be open but DC not switched on, so extra wiring needed all the way back to DC switch.
The BS Gate could be open and DC switched on, but an intermediate gate could be closed.
Other gates in the ducting system might be open when they should be closed, more switches and more wiring.
Before you knew it you'd soon get lost in the the tangle of cables and switches.
An adjustable vacuum switch could senses the situation direct and requires only a short length of wiring to act as a lock out.

Thanks BobL, this is what I'm looking for, your answer to the blast gate switch question is exactly why I'm going this way, it's just so simple.

Multiple blast gates are simply wired in series. The system only allows the machines to run if the DC is on. Still cheaper to organise for multiple blast gates or machines.

Originally Posted by Bohdan

Multiple blast gates are simply wired in series. The system only allows the machines to run if the DC is on. Still cheaper to organise for multiple blast gates or machines.

OK - Lets look at this relatively simple system of 4 machines and 6 blast gates.
The truth table is listed below. For machine M1 to work, B2 and B5 have to be closed (C) and B1 and B6 open (O) B3 and B4 don't matter.
How does wiring the 6 blast gates in series work for all machines

System.jpg

For M1 B!, B6 are in series,
for M2 B1, B5
for M3 B2, B4
for M4 B2, B3
etc.

Power to the circuit is supplied off the DC switch to close the various machine contactors and if the DC is shut down the machines stop.

Originally Posted by Bohdan

For M1 B!, B6 are in series,
for M2 B1, B5
for M3 B2, B4
for M4 B2, B3
etc.

Power to the circuit is supplied off the DC switch to close the various machine contactors and if the DC is shut down the machines stop.

Sure, for M1, switches at B1 and B6 can be in series, but how do you know if B2 and B5 are closed, or open?
If they are both open you wont get sufficient suck at M1 so you have to monitor their state as well.

The simplest way to do it is to run every BG switch back to a basic logic analyser.
I've looked into this for a automated blast gate activator.
One of the members here has designed an Arduino board for this and I have one of his prototypes and it's on my long todo list to play with

I'm not suggesting you are saying this, but in the case of the OP I don't like putting ANY micro switches (door, tensioner. or vacuum) in simple series to control a machine.
Micro switches can be used to deactivate, but final reactivation should where possible be under operator control.
A machine should not be able to start if the DC is switched on, or or the cabinet door is closed etc
All my setups use an EM switch in series with micro switches - the micros can deactivate a machine but cannot energise a machine until I push that EM switch.

Aliexpress also has a range of low air pressure sensor switches.

$17 and $4 delivery for this one which is adjustable for 0,2 to 2" of WC
https://www.aliexpress.com/item/Adju...2-63f47631ba85

Given the possibility of having to allow for excess blast gates being left open I agree that your vac switches are probably the only really viable option. They would also be a good monitor for blockages and other faults that may kill the DC performance.

The switching that I suggested should only supply power to the ON button and coil of the EM contactor so there would be no possibility of a self start.

It would be nice if someone came up with an electrically operated automated blast gate that was not expensive as I would love to install them.

Just to add a bit to this, in the past I programmed microcontrollers for various automation tasks and also designed the hardware interface components, so doing this with reed or micro switches and a bit of microprocessor logic is a task simple for me. The thing is, our small workshop layout changes due to upgrades, dust collector mods and as other dust making tools get added, so I'm trying to keep this simple and neat.

I understand what Bohdam is saying and it would likely be a lower cost option, however you still need to switch (disable/enable) the various equipment, that part is common to both methods. With the vacuum switch method you don’t need any logic (that could change the balance of cost for some) and you don’t have to run any cabling from the DC or gates, in my case there is NO cabling at all, just a small vacuum hose connection.

The only real disadvantage of the vacuum switch is cost, but it's elegant simple and if set correctly, it automatically covers loss of DC flow to the machine for ANY reason, from blast-gate closed to more than one open and all the way to low flow/pickup rate.

As it only requires a simple vacuum hose from the DC duct close to the machine, the sensor could be fitted inside the machine if desired, however in my case it will be fitted into a small power adaptor housing that is simply plugged into the wall socket, the machine is then plugged back into this adaptor. Just a plastic hose between the duct and the adaptor, BTW the EM relay in the adaptor will need to be manually set to ON if vacuum fails or suddenly appears.

In any case, all our machines use EM switches, so as BobL indicated, they cannot reenergise if vacuum suddenly appears and the machine happens to be powered on.


Bob I found some and ordered them before I saw the alliexpress units you posted, the ones I ordered were $25 AU, I have all the other bits in the parts bin.

Update: Got around to fitting the BS vacuum sensor, realised I could pop it in the lower cabinet and simply vent the HI side to the outside of the cabinet.

The switch detects the lower cabinet vacuum and simply plugs in series with the existing foot switch / disk brake system. The vacuum switch is held in place with two rare earth magnets, no holes to drill and no cabling was cut, and only took a few minutes to fit.

With the Vac switch installed, the Band Saw won't start when:

A: The lower door is open
B: The blast gate to the BS is closed
C: The blast gate to the BS is open AND another blast gate is also open
E: The Dust extractor is not running or there is no / low airflow for any other reason.





NOTE: Yes I have quite a high vacuum in the lower cabinet - The dust pickup is handled primarily by a below table pickup, the lower port on this cabinet is blocked, only the upper port is connected to the DC.

Opening up the lower cabinet is a mistake in this installation, not only does it rob airflow from the below table intake, it will also reduce the effectiveness of above table DC pickup, and finally, you virtually eliminate any air flow into the small blade entry opening in the top of the lower cabinet, this high speed, limited volume air, effectively removes chips from the gullet of the blade as it passes through the zero clearance at the top of the cabinet and as the blade starts to flex over the lower wheel, it also effectively removes any residual fine dust that gets by the under table pickup.

The picture above was taken after a few weeks of cutting on the BS, there is nothing in the lower cabinet, I just opened the lower door, fitted the vacuum switch and took the picture.

EDIT: The vacuum (pressure) switch is the round silver and black object at the bottom left of the cabinet. The clear plastic tube connects the HI side of the sensor (positive pressure) to the outside of the cabinet through the blocking plate over the lower port. The open 2nd port (low pressure) now senses the pressure difference (vacuum) between the inside of the cabinet and the outside of the cabinet.

EDIT2: I wanted to clarify the operation of the pressure switch for new users:

This BS has a foot brake, the micro switch in the foot break housing cuts the AC supply to the Bandsaw magnetic safety switch (relay), and the pressure switch in now in series with this foot break micro switch. When ever this circuit is interrupted, the magnetic power switch disengages, disconnecting 240v AC power to the BS. The only way to start the BS again is to press the BS power button which in turn engages the magnetic safety relay. This is the same condition as if 240v AC has been interrupted - the BS can not restart itself unless the power button is physically pressed again, so vacuum suddenly appearing and closing the vacuum switch, or AC power suddenly appearing, will NOT make the BS start by itself.

Originally Posted by BobL

Aliexpress also has a range of low air pressure sensor switches.

$17 and $4 delivery for this one which is adjustable for 0,2 to 2" of WC
https://www.aliexpress.com/item/Adju...2-63f47631ba85

FYI - I also purchased one of these for the modified Router table build and it's a really nice looking unit. I have just finished fitting and testing it. Easily detects another blast gate open or low DE flow (vacuum) for any reason. Only negative is the switch contact is only rated around 1A, however in this installation I'm using this sensor to control a 24V DC winding on a 3HP 240V AC relay, so it's perfect for the task.