TL;DR I need an air pressure sensor, to measure both positive and negative pressure between two fans.
I got an indoors transportable AC unit. To get the hot air outside, I extended the hose by 10 meters, which caused the pressure to go up, and the flow to go down. To help the air along, I added an in-line fan, which completely removed the air flow problem, but instead gave me a new problem/project. How to make the fan turn on and off?
My first thought was to have some kind of wireless link between the AC unit and the fan, but then I got the idea of using a pressure sensor and an arduino instead.
My idea is now to have a pressure sensor on the intake side of the in-line fan, but it needs to both measure positive and negative pressure.
When there is a set amount of positive pressure, the fan must turn on, and if there is a set amount of negative pressure, the fan should turn off again. That way the fan inside the AC unit can build up pressure in the hose, and the arduino on the in-line fan detects it. When the in-line fan turns on, it will probably cause a negative pressure at the intake side, so I need to figure out how much negative pressure there will be, when the fan inside the AC unit also turns on.
But first of all, I need to find the correct sensor, and I have no idea about air pressure. Anyone got any experience with those?
You don't need a negative air pressure. There is no such thing. The most negative you can have is zero! A complete vacuum, which you don't have.
What you want is to measure the two air pressures and subtract one from the other. So, find two sensors that are accurate enough to actually allow you to measure the two pressures.
Paul_KD7HB:
You don't need a negative air pressure. There is no such thing. The most negative you can have is zero! A complete vacuum, which you don't have.
Paul_KD7HB:
What you want is to measure the two air pressures and subtract one from the other. So, find two sensors that are accurate enough to actually allow you to measure the two pressures.
When using two baromic pressure sensors, you might have to calibrate them. You also need to put the sensors inside or create a special air tight chamber.
It is easier to use a differential pressure sensor. There are differential analog pressure sensors with a dual port and integrated amplifier.
Connect two small tubes, one before and one after the fan. Then two tubes to the pressure sensor.
Do you know how much pressure difference there is ?
There are a lot MPXV7002DP 5V analog dual port pressures sensors on Ebay and AliExpress. It is -2 kPa to +2 kPa (relative).
A "negative room pressure" is a term for a certain situation.
A "negative pressure" is not possible. If you can make a vacuum with an absolute pressure of -1 kPa, then you broke our universe.
If it were my project, I’d go simple, not complex. An optocoupler on the A/C fan driving a relay, no Arduino required. That’s a five by five solution: five minutes and five dollars.
Another option is a use a simple hvac air proving switch on the cold outlet side of the A/C rather than trying to measure hot side outlet pressure. IMO measuring pressure is unnecessarily complex and expensive, it’s much easier to prove flow/no flow with a mechanical switch than trying to measure a small delta pressure electronically.
With either approach, I’d modify the A/C with a socket for the auxiliary fan in plug into, scabbing the power from the A/C supply. Simple and cheap or complicated and expensive? I know which I’d choose.
What the authors should have explained is that the strictly positive room air pressure is kept slightly lower than the outside air pressure, to prevent exfiltration of particles from the room.
