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Negative pressure reading

Ca++

Well-known member
Has anyone metered their negative pressure? I think my brain hurts. I want an inlet fan, sensitive to the rooms pressure. So I set about measuring said pressure, and things got a bit wrong.

First up, we know most of us have an atmospheric pressure sensor in our pockets, right? To use my old midrange samsung, I must dial *#0*# to open the service menu, them select sensors. My Barometer is there on the list, bouncing around the 1002.46 mark, near an open window. In my lounge 1002.39 and as I move up the stairs towards the room my extract is in, 1002.14 by the door, which has a 15mm crack. So into the room, I push the door to, and it barely makes a difference. 1002.11 or something. I thought my eyes should be bulging or something. That door makes less difference, then I ever expected. I know my full fan pressure is really upon the tent, who's pressure I wanted to control, and have not measured as it's dark. However, these are small fry numbers.

I don't want to use just a pressure differential switch, I want a couple of sensors with fan speed control. My goal is unloading the extract fan. My extraction isn't really about temp or rh primarily. It's about negative pressure for smell containment. before anything else. With that covered, RH only ever needs increasing, which can't be achieved by slowing the fan, who's already at the minimum for containment. I just increase the fan for temp in summer, but not by much. If I could instead control the fans based on pressure, it would allow me a bit more leeway. I could lower extract speed, or even let my typical vpd tracking controler, actual do something. Knowing my containment is in order. Which eases the extracts job. It all just comes together better.


Has anyone else done such atmospheric pressure testing? Perhaps they would like to, having found it on their phones. While writting this, that 1002.39 has moved to 1002.74 as the weather has changed. It seems to use the baro for altitude measurements. Blowing on the phone, finds it in the earpiece. Waterproof phone.


So little difference shutting the door. It's just not what I expected. Hence I'm reaching out. Though I suspect atmospheric pressure changes over the height of my stairs, may simply dwarf my fan. Then the room is simply not as sealed as it looks.

Next up, I have an air speed sensor to go in the ducting. That might shine some light on how sealed my room is.
 

mudballs

Well-known member
Veteran
Has anyone else done such atmospheric pressure testing
yeah, but not to the degree you want i fear...i wanted negative pressure and used toilet paper pieces to see if inlet draft by intact ports/vents was strong enough to move the toilet paper.
Just for odor mostly, same as you. You'll burn motor off/on using a sensor trying to keep a steady state atmospheric pressure in a room not ISO Class 8 or above sealed i would think
Link to barometric lab shit
 

Ca++

Well-known member
930.jpg

Seen these? I'm guessing you may of. It's a diaphragm switch, with a hose attachment for each side of the diaphragm. A hose here, a hose there, it can offer a simple switching action, as low as 0.2 on the scale my phone talks. Though that online calc seems like it needs a second opinion.
Edit: This one isn't the best, they do a 20Pa one. This is 200Pa, but a better photo angle
I have access to these, and they are only $20 anyway. I might borrow one and see how it plays as a testing environment. As you say though, On/off isn't good. I guess I could translate that to high/low but I really want a variable fan speed. I find these sensors like the phone has, on boards, for 50c. That's basically free. I could look at a pair with a differential amplifier, and control my EC fans without too much effort. It just seems I will be responding more to cloud cover than room depressions though. That's not good control of an inlet fan.

On a different note, I would like one of these mechanical switches across my extractor. I could wire it to my flood alarm, which uses wifi to message my phone. The flood alarm was again, about 10$. However.. it uses a battery. I guess the battery compartment offers room for a power convertor, to run it off external power instead. It also has an audible alarm. Perhaps I won't do any better, concerning internet warnings of extract failure.

I wonder if I can contort myself around my pots, and shut the flap. Hmm..
 

Ca++

Well-known member
Oh.. my phone might be broke. The altitude reading was 80 meters, and having put it in my tent and the phone flashing green at me and vibrating, It now thinks 20 meters. 1010.50 which is very different. Like full scale different, on the image above. I need to learn my numbers, that calc must be wrong. 1000 hectopascal = 10 pascal it suggests. 100=centi surely, but hecto is off my scale of knowledge.
I think my phone could be off, but can the weather just be that different. I just know nothing of barometers. Except I need a weather station up my exhaust. Which is something I never thought I would say.
 

Ca++

Well-known member
This may be the droid I'm looking for
https://www.ali express.com/item/1005004668884481.html

It's like the one pictured before, except alloy, with a digital readout, and instead of a switch, it's 0-10v output. Just like an EC fan. £30 delivered.
No pressure difference, no inlet fan. More difference, more fan. All that's left is a little scaling.
This also suits my room inlet air scrubber. I would like to provide clean air. I can't supply more than the exhaust takes, or I go into positive pressure. Or jail. If Instead I supply less, the exhaust must work harder, and will find other gaps to bring air in from. Dirty air. With atmospheric pressure monitoring, I can have real time inlet air adjustments. Ones that track exhaust speed, and the opening and closing of doors.

I might pop it on the back burner for now, but it's interesting. I have also been looking at linear actuators for louver grills, so my exhaust fan still does all the work, but I make it's job easy (and quiet) as possible.
 

truuuedef

New member
I can measure barometric pressure with a sensorpush device.

I have an intake with a hepa filter and then a seperate fan for exhaust with a carbon scrubber.

I'll circle back to your post

-tru
 

944s2

Well-known member
ICMag Donor
Veteran
I know I have negative pressure when the grow door slams behind me😄,,
As for the science?
sorry I haven’t a clue,,,
good luck though Ca++✌️
 

truuuedef

New member
While I am not doing what you are doing exactly, I could, very easily.

With home assistant, I have my smart thermostat connected to that smart home platform.

From there, I have all my AC infinity gear integrated into home assistant as well.

From there, I can implement any other sensors I want into home assistant (so long as they are bluetooth or zigbee, etc).

A cool feature of home assistant that it populated for me today was a Mold Indicator.

It asks for the digital temperature sensor, then it asks for a digital humidity sensor, then it asks you to find the critical temperature point. This point is the coldest item in the room. Such as a glass window. You enter this figure into the setup, and from there on out it will calculate mold risk. It runs live calculations in the background of the temp, humidity, and the critical temp set point, and then I can just glance at that time to time and know everything is peachy rather then wondering if my temps or humidity were too high. It also logs all this data. I can go back all last year and see all the data recorded for everything. How long fans ran, lights were on, off, how long the hvac ran, how long the supplemental heating ran, etc.

What you want to do is really cool and possible
 

Grow44DE

Well-known member
Differential pressure transducer is what you are searching. sealed gauge and phone are relative pressure transducers. so they wont work for your application.

It's about negative pressure for smell containment.
Isnt it achieved by simply running a big ass fan at any speed?
But maybe I don't understand your intention behind it. making it more complicated than it needs to be?
 

Ca++

Well-known member
I have enough extraction to give me good negative pressure, but in doing so, I loose too much heat, and need quite a bit of humidification. If I back off, there are times when opening doors and such, allows smell out into the rest of the property, which drifts down as it's heavy with water. I would like to blow filtered air into the room, to meet extract demand, while the door is closed. However, the moment I open the door, it all falls apart. I would need to decrease the inlet, and increase exhaust, to retain negative pressure, which must then be over the entire property. Enough to defeat weather conditions, and the wind can be strong.
Big fan used to be fine, with HID supplying more heat than we wanted. Today, we have to add heat and humidity. It's not viable to use a big fan that pulls them straight out. The real complication is the balance of inlet and exhaust, as I try to minimise extract speed. While opening the door.
I'm moving towards heat recovery as an added twist. I'm trying to save electricity, and stop cold drafts coming through every window. It's -3c out there. Coming in, as I'm chucking away 30c air. It's illogical. I need better control of the situation. I need confidence in my negative pressure, while minimising it.
 
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truuuedef

New member
It sounds like you are scrubbing the room with a carbon filter. And when you turn the fan down to keep some heat in the tent and your humidity, you start to smell things.

This happened with me. I tried to turn my exhaust fan lower during lights off to keep some heat in the area. But then I began to smell things.

Ultimately you have to get some more filtration, another scrubber that runs within the area.
 
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Ca++

Well-known member
I'm looking at something similar. A second extract for the room, to accompany the one for the tent. It's getting towards that big fucking fan approach though. My feet are freezing because of all the cold air coming through the building. The more fans approach isn't as well metered as actually metering the negative pressure.

I'm absorbed into another electronics project at the moment, but have some pages open to circle back to. Differential pressure monitors with a nice suitable 10v output. I can add that to my fan controls without a huge amount of bother. It's under £30 so this isn't really a major project. It's something that others could adopt, if it turns out to be useful.
Heat recovery is the real game changer, and I have some duct routes planned. It's been suppliers backing out that's hampered that job. I should have it running already.
 
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Hiddenjems

Well-known member
Differential pressure transducer is what you are searching. sealed gauge and phone are relative pressure transducers. so they wont work for your application.


Isnt it achieved by simply running a big ass fan at any speed?
But maybe I don't understand your intention behind it. making it more complicated than it needs to be?
He could run two cheap map sensors for cars. One inside, one outside. The difference is the vacuum.
 

Ca++

Well-known member
Edit: Seems I already posted this in November.

I have had my eye on these
https://www.ali express.com/item/1005004668884481.html
A differential sensor, with a readout, so it can be used as a standalone meter. It comes in a number of ranges, and has a number of output types. Including analogue in a range useful for EC fan control. Not directly, but a very simple board completes the arrangement.
Having the readout inbuilt makes it very useful. That alone is worth the price.
 

FTL

Well-known member
I'm looking at something similar. A second extract for the room, to accompany the one for the tent. It's getting towards that big fucking fan approach though. My feet are freezing because of all the cold air coming through the building. The more fans approach isn't as well metered as actually metering the negative pressure.

I'm absorbed into another electronics project at the moment, but have some pages open to circle back to. Differential pressure monitors with a nice suitable 10v output. I can add that to my fan controls without a huge amount of bother. It's under £30 so this isn't really a major project. It's something that others could adopt, if it turns out to be useful.
Heat recovery is the real game changer, and I have some duct routes planned. It's been suppliers backing out that's hampered that job. I should have it running already.
Two exhaust fan filter combos.

one coming out of tent blowing into lung room 24/7 on.

Another one on a thermostat and ducted out of the room. This is the lung room exhaust/scrubber.

This should keep your temps stable and smell taken care of.
 

zachrockbadenof

Well-known member
Veteran
i don't have a problem with smell, i don't use a scrubber, tho i do have one... i am growing in a tent, the back of the tent has the flap, which is open - the back of the tent in up against a window which has a fan and the fan goes on when the temp reaches a certain temp.. on the front of the tent, i have a floor fan that blows over the top of the plants to push the heat from the lights (tent is 36''x42'', with 2 315 cmh's) away from the top of the plants... as u can see i don't understand this neg. pressure...

i have grown this way for some years, without problems, but if there is a better way, i am all ears... thanks
 

FTL

Well-known member
i don't have a problem with smell, i don't use a scrubber, tho i do have one... i am growing in a tent, the back of the tent has the flap, which is open - the back of the tent in up against a window which has a fan and the fan goes on when the temp reaches a certain temp.. on the front of the tent, i have a floor fan that blows over the top of the plants to push the heat from the lights (tent is 36''x42'', with 2 315 cmh's) away from the top of the plants... as u can see i don't understand this neg. pressure...

i have grown this way for some years, without problems, but if there is a better way, i am all ears... thanks
The negative air pressure is slightly more air going out than in.

For example: Your tent will be sucked inwards by the negative pressure as opposed to being inflated like a ballon with positive air pressure.

Negative air pressure guarantees two main benefits.

1. Fresh Air is being brought in and co2 is being replenished.

2. You know you are exhausting all the odour particles through your exhaust filter.

Everyone’s situations are slightly different. What works for you may not work for others due to many external factors.
 

Ca++

Well-known member
No scrubber?
I have never found a scrubber good enough to recirc, like the two fan system outlined. I have double filtered with an ozone gen following, but it's just not enough. I have been through the makes, and had carbon by the pallet to get importers to send the lorry. Giving me access to a world of grades, from the two biggest names in the business. I have 100% exhausted the choices. They are very useful, but just as the moisture passes through them, so does anything dissolved in that moisture. They have defined limits. The difference in effectiveness starts to come down to who it is that's smelling it.

I can do something similar. The lung room can't use a stat, it would run all the time to ensure negative pressure on the whole space. It would be the tent extract with the temp/rh control. Both systems would need a filter, as I need to open the tent and trim/dry in the room.
This need to keep the lung under negative pressure, doesn't reduce cold through the property. It's an improvement on what I have, but still leaves me looking at the actual pressure and fan control to achieve it. One ideal, is a fan speed controler on that lung extract, that's not interested in temp or RH, but modifying the pressure. Set low, it's for normal. Set high, for trimming days. However, its the pressure I'm turning up and down.

If this can be made simple, and works, then people will realise it's useful.

I have other ideas, with duct divertor valves. When a set extract fan, moves duty between tent and lung, incrementally as needed. I really need to free up £50 for a mess about, but this month is my taxes, my cars shot, and my phone's well overdue replacement. The boiler need tending to regularly, just to run on low, and child support for 2025 is due.
I might have to take the tent out, and get commercial about this. Them divertors are not the right thing though, as they just aint cheap. Louvers over passive filter on the lung intake are also too expensive. I need to focus on fan speed, not actuators.
 
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