Measuring cfm for optimal ventilation

Fellow growers. What's happening!

I'm almost don't setting my indoor back up, been waiting on some new pieces. I just bought an anemometer to measure cfm and hoping someone in here can point me in the right direction for use.

I have my exhaust fans on speed controllers and I want to get them set perfectly to vent the room ROUGHLY 1X/m using the anemometer to measure.

It is my understanding that 1sq ft of intake vent is required per 300cfm. I've done the math and have 1.06sq ft of intake vent size. My room is 224 cubic feet. Thus I want to get my fan set to 224-300cfm precisely.

My question - where do I take the measurement with the anemometer to do so?

First thought is at the carbon filter with all ventilation hooked up, however, given it pulls from every which way. I don't think that would be accurate? Do I take it from directly behind the fan once the fan is attached to the carbon filter? Do I take it at the intake vent? At the end of my exhaust run?

Any help greatly appreciated. Thanks and happy growing!

I’m trying to connect using an anemometer with calculating volume.

My fan speed is adjusted by an AC Infinity controller to maintain desired air temp/humidity.

Why would one exchange per minute be the magic number? Where does that come from?

hotrodharley said:

I’m trying to connect using an anemometer with calculating volume.

Click to expand...

What he said! Anemometers measure wind speed, not volume of air flow.

T.H.Cammo said:

What he said! Anemometers measure wind speed, not volume of air flow.

Click to expand...

you can calculate it from that.
wind speed thru a defined opening.

Humidity and temperatures gauges work well for optimal ventilation, adjust fan speed accordingly.

J. Rocket said:

you can calculate it from that.
wind speed thru a defined opening.

Click to expand...

Post the formula please.

Never mind. Here’s a calculator for it. Seems quite ridiculous. It’s not that critical at all. But for the rat turd crackers here’s another trail to follow.

The main issue after removing moisture down to the desired level is CO2. I bought an ink bird meter on special to test and see when the CO2 wasn't enough. I found even at 3 out of 10 with 400cfm max (120cfm max without factoring loss) that my CO2 level was still at regular ambient levels. Meaning even with 50 gallons of soil and large plants in an almost identical sized room CO2 supply was not a limiting factor.

So imo, even though you could calculate airflow speed through say your 4 or 6 inch exhaust in reality you just want consistent gentle airflow and the correct VPD. That is imo perfect or close enough.

hotrodharley said:

Never mind. Here’s a calculator for it. Seems quite ridiculous. It’s not that critical at all. But for the rat turd crackers here’s another trail to follow.

Air Flow Conversion Calculator - engineering.com

Air Flow Calculator - Determine the air volume flowing past a point in the duct per unit of time

www.engineering.com

Click to expand...

yup, going at it from the wrong end.
use fan speed to control RH and temperature first, if possible.

You need around 15 air changes per hour.

An easy formula is fan cfm multiplied by 60. Divide your answer by the volume in cubic inches. The answer is the ach calculated off the stated max cfm of the fan. Use your fan controller to dial lt back.