How are fan static pressure measured? - Page 1
Forum section: electronics / projects

author: tske
author_rank: Regular Contributor
timestamp: March 29, 2025, 02:31:15 pm
content: I'm looking at industrial DC fans on various parts distributor website, and I found static pressure generally go down with physical size increased, which is really strange.I was assuming bigger size will give more efficiency, but seems bigger size only help air flow, but decrease static pressure?Or is the measurement method messing with me? When measuring static pressure, do they calculate in such a way that physical size are compensated?

author: TimFox
author_rank: Super Contributor
timestamp: March 29, 2025, 02:38:52 pm
content: My take is that the "static pressure" rating is how much back-pressure is required to stop the airflow: with larger area, less pressure (lb/in2) is required for a given flow rate.

author: MathWizard
author_rank: Super Contributor
timestamp: March 29, 2025, 03:12:49 pm
content: You know how they measure thrust with little rockets/fireworks in a lab, attached to some fixed 'strain gauge' or some such thing ? How much thrust do little fans make ? Would they call that static pressure ?

author: TimFox
author_rank: Super Contributor
timestamp: March 29, 2025, 04:39:08 pm
content: “Thrust” is force.“Pressure” is force per area.

author: CaptDon
author_rank: Super Contributor
timestamp: March 29, 2025, 11:51:33 pm
content: Static pressure and flow rate are related so a graph would be proper. If you dead head most fans the fan will unload, the pressure will drop and the RPM's may increase. As you begin to unblock the fan the pressure goes up rapidly then drops off. As you can see you would need a graph to accurately predict back pressure vs. airflow. You can usually gain a substantial increase in pressure by connecting two fans in series as close to each other as physically possible. Also worth investigating is the characteristics of forward and reverse incline squirrel cage blowers. They are radically different!! Eimac 'external anode' transmitting tubes (4CX250A, etc.) datasheets list both an airflow requirement based on anode wattage as well as the backpressure the tube will present at that airflow. Backpressure for small fans is usually measured as inches of water column or simply 'Inches W.C.' on the graph.

author: Konkedout
author_rank: Frequent Contributor
timestamp: March 30, 2025, 05:24:31 am
content: I think that static pressure is measured without airflow, right? So with the airflow completely blocked. Measured in Pascals (I just Googled the question of metric units of pressure). I do not know what the pressure meter looks like, but the measurement sounds conceptually simple.This question piqued my interest because we bought a new heat pump system 2 years ago. The air filter is 20 inches square x about 5 inches thick and is relatively expensive depending upon the "MERV" grade. (The "MERV 13" is about $60 US each. That was before tariffs.) The heat pump system has what is supposed to be some sort of intelligent system for telling me when to replace the filter. But apparently this does not work well and the local contractor tells me to hold the filter up to daylight and see how much light gets through. Hmmm.I had the idea of constructing a large box with a fan in the bottom, the filter under test would be sealed against the inside walls in the middle, and some sort of gauged flap on top which lift angle depends upon airflow. While this would not be accurate in any sort of absolute sense, I think it would be possible to make a comparison (relative measurement) between a brand new filter and a used one, or a brand new one which is 50% blocked by a sheet of paper for example. Given the size of the filter, this contraption would be large. This would not all be top notch measurement accuracy but might be much better than just holding the filter up to see how much light passes through.

author: coppercone2
author_rank: Super Contributor
timestamp: March 30, 2025, 05:32:43 am
content: Quote from: MathWizard on March 29, 2025, 03:12:49 pmlol what if you put a pinwheel firework on a torque wrench?I imagined a new technology, a pyrotechnic fastener, you light a fuse and it tightens a lug nut on a truck, or loosens onemaybe its the missing link for large scale heavy space construction? Then you don't need to worry about trying to hold a drill, wrench or special fixturing, just finger tight and igniteput a tightness OK indicator of some sort on it and send every box out with a magnifying glass to light the fuse and its goodor maybe it would come with a tool that you attach to your threaded rod with a magnet so it can spin down a track to build up speed and momentum and fly into the threaded rod. Just a index hole and a few pins + magnet. On the other side, pyrotechnic rocket bolt could just spin into the hole...its the next step beyond those primitive pyrotechnic nail guns.And if humans sucked alot less, you could get small shaped charges for drilling holes and then rocket taps for threading them.... i think that's what you need to put people back in the trades, instead of trying to entice them with a magnetic drill they can have a celebrationconstruction workers of ringworld anyone? niven you wanna write another book?is that what a self sealing stem bolt is?

author: CaptDon
author_rank: Super Contributor
timestamp: March 30, 2025, 06:19:17 pm
content: Static pressure entirely depends on airflow as I mentioned above. As for filter change outs, in industrial settings there is a device called a 'Magnehelic Gauge' generally about 5" in diameter and usually marked with 'water column' references. They are sealed units designed to measure differential pressures and as such can measure a tiny 0" to 4" water column differential even when working at 10 P.S.I. They work equally as well to measure differential vacuum. The absolute measurement of a fans 'static pressure' must be based on airflow as no fan is designed for zero airflow so a dead headed measurement is of no value. This is also true of fluid pumps. When comparing fans that simply state 30CFM you need more data such as a standard reference often 2" water column. A standard of reference is required otherwise you are comparing manufacturer 'A' Apple ratings to manufacturer 'B' Banana ratings. My most serious fan was a 300HP electric motor with a VFD driving what for the most part looked like the compressor section of a 5000 pound thrust JP4 fueled jet engine. It had a 24" diameter upflow inlet and it could hold a red brick against the inlet screen. The 36" inlet at the service point could ingest a human being. It was part of a unit called the R.T.O. and measured about 40 yards by 20 yards and tall enough to have catwalks.

author: timeandfrequency
author_rank: Frequent Contributor
timestamp: March 30, 2025, 06:48:58 pm
content: Hello Konkedout,Quote from: Konkedout on March 30, 2025, 05:24:31 amI'm not convinced either. Actually a HVAC guy should know how to accurately check if a filter needs replacement.And what if the replacement has to be done during the night ? Will he wait until sunrise ?Differential manometersare really useful for measuring filter clogging. Last digit has to go at least down to 1 Pa (1 Pa = 0,00401865 inch W.C.).Sometimes the filter manufacturer might give a diff pressure ballpark value to indicate when the filter has to be replaced. You may contact him and ask for that information.Physics-Based Degradation Modelling for Filter CloggingBack to the thread's subject :Understanding the Basics: Static Pressure and Airflow

author: Perrold
author_rank: Contributor
timestamp: March 31, 2025, 05:15:44 am
content: I've always assumed all those fan curves are fit to actual data generated with a pitot probe with an additional dedicated dP transducer between the static port and ambient/fan inlet. Put a fan in a duct test section, vary backpressure, collect data, make a graph. Two dP ducers give you flow rate data and fan static pressure, enough to fully quantify fan behavior that the user cares about.

author: Berni
author_rank: Super Contributor
timestamp: March 31, 2025, 05:35:43 am
content: Yep there are machines that are used to test the pressure/volume curve of fans.But for just measuring static pressure, that's easy. Just mount the fan onto a closed box and then use a pressure gauge to measure how much higher the pressure inside the box gets versus ambient pressure. Tho you need a very sensitive pressure gauge since regular fans can only produce a tiny amount of pressure (they sell these gauges for HVAC servicing). Tho if you use a powerful blower style fan you can get some pretty impressive pressures.The reason static pressure is a useful piece of info is when fans are mounted to heatsinks/heatexchangers with dense fins (or have to blow trough densely packed components). There a high static pressure fan is able to generate more airflow. A graph is better to have, but generally high static pressure tends to mean more pressure across the board.

author: tske
author_rank: Regular Contributor
timestamp: March 31, 2025, 11:24:51 am
content: Thank you all!Excuse me for my fool, but I still don't understand something.Let's say I have two boxes with exact same dimension, each box have an same-size opening A and B on the same place.And I have two fans, they have the same static pressure rating, but different in size.Now I mount the two fans on the opposite surface of each box, with the mounting-opening C and D accommodate to the fan size, so fan blowing area is not blocked, and sealed well.Now when testing from opening A and B, do they have different or the same static pressure?

author: Berni
author_rank: Super Contributor
timestamp: March 31, 2025, 02:49:07 pm
content: The static pressure rating is measured at 0 airflow.So you will only be able to reproduce the spec numbers if your box is fully sealed.Tho you could have two fans "battle it out" by configuring them to both blow into the same sealed box. If they have the same static pressure rating then there will be 0 flow trough both of them. Does not matter if one fan is larger than the other. But if one fan has a higher static pressure rating then the air will start moving backwards trough the lower pressure rated fan as the more powerful fan will overpower it (again size does not matter, you could have a small fan overpower the pressure of a large fan)In general fan diameter doesn't give any particular advantage in static pressure. Larger fans mainly have a big advantage in flow rate as they are pushing air along a larger area. This area goes up quickly with the square of diameter, so you can often also run large fans at a lower RPM and still get the desired flow rate. This is why bigger fans are usually desired in PC cooling as running them at a lower RPM makes them a lot quieter.

author: tske
author_rank: Regular Contributor
timestamp: April 01, 2025, 01:05:03 pm
content: Quote from: Berni on March 31, 2025, 02:49:07 pmThis is a clear answer, thanks a lot!