do potential dividers work pneumatically? - Page 1
Forum section: electronics / beginners

author: Capernicus
author_rank: Frequent Contributor
timestamp: February 25, 2022, 01:26:18 pm
content: I was just wondering if voltage dividers also work pneumaticly with the same equivalent circuit, except for electrical conductors you have air pipes?

author: Swake
author_rank: Frequent Contributor
timestamp: February 25, 2022, 04:58:57 pm
content: IMHO, The basic workings are certainly comparable. Same for hydraulics.

author: TimFox
author_rank: Super Contributor
timestamp: February 25, 2022, 05:28:26 pm
content: In most hydraulics, the pressure drop from one end to another of a pipe (with an incompressible fluid) is that due to gravity (if the pipe is not horizontal) plus a pressure drop due to flow that is proportional to the square of the flow rate, while Ohm's Law states that the voltage across a resistor is proportional to the current (to the first power).https://www.engineersedge.com/fluid_flow/pressure_drop/pressure_drop.htmThe situation is quite different at very low pressures (low density), where the molecules interact with the walls more than with each other. There, the flow rate is proportional to the first power of the pressure drop, a better analogy with Ohm's Law.In vacuum systems, this usually means at pressures below 0.1 mm Hg (Torr), or roughly 10 Pa.With two identical pipes in series, the pressure drops across each will still be equal, since the flow rate is the same.Note that this ispressure drop, the difference between gauge pressures or absolute pressures at the two ends of the pipe.

author: golden_labels
author_rank: Super Contributor
timestamp: February 25, 2022, 07:26:04 pm
content: In a pneumatic system, unlike with a hydraulic one, there is little flow. In pipes with no flow the pressure is the same everywhere,(1)which means the potential energy is the same everywhere, and that implies there is no potential divider.(2)I believe a more accurate analogy would be a single capacitor with some small (but complicated) parasitic resistance and inductance visible during charging.(1)Ignoring variations due to gravity and temperature.(2)At this point I make an assumption you do not mean a 20m tall vertical tube with tap at some height, but just any pipes.

author: CaptDon
author_rank: Super Contributor
timestamp: February 25, 2022, 08:01:04 pm
content: Short answer NO!! A resistive divider made with two equal resistors will show a mid-point voltage at the center connection which will vary a bit with load. At no load it will be at the mid-point always. How do you derive a mid-point pressure if there is no 'load' (flow). The pressure will be the same everywhere. With steady flow rates and valves you could do some adjusting to create a mid-point but that point will fluctuate wildly with varying flows. So is this a general question or do you have plans for something??? Fluids get real angry and turbulent going around sharp 90 degree bends and also can howl and whistle when passing through the straight section of a TEE. Kind of works like a real whistle. Electrons don't seem to care if your wire has a 90 degree bend or a TEE.

author: eugene
author_rank: Frequent Contributor
timestamp: February 25, 2022, 08:58:49 pm
content: The bookDynamical Analogiesby Harry Olsen 1943 covers the analogies between electrical, mechanical, and acoustic systems. It's an interesting read, especially if you're interested in acoustics.

author: IanB
author_rank: Super Contributor
timestamp: February 25, 2022, 09:13:00 pm
content: Quote from: Capernicus on February 25, 2022, 01:26:18 pmYes. And no. And it depends. Which answer would you like?If you consider air flowing through a tube, then pressure is analogous to voltage, and flow is analogous to current. If the tube is horizontal, then gravity will not affect the results. If the tube is very narrow (like a capillary), and the flow is low enough, then it will be in the laminar flow regime. Under laminar flow conditions in a horizontal, uniform tube, the pressure loss in the fluid will be linear along the length of the tube: it will obey "Ohm's law".[Edit: the fluid must also be incompressible, or at least the density must not change very much.]Under these circumstances, the flow of air is similar to the flow of current in a wire.However, if the diameter is large and the velocity is high, then turbulent flow will occur. Under these circumstances the drop in pressure will be proportional to the square of the flow, and it will not be linear any more.If the pipe is not horizontal then gravity will also affect the pressure. This can be neglected for air, but cannot be neglected for water.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 25, 2022, 09:14:13 pm
content: Pneumatics has the additional complication that the air expands when the pressure is reduces. With the usualy pneumatic pressure in the 2-8 bar range this change is significant. The analog works better with hydraulics.The transistion between laminar and turbulent flow is another complication.

author: IanB
author_rank: Super Contributor
timestamp: February 25, 2022, 09:16:04 pm
content: Quote from: TimFox on February 25, 2022, 05:28:26 pmNot always. The OP specified air (pneumatics), not water (hydraulics). When air is the fluid, the pressure drop depends on both the flow rate and also on the absolute value of the air pressure along the pipe.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 25, 2022, 09:17:13 pm
content: Yes, but: and, I think the above posts can be condensed as -- you don't, usually. The value for them is in transmitting pressures, and for hydraulic, volumes as well.One of the kind of interesting things about mechanics is, you're often working more on the positional side of things. Analogous to electric charge, whereas we more often work with current (flow of charge).That said, fluid transistors are a known and standard-ish sort of thing. The usual downsides apply: it requires bias flow, pressure drops in connecting tubes add in series, you need relatively large spaces (where still flow and less turbulence are present) to act as unipotential nodes for analysis and measurement purposes, etc. (Remember not just hydrostatic (gravitational) pressure, but also dynamic (ram pressure) and Bernoulli effects too.) And transmission line effects (acoustics) are much worse than for electronics (speed of sound is relatively low in relation to the size of components), as well as dispersion and losses. So, long lines are lossy, but reasonable quality resonators can still be made from cavities in either much stiffer, or much more flexible, materials. (Hmm, yeah. Come to think of it, bellows, elastic bladders, trapped air headspaces or gas pistons, etc. act much like lumped equivalent components. And like electronic components, will suffer from wave effects at high frequencies, so, frequencies well below there are fair game, given of course you don't run afoul of the numerous other modes that apply: splashing of free surfaces, turbulence, etc.)Tim

author: IanB
author_rank: Super Contributor
timestamp: February 25, 2022, 11:21:10 pm
content: It's worth noting that analog and digital pneumatics are a thing, similar to analog and digital electronics. Although pneumatics have today largely been superseded by electronics, they do still have niche and historical uses.I have worked with pneumatic logic circuits that have AND gates, OR gates, relays, switches, actuators, and all the usual stuff.In the analog world, pneumatic PID controllers are a thing. At one time, they were the main or only thing.https://www.emerson.com/en-us/automation/valves-actuators-regulators/controllers-instruments/pneumatic-and-process-controllers

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 26, 2022, 01:33:17 am
content: Indeed control theory itself was developed by none other than James Clerk Maxwell, studying mechanical governors; while an engine is a bit more comprehensive (multidisciplinary if you will) than say pneumatics alone, for sure the engine uses gas or other fluids, so is very much a relevant example in that sense.Tim

author: Capernicus
author_rank: Frequent Contributor
timestamp: February 26, 2022, 05:32:06 am
content: Thanks for the advice from everyone.So I take it in a certain situation a potential divider will convert across to gas, but u have to set it up right.That's cool then, thankyou.I'm interested in doing a pneumatic logic system, because not having to use a conductive medium for the hardware might make things easier in the long run for my business during the fab process. (in the near term electricity would in fact be easier for me, because its all more known to me.)I'm thinking of doing a pull system because it would be more like electricity. (Pull from the end, instead of push from the start.)And also doing things with "ac" instead of dc would simplify it by only making it a really short stroke for a cycle (similar to running it on a plumbers plunger), that might make things easier, (so maybe i could include some kind of capacitor type membrane, which would only be a really puny capacity. less than a femto if it were electrical.) but because air is soft the whole thing would have to be pumped up to high pressure to get it more like hydraulic response, which would be ideal.I need an equivalent pneumatic microphone, so that's where all my research is going, if I could get that, I think I'd free sailing then.so thanks for help, so voltage dividers do have an analogue in penumatics.

author: IanB
author_rank: Super Contributor
timestamp: February 26, 2022, 07:23:26 am
content: Quote from: Capernicus on February 26, 2022, 05:32:06 amYou mean like a speaking tube?

author: Capernicus
author_rank: Frequent Contributor
timestamp: February 26, 2022, 07:49:57 am
content: That's excellent, I bet understanding the workings of that machine would help me allot.Does it involve amplifying the voice to help it dig into the wax or does it do it completely passively?I'm looking more for of a pneumatic megaphone, that it takes in the signal, amplifies it then reproduces it.For sonic delay lines and a sonic based depth camera, then the rest could be mechanical it doesn't matter so much.Because I want to make a delay line with it, it will probably go back through a dac before it goes back into the feedback loopto get the digital error correction it would need.