The Physics of Water Pumping

After much browsing around about pumping it has become clear to me that while many members understand the mechanics of pumping, many (possibly most) do not. I was writing this post as part of a larger project designed to illustrate how connective tissue is permanently elongated/expanded in the body, which stretching modalities work best to achieve it, and the misconceptions we often have about the process. This ongoing project is based on research and empirical evidence (not specifically regarding the penis, of course, but connective tissue as a whole), and originally I was going to include what I am about to post here as a section regarding pumping. As it worked out I could not find any research examining the effect of vacuum on permanent deformation of tissue. In fact, most research involving tissue and applied vacuums considered closing wounds faster and more efficiently - interesting, but far from our purposes here. So, without finding any research, all I was left with was the application of some physical laws, creating a theoretical model for the effect of pumping. I do believe this theory is sound because it is completely based on physical laws and principles that are, I feel, correctly applied. I hope this can help shed some light on how pumping actually works and the reasons why pumping with warm water in the cylinder is probably preferable.

Water pumping is usually considered superior due to two properties of the water used to fill pump cylinders:

1) The water is warm – the benefits of heat are basically a given when talking about tissue expansion/elongation/deformation. So many here at Thunders have discussed this and presented evidence backing it up that now the bottom line is simply: heat works, use it. In terms of expansion, heat allows it to happen more readily and makes the whole process safer. That is really all I’ll say about heat here as a full discussion of heat will be beyond the scope of what I am trying to show here. Any further mention of heat will merely be to supplement my analysis of the second property of water used to pump.

2) The kinetics and attraction of water molecules in the liquid state (what most people here are talking about when they say, “Water works better because it is incompressible”) – this will be the focus of my analysis here. The basic fluid dynamics of water (or any inert liquid, really) makes it an ideal pumping medium. Hopefully this will explain why and put some misconceptions to rest. So without further ado, the physics of water pumping:

I’d like to start with a common misconception: vacuum pressure is vacuum pressure, regardless of the pumping medium. This idea is both true and false, but what is true about it is rather irrelevant. The idea that “pressure is pressure is pressure” is based on Pascal’s Law which states that wherever pressure is measured in a system it will be equal if the system is closed and the fluid (either liquid or gas) within is at rest. Thus, it can be concluded from this that as far as the measurement of pressure goes, pressure is pressure. As pressure is defined as the force exerted equally in all directions against the walls of a system, it doesn’t even matter whether the system is filled completely with a liquid or gas, pressure will still be equal throughout. Even though gasses are very compressible and liquids are almost completely incompressible, the only difference will be that a much smaller volume of liquid will need to be evacuated from the system to create a similar level of vacuum when compared to gas.

The important point to make to the pumper - where pressure is not simply pressure - has nothing to do with measuring pressure, but instead has to do with the way these two fluids (air and water) each behave kinetically at the molecular level causing them to manifest very different properties under a vacuum. It would still be nice, however, to base the discussion on something that can be measured so that we can at least see something roughly quantifiable. So if pressure is not a good measurement, what is another measurement of vacuum quality that will work? We need to examine Mean Free Path (MFP), a common measure of vacuum quality in physics and aerodynamics.

Mean free path is essentially the distance one particle of a fluid must travel, on average, before encountering another particle. This measurement is closely related to density, but takes into account some other factors which make it more useful in analyzing vacuums, namely temperature/excitation (the speed at which particles are moving at), particle size, and the size and shape of the system. MFP is seldom used when speaking about liquid fluids as their MFP is so low it can generally be considered to be zero, the particles are always touching or extremely close. This, however, definitely is not the case with gaseous fluids. As a partial vacuum is created in a closed system filled with a gas by evacuating some of the gas molecules, fewer and fewer molecules are left behind, the average spaces between them become greater (lower density), and the distance each individual particle must travel in order to encounter another particle becomes greater (higher MFP). To give you an idea of how widely the MFP of partial vacuums filled with gasses can vary check out this example: the MFP at low vacuums (like in our air pump cylinders) can be measured in nanometers, while the MFP in the near-perfect vacuum of deep space is larger than the diameter of many planets. The point here is that gasses will always expand to fit their container no matter how low this brings their density or by how much it reduces their particle interactions (MFP-derived). Liquids, on the other hand, will retain their density and volume, and have a mostly constant, near-zero MFP in a vacuum – they will be in a constant state of particle interaction.

This constant, near-zero MFP of water is due to the molecular attraction and molecular interaction of water. When water freezes (becomes a solid) there are strong attractions between the molecules, they form an orderly lattice structure and just sit there and oscillate a bit – no moving around one another. As energy is applied to the ice and it begins to melt, the molecules begin moving enough to break free of the lattice structure and gain the ability to move around each other in a disorderly pattern, but the molecular attraction remains strong enough to hold the water molecules very close together, allowing almost no compression or expansion and keeping the MFP at basically zero. Hence, liquids take on the shape of their container (molecules can “flow over” one another) but do not expand to fill it (strong molecular attraction), while gasses do both.

The molecular attraction of liquid water is so strong that as a partial vacuum is created in a closed system filled with water the MFP will remain nearly zero, increasing only imperceptibly with the vacuum level, until the vacuum level becomes strong enough to begin overcoming the molecular attractions completely, changing the liquid water into water vapor (i.e. as the vacuum level rises, the vapor pressure of the water equals than exceeds the ambient pressure of the system, to put it in physics terms). So, if you have water in a closed system at room temperature and steadily increase the vacuum level in the system, the water will expand only an imperceptible amount until it begins to boil at room temperature as it vaporizes into a gas. This relationship is also why water boils at a lower temperature at higher elevations where atmospheric pressure is lower. I bring this up to show that at no vacuum we are likely to use while pumping, will water lose its molecular cohesion and desirable properties.

Water’s inability to appreciably expand is what makes it such an ideal pumping medium. To illustrate this, let’s take a look at how penile expansion happens when you pump with only air filling the cylinder. First, however, we need to dismiss another misconception. When pumpers talk about vacuums they like to talk about “pulling” the penis into an expanded state, like thousands of little hands grabbing and pulling out laterally. Though this language is perhaps useful in general discussion, it is not the true pathway which causes expansion. In reality, expansion takes place when pressure within the penis (standard atmospheric pressure) becomes greater than the pressure in the tube. The higher pressure in the penis pushes outward but encounters less resistance, causing expansion. That is, when a vacuum is created around the penis, fluid is hydraulically pushed into the penis, the expansion force is both pushing and internal. This is in accordance with Bernoulli’s Principle. Understanding this, we can examine the mechanics of pumping with air:

1) As the partial vacuum is created, fluid is pushed into the penis through the most immediately available pathway: blood from the central cavernosal arteries floods the erectile chambers. The maximum quick expansion of the erectile chambers, however, is dictated by the elastic expansion range of the tunica albuginea. That is, the penis expands until a maximally erect state is achieved. This happens very quickly as the vacuum is raised higher and higher. Beyond this point (maximum elastic stretch of the tunica), the erectile chambers of the penis will no longer expand so readily, so fluid begins to be pushed into other areas as well.

2) If a vacuum still exists around the penis, the penis will continue to expand. The expansion, however, cannot come from simply filling the erectile chambers with more and more blood due to the resistance of the tunica. So where does the extra fluid come from? Well, if enough expansion force is being exerted constantly on the tunica, the tunica will undergo viscoelastic creep, continuing to slowly expand and allowing more blood into the erectile chambers. If you are able to make permanent gains from pumping, this is probably the mechanism that will prompt the necessary tissue remodeling. Also during this time period, lymph and other fluids are able to seep into and fill the soft tissues between the tunica and skin, creating further – and usually undesirable – expansion. This is what is generally referred to as fluid retention. Because these other fluids must cross semi-permeable membranes to fill the penile tissues, their accumulation is more difficult to achieve at a fast rate and is therefore usually associated with vacuums that are too high and spending too long in the cylinder in a single stretch.

Note: both of these processes are happening simultaneously, but the proportion of fluid retention per unit time will increase greatly after the elastic threshold of the tunica is met and the penis is allowed to continue expanding.

Knowing the mechanical process, what is it about water that makes it so much more desirable as a pumping medium? It is primarily its almost complete inability to compress or expand, as illustrated in the discussion of MFP, which makes it ideal for maximizing the desirable mechanics of pumping.

When air pumping, the penis will expand outward, encountering only a soft, extremely compressible “cushion” of air. After achieving a fully erect state, the vacuum will continue to cause the penis to expand into this cushion, compressing the gas and reducing the pressure differential. This continued expansion, depending on the relative strength of the tunica, will likely be caused by a high proportion of fluid retention/inflow in the soft tissues between the tunica and skin.

When water pumping, on the other hand, the penis will expand outward to a fully erect state, and encounter an incompressible “wall” of water. This wall essentially stops further expansion from happening at an unrestricted rate. If not overzealous with your pumping intensity, this will prevent most fluid retention as there is really no place for the fluid to go – significantly more force is required to shove fluid across membranes into these constricted spaces. Water can slowly be evacuated from the cylinder at this point with the aim of creating a slow further expansion, just fast enough to keep up with the viscoelastic creep of the tunica. Here, the heat of the water also aids in further and easier tunica expansion. In this way, water pumping doesn’t directly “pull” more on the tunica, as many have said here, instead it limits fluid (other than blood) buildup/inflow, allowing more of the total expansion achieved during the pumping session to be from tunica creep.

Just because water retains its density and near-zero compressibility factor in a vacuum, there is still a vacuum, so the penis will continue to expand. If this expansion happens too quickly, there will still be fluid retention, though still probably less than pumping with air at similar pressures. What this means is that water pumping can bring you to a fully erect, maximal elastically expanded state rather quickly and at a slightly lower vacuum than air, but to reap the most possible benefits from the use of water, the vacuum should be increased very slowly from this point to the maximum negative pressure you are going to use for that individual pump session. On subsequent, successive pumps, because connective tissue will retain a great deal of viscoelastic relaxation during a short break, the preliminary vacuum and expansion can probably be a little bit higher each time as the tunica has increased ability to initially expand. After this point, however, the vacuum should be raised slowly once again until you achieve your maximum vacuum level once more.

I hope all of that made sense…

Roots

Good read! Pretty technical but very informative. Makes me want to pull my bathmate out again!

Excellent Roots, hopefully this will stop guys talking about partial vacuums pulling on the skin and the force not making it through to the tunica etc.

It’s only ever internal pressure that’s at work as you’d discover if you tried pumping your dick in outer space :D

Also the compression effect water has on the skin, you can feel it if you put your hand under water, it’s exactly why you get less bruising and edema from water pumping.

Great contribution.

Your phrasing seems to imply that for equal pressure, water exerts more force on a surface than air, which is impossible, as per the definition of pressure. Additionally, more often than not there will be air somewhere in the system, negating the proposed effect of the incompressibility of water.

Pressure very much IS pressure. While using water as a medium results in a total of more molecular collisions, we’re really only interested in the average effect, which is exactly what the gauge measures. The molecular behaviour here is irrelevant.

The largest macroscopical difference between the two lies in the pressure profile. Whereas air pressure will be equal all throughout the tube, the mass of water creates a pressure gradient (just about 17cm of water is required to raise pressure by 0.5”Hg). The gauge reading is equal to the partial vacuum at the surface of the water, below which it drops linearly.
At equal gauge readings, water pumping creates a smaller pressure difference than air. This effect becomes relatively smaller as you increase the partial vacuum.

So at a gauge pressure of 3”hg the actual pressure if most of the system is full of water will be slightly less ?

I have always thought the gauge measure the pressure at any point inside the tube. If what Serenity says is true, than with water pumping you should feel the pressure more around the tip and less at the base. Does this match with the experience of pumpers? I have to say that air pumping gives a more even pull compared to water pumping actually. The problem with air pumping is that pressure is not constant, it changes from a second to the next so you have to always pump.

Roots very impressive post and the sort of stuff to get a better understanding

I would like to examine the difference between air and water pumping in more detail.

From a static point of view what you write in the quote extract above looks correct - ie the interaction of the penis expansion and the vacuum differs between air and water vacuums. However even in static pumping any reduction in vacuum caused by the penis expansion is nullified by continued pumping to maintain the desired vacuum so I am not sure the above tells the whole story.

However I suspect the differences you describe between water and air pumping related to compressability might be much more relevant in cyclic water pumping as you can achieve with the Mr Fantastic water pump. This type of pumping presumably causes pressure waves which would impact much more on the penis in the case of the water pump because of the incompressability of the media. What do you think?

Regards
Austfred

Marinera,

I have not found that with the Mr Fantastic water pump. I feel more pressure further back towards the base after I have been in for a while (usually after 30 mins). With this pump the pressure is varying constantly with a cycle period in my case of less than 1 minute.

Austfred

Yes. If you have 20cm of water in a tube placed vertically and pump to 3”Hg, the partial vacuum at the bottom should be around 2.4”Hg.

Pressure distributes itself uniformly in the absence of body forces (like gravity!). When you account for gravity, pressure will always be higher below a volume of any mass (including atmospheric air). For most gasses in small systems it’s a negligible difference. Water, however, is heavy enough to have a noticeable impact.
An experiment to try: Put on a pump completely flaccid and pump up to 5”Hg. Keep the gauge there and see how long it takes to reach an equilibrium (where you stop getting longer). Now try the same with water in the tube and compare the times. You’ll probably notice that not only does it take longer, you won’t reach as far into the tube.

Once you get to higher vacuums, like what’s often used in dynamic pumping, you probably won’t notice the difference (10”Hg vs 9.5”Hg). I personally always felt pumping mostly in the glans but I suspect this is just because it’s more sensitive.

Wait Serenity please, let’s try to explain this with the most simple words. The same (de)pressure read by the gauge will be measuring the force the penis is pulled with, at any point in the case of air, whereas, in the case of water,it will measure the force that is pulling the penis only where the is no water (it will be lower where there is water). Is that right? So, contrary to what many believe, the same nominal pressure will be more effective with air pumping than with water pumping? Am I getting this right?

If so, this should lead toward the consequence that air pumping is, ceteris paribus, more effective than water pumping, because the amount of pressure in the latter will be limited by the amount of force that the glans can stand (supposing you are pumping vertically).

Exactly!

I don’t know if it’ll be more effective, though. The pressure rises gradually from the surface of the water to the bottom. There could be some therapeutic effect to having this difference along the length of the penis (though I don’t know what that would be).

And again, it’s only some 0.5”-0.7”Hg. For high gauge readings I don’t think it makes much of a difference. But yes, if you want to reach the max pressure you can tolerate and you’re limited by the glans, air will affect your whole penis a little more than water.

I do think, though, and that’s the reason I brought it up, that when people first try out water/air at 3-5”Hg and realize that they can go longer with less edema/capillary damage in water, it’s because of this effect.

Yes, the ‘felt’ pressure changes with cycling. Lower pressure is felt lower at the base, while as pressure goes it’s felt at the glans. Although, as Serenity stated, this might be due to the glans being a sensitive area to start with.

Neverthless pumpers seem to report longer length with the water than with air.

Water exerts more force: this is exactly what I’m saying does not happen. They exert the same force because of Pascal’s Law and the definition of pressure, but my point is that this is largely irrelevant as more than expansion force is important here.

Air in the system: basically true, but I spoke about how I was talking about systems completely filled with one type of fluid or the other (liquid or gas). I got my pump out the other day and managed to keep the air out completely, or at least visibly, and when I removed the brake bleeder there was no longer air in that system. Anyway, even if there is air, the density of water is still important as expansion force must be transmitted outward through a dense and heavy medium which is pushing down due to gravity and is very incompressible. Also, the amount to which air mitigates the true incompressibility of water depends on how much air you allow into the system as that will dictate the TOTAL compression that can occur in the entire system. So less air = less compression system-wide.

Water pressure/weight: exactly right. Because we are not doing this pumping in a vacuum, gravity comes into play. The weight of Water creates positive pressure just as it does in the deep sea, and this adds compression to the penile tissues. This follows from the high density of water -it is much heavier than air as a result. So this is important, but not the only factor here.

Great post, Roots. Thanks for all the time I’m sure it took.

You’ve detailed what was basically my assumption about the differences between the two types of pumping. I’ll mention too that this lines up with personal experience: over the same duration and vacuum level, water pumping results in less fluid build-up than air pumping.