Wednesday, June 29, 2011

The Modified Suction Pump


Low-cost negative pressure is only feasible with the availability of a suitable negative pressure source. Dressing materials, tubings, and reservoir bottles can be easily found or cheaply procurred. The most technically challenging part is getting the right kind of pump.

There are several sources of negative pressure:

a) Wall mounted suction ports
b) Commercial NPWT vacuum pumps
c) Large suction pumps designed for other uses (like suctioning secretions)

d) Modified, DIY (do-it-yourself) suction pumps

A satisfactory negative pressure source have the following qualities:

1. It can deliver sufficient suction pressure and flow rate;
2. It has a pressure gauge and regulator to allow the operator to control the pressure being delivered;
3. It can operate continuously for days or weeks on end without overheating or breakdown;
4. It is easy to operate, rugged, and requires little maintenance;
5. It has a low noise signature;
6. It has alarms to alert the operator in case of excessive suction pressures, reservoir overflow, etc.; and
7. It is light and portable

We now then proceed to how one can modify an electric air pump for NPWT uses. An aquarium pump is recommended because it can be easily found, it is cheap, and it is designed for continuous use for weeks or months on end using minimal electrical power and emitting minimal noise.

The most important part is finding the RIGHT pump. This means that the pump is easily converted into a suction pump. Some pumps only require you to attach a tube into the diaphragm pump's suction port in order to convert it. Other pumps will require extensive modifications.

Let us first look at the anatomy of an aquarium pump so we'll understand each

The typical aquarium pump has the following basic components:

a. Oscillating coil -- This is the main "motor" of the pump, although it has no moving parts. It generates an electromagnetic field that switches polarity 50-60 times a second (depending on the power supply in your locality). The magnetic field will cause the lever in the diaphragm pump to swing to and fro, by alternately repelling and attracting the magnet at the end of the lever arm.

b. Pump lever -- Has a pivot point on one end and a magnet at the other end. The magnet will be alternately repelled and attracted by the oscillating magnetic field, and as a result, the lever swings from side to side very rapidle. This motion will drive the diaphragm pump, just like a bellows.

c. Diaphragm pump -- It is called by a diaphragm pump because of a rubber cap connected to the lever arm. As the lever moves, the diaphragm also contracts or expands.

This diagram will better explain the pumping mechanism.

During the EXPANSION PHASE, the cavity within the pump expands because the rubber diaphragm is pulled up, thereby sucking air INSIDE the pump via the suction port. Air is not sucked inside via the Outlet port because of the one-way valves present.

During the CONTRACTION PHASE, the rubber diaphragm springs back in, making the cavity smaller and causing the air inside to get pushed OUT via the Outlet port. Air does not get squirted out the Suction port because of the valves.

Now that you understand the basic mechanism, the next step is to look for the SUCTION & OUTLET ports. In aquarium pumps, the Outlet port is very easy to find. Just look for the plastic tube where the rubber tubing is attached. If you're not sure, simply turn on the pump and see where the air is jetted OUT.

The tricky part is looking for the Suction port. If you are lucky, the suction port is also a plastic tube. If this is the case, all you have to do is to attach a rubber tubing to the suction port and you will now have a suction pump.

In most pumps, however, the suction port isn't a tube; it's usually just a small hole at the bottom of the pump. For you to convert it into a suction pump, you must:
a) Plug up/cover the old suction port; and
b) Place a new suction tube and seal it properly.

You can do both in one step, but you have to do both PROPERLY. Otherwise, you pump will leak and will have greatly reduced suction pressure.

The first thing to do is examine the layout of the pump. What you want to do is place the NEW suction port in the RIGHT place. Examining the pump's internal layout will tell you where to put it. It should be easily accessible so you can easily attach (or RE-attach) a rubber tube and where it won't get in the way of other parts, especially the Lever Arms.

Mark the spot using permanent ink. Then remove the whole pump and drill an appropriately-sized hole in the pump body. Before you drill the hole, check the inside of the pump to see if the hole will not disrupt the internal valve layout. Doing so will cause problems with the pump's proper functioning. It's also wise to have the suction tube/port of choice ready, so you'll know what drill bit size to use. A 3/16" copper tubing is routinely used, because it's commonly available, bendable, and fits most rubber tubings. If you are not sure of the size, it's wiser to use smaller drill bits, then progressively enlarge it until you get the right-sized hole.

Make sure the copper tubing and pump body is clean and free from dirt, oil or grease so that the glue will stick properly. I use a 5-minute epoxy because epoxy doesn't expire after a year or two like super glue. After placing the glue to secure the tube, place some glue to cover and seal the smaller original suction slot. Once the epoxy sets, apply a layer of silicone sealant over the epoxy. This ensures that even if the epoxy bond fails with prolonged vibration, the silicone will maintain the airtight seal.

Once everything is dried, re-attach the pump body to the casing. You may need to check if you have done the conversion properly, so attach a rubber tube to both suction and outlet ports and turn on the pump. Cover the suction port tube, then see if air comes out of the outlet port tubing. Do this by immersing the Outlet port tubing in a glass of water, so even minute amounts of air can be detected as bubbles. Theoretically, if you have a perfect seal, once you completely cover the suction port, no air will come out the outlet port.

The basic conversion is then done. you attach the suction port tubing to the reservoir, which is in turn connected to the dressing in the patient.

Additional features you might want to add:

1.) Vacuum Gauge -- Ideally calibrated in mmHg, this allows you to find out how much suction pressure is actually being delivered.

A vacuum gauge can be found in stores selling pneumatic and hydraulic fittings. One will cost around $10-15. You may need to by additional brass fittings that will allow you to attach a tubing at the end of the gauge (and additional $2-5). If you want to save money, attach a 3/16" copper tube at the end and secure with epoxy.

2.) Pressure Regulator -- It allows you to control the amount of suction pressure being delivered. It does this by bleeding air in from the outside. When fully closed, you get the maximum pressure, when totally opened, suction is almost down to zero.

Simple, cheap regulators can be found in petshops, the same source of the aquarium pumps. They cost as little as $0.25-1.00 each. Fancier metal ones cost more.

To connect everything together, use a y-connector or cross connector: Attach the pump's suction port tubing, gauge, and regulator to the connector. Please take note that one end of the regulator must remain free (i.e., not connected to anything).

Now you have a basic NPWT pump.

Have Pump, Will Suck.


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