The Low Pass Filter Effect

Low Pass Filter Effect on High Speeds

The Venus 2000 operates over a wide speed range. At low speeds the basic design of the system is adequate to handle the full range of movement of the diaphragm and the amount of air displaced, even with the longest of the stroke settings on the drive wheel. At slow speeds, these air pressure fluctuations are efficiently delivered to the receiver through the 3/8 I.D. of the receiver hose and the constrictions of the tubulations on each end..

But as stroking speeds increase, you can see a noticeable change in the length of the produced receiver stroke and a change in the stroking efficiency. This is because of an effect called "low pass filtering". Basically this means that when viewed as a dynamic system, the system is different than when viewed as a static or fixed system. And the difference is more pronounced at higher speeds.

Skippable Grizzly Details

The air volume changes produced in the diaphragm motion are transmitted to the receiver volume through a length of hose. This hose can be viewed as an air flow resistance. The volume of the diaphragm can be viewed as a capacitor. The volume of the receiver can be viewed as a second capacitor. The motor can be viewed as an A/C voltage generator. The air flow back and forth as an A/C current generated.

The way this is hooked up, this combination adds up to what an electrical engineer would recognize as the equivalent of a low pass filter circuit.

When running at low speed, the hose has no problem moving virtually all the air being displaced by the diaphragm's back and forth movement, through the receiver hose and into and out of the receiver. But as stroking speeds increase, the resistance of the hose becomes too large, and instead of moving all the air back and forth through the hose, more motor energy goes into generating back pressure in the diaphragm, compressing the air in the diaphragm since it just cannot be squirted in and out through the receiver hose resistance fast enough.

Less total air movement at the receiver end of the hose means less response from the receiver regardless of how furiously the power unit slugs away. If you wondered why the hose on the Venus 2000's air line from the power unit to the receiver was so large, a full 3/8 inch I.D., this is why. To keep the air flow resistance down and maintain reasonable performance at high speed. It is a tradeoff between something massive like a 1 -inch I.D. hose which would have great high speed performance but be completely unworkable because of its size and strain on the receiver, and a small 1/8 inch I.D. hose that would be great in terms of size and easily ignored by the receiver, but strangle all but the slowest speed performance.

The use of the 3/8 inch hose obviously was selected carefully as it is a good match for overall performance. If one is really fond of the higher speed ranges on the Venus 2000, there are several things you can do to optimize performance at the faster speed ranges.

First, you can use as short a hose as is reasonable. Shorter length means less resistance to the reciprocating air flow from the same diameter receiver hose. This should help a bit. The diameter of the stroke control hose, the 1/8 inch one, is largely irrelevant since it does not move any of the air used in the power stroke and adds little to the total trapped air volume.

You can gain a tiny additional bit by using a taper reamer or round rat tail file to clean out the inside of the hose tubulation on the receiver body itself. There is a nasty burr at the entrance that constricts the diameter there for no good reason. I have reamed out all the tubulations on my receivers. The gain is probably small, but I like the cleanliness of the reamed out tubulations.

Also you should use one of the shorter stroke settings on the drive wheel for faster speeds. The longer stroke settings at high speeds are just going to back up more air and more backpressure. The motor will run better at higher speeds with the shorter stroke setting, and be more tuned to the amount of air it can move in and out of the 3/8 inch hose on each cycle.

More trouble than it is worth category

Changing to a larger receiver hose would be a big deal! Simply kludging on a fatter hose with a couple of adapters is not going to work.

The fitting (tubulation) on the diaphragm housing would have to be changed (enlarged) and the fitting (tubulation) on the receiver would also have to be somehow made bigger in I.D. Also the weight and strain of the larger hose would probably be such a hassle as to prove to be a really bad idea. There would start to be a serious problem with wall thickness on the larger hose as well. If kept reasonably thin to be flexible and light, you would end up with the hose itself acting as a receiver liner and soaking up a significant portion of the pressure fluctuations, trying to collapse and balloon, instead of transmitting the pressure fluctuations efficiently.

It is hard to imagine you could change to anything much larger than 7/16th inch. Probably not worth the large amount of modification to the power unit and receivers to use the larger hose. If you did not change the fittings themselves, simply going to a larger hose would not help that much. All the constrictions from diaphragm to receiver liner would have to increase in size.

One significant thing you can profitably try is to decrease the total trapped air volume. To keep lively movement of the receiver, decrease the trapped volume in the receiver itself, the volume or capacitance which is at the end of the hose. Since reducing the air flow resistance of the receiver hose is not all that practical, you can attack the "capacitance" at the "load", the receiver end of the hose. This is most easily done by using a 2-inch housing if at all possible. You should also consider cutting down the housing to the minimum reasonable length for your particular fit. For instance shortening a seven inch housing to 6.5 inches. The low pass filter is made up of the combination of the receiver capacitance and the hose air flow resistance.

Decreasing the trapped air volume at the receiver end of the hose should noticeably change the responsiveness of the receiver at the higher speeds. It will be livelier and keep up with the increasing speed a bit more effectively. It will reach a higher stroking speed before its stroking length starts to decrease due to the low pass filtering effect. There is a limit to how effective any of this can be.

So if you are into high stroking speeds:

	Use as short a length of hose as reasonable.
	Clean out the tubulations with a taper reamer, or round file.
	Use one hole shorter stroke on the mechanical drive wheel setting.
	Use as small a diameter housing as possible to keep down trapped receiver air volume
	Consider whether or not the receiver housing could be a bit shorter.

Even if you are mainly into the slowest stroking speeds, you will find that reducing the trapped air volume in the receiver, at the far end of the hose, will stiffen the action, it will make it less "mushy" and more responsive.

If you find you are able to move from a 2.5 inch to a 2 inch housing, you should find you can also change to a more inner, and shorter mechanical stroke length hole setting on the drive wheel, changing the mechanical advantage on the pump mechanism itself. This change will slightly increase the power on the stroke and should even decrease the low end stall speed.

For the same reason you will find it worthwhile to not use a receiver any longer than it really needs to be. With care and a decent hacksaw, you can easily cut a bit of one end or the other of a receiver, though you need to be very careful about shortening the end with the tubulation. There needs to be a fair amount of plastic past the tubulation to allow for mounting the constrictor and the liner on that end.

08/04

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