Diaphragm-type vacuum pumps are often ideally suited for a variety of medical devices. Unfortunately, flow pulsation can offer a challenge to developers, specifically in flow measurement and control. This article will focus on some simple techniques for reliable and repeatable measurement of flow and pressure, particularly where the pumped media is air or an otherwise compressible gas.
Diaphragm-type vacuum pumps and compressors are well suited to a variety of medical device applications due to their ability to operate efficiently over a broad range of flows and pressures while keeping the fluid path completely sealed. A downside is flow pulsation—a limitation characteristic of most reciprocating pumps—which can create challenges for systems integrators in terms of flow measurement and control.
|Figure 1: Pressure test setup|
A proper test setup for evaluation flow and pressure performance for a pump working against a compression load is schematically depicted in Figure 1. For flow measurement, a conventional rotameter is depicted. Rotameters are simple, reliable and reasonably accurate (plus/minus a few percent) devices that typically utilize a ball, or “float,” that rises in a tapered tube as the flow increases. As reliable as rotameters can be however, to produce accurate results, they require smooth flow. They are also typically calibrated for operation at standard conditions. The test set-up depicted achieves the former by including a rigid volume, or “tank,” of sufficient size to damp out the inherent pulsations in flow produced by the compressor.
Note that the tank is positioned downstream of the loading valve so that it is not subjected to the same pressure load as the compressor under test. This ensures that the tank itself need not be rated to withstand a high burst pressure in order to be safely used in most application. Sizing of the tank is another consideration. Too much pulsation in the flow stream will show up as apparent vibration, or bouncing, of the rotameter's float, and this is an indication that the observed flow cannot be relied on as accurate.
As a rule of thumb, a tank volume of about one third the maximum flow being measured—in terms of magnitude—should be sufficient. For example, if the pump's free flow is about 30 LPM then a 10 Liter tank should provide a good starting point. Another approach is to start with a relatively small tank and increase its size until no change in the measured flow can be observed. At the other end of the spectrum, a tank much larger than necessary will certainly provide accurate results, but response time may become an issue. Given that some small but finite amount of pressure will be required to raise the float and move the gas through the connective tubing and fittings, there will be some slight compression of the gas within the tank and even small changes in tank pressure will take time to re-stabilize, particularly where the measured flows are relatively small.
Once tank volume is determined, accuracy of the rotameter readings can usually be relied upon over the full range of applied loads since the added load—applied via a restrictive or throttling device (in this example, a simple inline valve, or “Variable Restriction”)—will act to further damp out flow pulsation.
Another important aspect of test set-up that can be seen in the schematic is that the rotameter is the last element in the flow path. This is necessary in order to keep one end of the device open to ambient, thereby ensuring operation under essentially standard conditions.
|Figure 2: Vacuum test setup|
A detail not shown in the schematic, but in some instances helpful, is the introduction of a damping element between the pressure gauge and the main flow path. In some cases, large pulsations in pump output can show up as oscillations in the pressure gauge reading. Since the leg leading to the gauge is essentially a dead end with relatively little internal volume, a passive restrictive element can be introduced to stabilize gauge readout.
Figure 2 depicts the corresponding set-up for measuring pump performance when subjected to loading on the inlet—vacuum load. Note that in this case the rotameter's inlet is open to atmosphere. Again, this ensures that the device will be operating at relatively standard conditions, and therefore return the most accurate results.
A final consideration for test set-up is the selection and sizing of connective tubing and fittings. These components should be selected so as to present as little pressure drop in and of themselves as is practical. A good test is to check for minimal pressure (or vacuum) reading at the gauge when the pump is running under no applied load—where the variable restriction is completely open.
William Fleming is the president of Dynaflo, a supplier of smaller, quieter, and more efficient vacuum pumps and compressors for the medical device industry. He can be reached at 610-200-8017, x7011 or firstname.lastname@example.org.