Precision fluid metering is demanded in many industries. A common denominator is the need to meter or dose working fluids at flow rates generally less than 10 liters per hour, sometimes referred to as the “micro flow” range. End users frequently require very high accuracy fluid delivery provided at a low cost. High accuracy is synergistic with the continual advancement of digital technologies that make it possible to achieve more precise control of electric motors and solenoids. Information transfer using wireless technology, WIFI internet, radio frequency identification tags, bar codes, etc., are also pushing system developers to offer more customized user interfaces that demand increased fluid delivery or dose precision.
Typical market segments and applications may include: medical diagnostics, medical fluids delivery, food dosing/packaging, beverage equipment, industrial dosing, paint and ink dosing, fuel cells, water analysis, semi-conductor electronics, chemical/gas analyzers, cleaning and disinfectant dosing.
Working fluids are as diverse as their respective applications and may include liquids and gases such as water, IV drugs and solutions, food and beverage concentrates, soaps and detergents, dyes, and analysis chemicals to name a few. Precise adjustment of flow rate is often required between as little as 1.0 ml/hr to as high as 10,000 ml/hr. Delivered fluid pressures are generally very close to atmospheric pressure, but may range upward to 15-30+ pounds per square inch (103-206+ kilopascal) in some applications.
Precision fluid metering solutions often fall into three (3) general categories. One category employs a variable speed pump, electronic flow meter and a closed loop feedback controller. The controller makes incremental adjustments to the pump speed to correct for flow rate deviations from a pre-defined set point. A second category may involve using constant speed pump, and applying an electronically actuated, variable orifice downstream of the pump. The controller makes incremental adjustments based the measured flow rate, but instead of adjusting pump speed, opens or closes the variable orifice to throttle the flow rate and maintain the flow at pre-defined set point. A third category applies open loop control using a variable speed pump that is powered using continuous or pulse width modulated DC supply voltage. Open loop control may be used where there is a known relationship between the DC supply voltage, pump speed, and the volumetric displacement rate of the pump. Open loop control is desirable because it is generally simpler to operate, has fewer components and is lower in cost.
However, the flow accuracy of open loop control is limited by the volumetric displacement accuracy of the pump and accuracy of the pump motor speed. Each pump type provides its own set of features and benefits that include trade-off in size, cost, power, material compatibility, reliability, and flow accuracy. There is generally a large trade-off between cost and accuracy. For example syringe pumps with precise stroke and volumetric precision may be used to deliver intravenous drugs and solutions with exceptional dose accuracy, but they are very expensive and not convenient to use. On the other hand, peristaltic pumps provide good value and are easy to use. However peristaltic pumps offer greatly reduced accuracy as compared to syringe pumps due to inconsistent tubing elasticity that may result in variable fluid delivery rate.
Each market application has its own set of demands and challenges. Some markets are also beginning to consider a new demand—disposability of the wetted pump components. Precision disposable pumps are of keen interest in the medical market where it is cost prohibitive to clean and sterilize recyclable components after contact with medical fluid media. While the tubing set of a peristaltic pump is disposable, it cannot deliver acceptable dose accuracy, especially in the lower micro flow range. Disposable medical applications may include but are not limited to drug delivery, IV solutions, peritoneal dialysis, hemodialysis, and anesthesia delivery. Disposable pumps may also be attractive in other markets, for example integration with disposable fluid containers such as the “bag-in-box” used in the food, beverage, and personal care products industries. The beverage market provides an added challenge wherein a precise amount of concentrate must be continuously mixed with flowing water to maintain an accurate volume ratio of water-to-concentrate for good beverage quality and customer satisfaction. A major difficulty is caused by the fact that water flow rate may vary widely due to variation in water supply pressure. However, a general theme across all markets is that customers increasingly demand high accuracy, ease of use, and reliability, all provided with a low cost.
Positive displacement pumps, such as diaphragm pumps or more preferably piston pumps, may offer precision dosing as long as a suitable control system is employed to trigger precisely timed linear, cyclic movements that drive the diaphragm or piston, respectively. In general the piston pump is more suitable for low dose and/or low flow rate because the stroke volume can be scaled down by reducing the diameter and stroke of the piston. Also the stroke volume of a piston pump is precise as compared to a diaphragm pump. Diaphragms being made from flexible elastomers may cause the stroke volume to vary with changes in the stoke speed, fluid viscosity and pressure rise across the pump. Conventional diaphragm and piston pumps are not considered disposable because the pump cost is too high for one time use. The most expensive component of the pump is the drive motor assembly. Recovery of the drive motor assembly from the pump head is cost prohibitive due to the high amount of labor needed to remove the motor from the pump head, and then reassemble and re-qualify a new pump head with the recycled drive motor assembly.
Many micro dosing fluid delivery applications involve liquids that are stored in a plastic bag at atmospheric pressure. Such bags are equipped with fittings that allow for a tube to connect the liquid contents of the bag to the inlet port of the fluid delivery pump. Examples are the common “bag-in-box” containers used in the beverage industry to store drink products and beverage concentrates. “IV bags” are also used to store intravenous solutions and drugs in the medical field. Of concern is the infiltration of air into the pump inlet or suction tube when the bag becomes depleted and must be disconnected from the tubing to install a new, replenished bag of liquid. Air bubbles pulled into the pump suction and then delivered into the pump discharge tube is problematic. In beverage applications this may result in poor delivered drink quality. In medical applications air delivered with IV fluids may be harmful to the patient under some conditions.