Variable Dispense vs. Air over Liquid Dispense

As a mechanical engineer working on the hardware design of IVD platforms for ISO:13485 medical devices, it's essential to understand the different fluidic architectures and their pros and cons. Two popular methods for dispensing fluids accurately and consistently are air over liquid dispense system (aka, time  pressure dosing system) and piston pumps. 


Variable Dispense vs. Air over Liquid Dispense


An air over liquid dispense system is a complex system that dispenses fluids at a specific pressure and time. It uses custom software to calculate the fluid's specific gravity, pressure sensors upstream and downstream to measure the pressure, and dispenses the required volume on the fly. While this system is suitable for many industries, including pharmaceuticals, food, and cosmetics, it can be a nightmare to integrate into manufacturing. The precision tips can become clogged as the pressure changes constantly. Recalibration is often required, resulting in downtime and waste in dispensing, which can be costly if expensive reagents or buffer fluids are used. 

An air over liquid dispense system also comprises approximately eight separate components that must work together, including a pressure pump, transfer pump, purging pump, pressure controller, software, pressure accumulator, dispense valve, and pressure sensor. If one of these components fails, the entire system can fail, and it can be challenging to diagnose the issue in final assembly. Additionally, if fittings are used, there are potentially 17 different leak points, including leak points on the pressure controller, pressure accumulator, dispense valve, tee for the pressure sensor, and pressure pump.

When using a custom manifold, burrs can also be challenging to identify and find, leading to difficulties  in final testing. Stack-up tolerance can also be a problem as dimensional variations or deviations can  accumulate when several components are assembled or mated together, leading to variations in the final  product's dimensions or performance. Furthermore, air over liquid dispensing requires constant pressure  from a pump, which can be noisy and negatively impact the pump's life.

Other factors to consider: 

  • Humidity can affect the viscosity of the liquid being dispensed, which can impact the accuracy of  the dosing or dispensing process. High humidity can also cause moisture to condense on the dispensing system or within the liquid being dispensed, which can lead to contamination or other issues with time pressure dosing.

  • Altitude can also affect the accuracy of time pressure dosing and air over liquid dispense methods, as changes in air pressure can impact the flow rate of the liquid being dispensed. This can lead to inconsistencies in the dispensing process, resulting in errors in the amount of liquid  dispensed. This makes it extremely challenging for an OEM to calibrate and quality an instrument on manufacturing floor and then air freight or deliver to a customer who might be located at a different altitude. In most instances it would typically take an extra two days for a field service engineer to get a customer up and running.

  • Temperature can also impact the accuracy of these dispensing methods, as changes in temperature can affect the viscosity of the liquid being dispensed, as well as the pressure within the dispensing equipment. This can result in inconsistencies in the dispensing process and errors in the amount of liquid dispensed. 

On the other hand, a valveless piston pump, such as the Fluid Metering Variable Dispense (FVD) Pump,  has only four wetted components and is a better fit for critical ISO:13485 applications. The risk to the  design significantly decreases, and the overall reliability of the design increases by using only one device  with two moving components. The FVD Pump does not require a separate priming or flushing pump,  allowing for a simplified fluidic architecture and software design. Also, the angle change of the FVD pump could allow for a quicker start up time by handling the priming, flushing, and dispensing within one  unit. Similarly, the low number of moving components also promotes a quieter operation, typically  between 40 and 50 dB. Furthermore, limited number of interaction points between the reservoir and  dispense site (only inlet and outlet of pump head) is less prone to clogging, and with fewer parameters to  manage (1) maintaining rotary speed and acceleration, (2) maintaining pump head angle diminishes the  need for endless recalibration, resulting in less downtime and waste. Reliability of the FVD Pump has  been demonstrated through minimal dispense drift over millions of cycles with verification consisting of  5 million dispenses and 2 million angle changes.  

Variable Dispense (FVD) pump


In conclusion, when choosing between an air over liquid dispensing system and a piston pump for the hardware design of an IVD platform, it's crucial to consider the pros and cons of each. While an air over liquid dispensing system may be suitable for some industries, it can be challenging to integrate into manufacturing and can be prone to failure. A valveless piston pump, such as the Fluid Metering FVD Variable Dispense Pump, is a better fit for critical ISO:13485 applications and has a lower risk of failure  and higher overall reliability.