Micropump or solenoid valve for dosing? When is which approach more reliable?
Two fundamentally different approaches are available for dosing small liquid volumes: a micropump that actively displaces a volume, or a solenoid valve that opens a passage while a pressure source drives the liquid. Both are available in inert versions, both are compact and both isolate the solenoid from the medium by means of a diaphragm. The choice therefore does not lie in material properties, but in the dosing principle itself, and that makes a big practical difference.
⏱ approx. 6 minutes reading time
Two fundamentally different principles
Both the Bio-Chem Fluidics micropump and the Humphrey iDP solenoid valve isolate the solenoid from the medium by means of a flexible diaphragm. That is the shared starting point. The fundamental difference lies in what happens to the liquid next.
Micropump: volumetric displacement
When the solenoid is energised, the diaphragm is pulled up and a partial vacuum is created in the pump chamber. This causes liquid to flow in via the inlet check valve, while the outlet check valve remains closed. When the solenoid is de-energised, a spring pushes the diaphragm back and expels a precisely defined volume via the outlet check valve.
Each complete on/off cycle delivers one discrete volume, regardless of the pressure in the system. The pump needs no external pressure source and is self-priming.
Solenoid valve: time-controlled passage
The solenoid opens the diaphragm poppet, allowing liquid to flow from inlet to outlet as long as the valve is open. The dosed volume is the product of opening time and flow rate: V = t × Q. That flow rate depends on the inlet pressure, the viscosity and the temperature.
The valve itself does not control volume; the system around it determines how much flows through. With a stable, well-regulated pressure source, however, time-controlled dosing is accurate and reproducible.
The core difference in one sentence
A micropump doses volumetrically: every cycle delivers the same volume regardless of system conditions. A solenoid valve doses on a time basis: the volume depends on the opening time and on the pressure and viscosity at that moment. That dependency is the key to the right choice.
The products: Bio-Chem Fluidics and Humphrey iDP
Inacom supplies micropumps from Bio-Chem Fluidics and inert solenoid valves from Humphrey Products. Both manufacturers focus on the same markets: analytical chemistry, clinical diagnostics, medical equipment and industrial fluid handling. As a result, the products are direct alternatives to each other in many situations, but each has its own optimal field of application.
Bio-Chem Fluidics micropumps
The Bio-Chem Fluidics micropump family consists of four series. The 120SP and 130SP are ported pumps with ¼"-28 UNF connections for volumes of 20 to 60 µl. They differ in body materials: the 120SP is available in PPS and PEEK, the 130SP in PTFE and POM, with the PTFE version offering the most inert combination for the toughest applications. The 150SP handles higher volumes of 100 to 250 µl via 5/16"-24 connections. The 139SP is the manifold version of the 130SP and allows direct mounting on a manifold block without intermediate tubing or fittings. All models operate on 12 or 24 VDC.
Humphrey iDP solenoid valves
The Humphrey Products iDP series comprises several sizes, each with a diaphragm poppet construction that completely isolates the solenoid from the medium. The 101 Series is the smallest: only 10 mm wide, with an internal volume of 3.6 µl in the actuated state, designed for micro-dosing in clinical and analytical instruments. The 300 Series is slightly larger, available as 2-port and 3-port versions for inline or manifold mounting, with a PEEK body and Viton® GF or EPDM seals. The 350/351 Series (2-port) and 370/371 Series (3-port) are the larger variants for higher flow rates and pressures. In addition, Humphrey offers the MiDP S390/S391 proportional valve for continuous, variable flow control via a stepper motor.
Comparison on the critical factors
Accuracy under variable system conditions: pump wins
Bio-Chem Fluidics micropump
Delivers a fixed stroke volume per cycle, regardless of system pressure, temperature or viscosity variations. Repeatability ±5% CV, structurally independent of external factors. Easy to validate: only the pump model and the number of cycles determine the total volume.
Humphrey iDP valve
Accuracy depends directly on the stability of the inlet pressure, the viscosity and the temperature. With a stable, regulated pressure source, high accuracy is achievable. Under variable conditions, the dosed volume drifts proportionally with the pressure fluctuation.
Chemical inertness and media compatibility: equal
Bio-Chem Fluidics micropump
Fully inert thanks to diaphragm isolation. Choice of PPS, PEEK, PTFE or POM body; PTFE or EPDM diaphragm; EPDM, FKM or FFKM check valves. The 130SP in a PTFE/PTFE/FFKM configuration offers maximum chemical inertness for the toughest reagents.
Humphrey iDP valve
Also fully inert thanks to diaphragm poppet isolation. The coil housing is encapsulated for use in corrosive environments. Compatible with concentrated nitric acid, sodium hydroxide, formaldehyde, hydrogen peroxide, biological solutions and bleach. The 350/370 Series also offers FFKM seals for the toughest media.
Independence from an external pressure source: pump wins
Bio-Chem Fluidics micropump
Needs no external pressure source. Self-primes from an unpressurised reservoir up to 1.3 m below the pump. Generates approx. 0.35 bar outlet pressure after priming. Changes in system pressure do not affect the stroke volume.
Humphrey iDP valve
Always requires an external pressure source: the 101 Series up to 30 PSI (2.07 bar), the 300/350/370 Series up to 40 PSI (2.76 bar). Without pressure, nothing flows. Moreover, for accurate dosing, a pressure regulator is not an option but a requirement.
Switching speed and cycle frequency: valve wins
Bio-Chem Fluidics micropump
Maximum 2 Hz for the 120SP, 130SP and 139SP; maximum 1.6 Hz for the 150SP. The fixed on-time (150 msec for the smaller series, 200 msec for the 150SP) must not be shortened without loss of accuracy.
Humphrey iDP valve
Considerably faster. The 101 Series achieves 600 cycles per minute (10 Hz) with a 15 msec response time. The 300 Series achieves 300 cycles per minute with a 25 msec response time. The 350/370 Series responds in 16–20 msec. For fast switching sequences, the valve is the only option.
Minimum dosable volume: valve wins
Bio-Chem Fluidics micropump
The smallest available stroke volume is 20 µl. Volumes below this threshold cannot be achieved with this principle. Nor is the volume adjustable per stroke; it is fixed per pump model.
Humphrey iDP valve
The 101 Series has an internal volume of only 3.6 µl in the actuated state. With short opening times and low pressure, volumes well below 20 µl are achievable. This makes the valve ideally suited to applications such as DNA analysis, blood testing and microfluidics.
Controlling flow direction and routing: valve wins
Bio-Chem Fluidics micropump
The micropump is a 2-port device: inlet and outlet. Routing flow to multiple destinations requires additional valves or a manifold with separate outlets per stream.
Humphrey iDP valve
The 300 Series and 370/371 Series are available as 3-port valves: one valve switches as a diverter (one inlet, two outlets) or as a selector (two inlets, one outlet). This is a function the micropump structurally lacks.
Continuous liquid supply: valve wins
Bio-Chem Fluidics micropump
Delivers a pulsating, discontinuous flow. Each dispense is a discrete pulse. For a truly continuous flow, multiple phased pumps or a pulsation damper are required, which increases system complexity.
Humphrey iDP valve
With a continuous pressure source, an open valve delivers a truly continuous flow without pulsation. Ideal for continuous rinsing flows, buffer supply or liquid transport where dosing is not a primary requirement.
Continuous liquid supply: valve wins
Bio-Chem Fluidics micropump
The micropump works in a binary way: a fixed volume is dispensed per cycle. Variable flow control is only possible by varying the cycle frequency, which affects the average flow but does not provide continuous proportional control.
Humphrey iDP valve
The MiDP S390/S391 proportional valve controls the flow continuously via a stepper motor. Hysteresis less than 2% of full scale, step response time less than 40 msec, service life of more than 1 million cycles. Open-loop control is possible. Ideal for dosing additives or as a mixing controller.
When should you choose the micropump?
The Bio-Chem Fluidics micropump is the better choice when the dosed volume must be independent of system conditions, or when no stable external pressure source is available.
No stable pressure source available
The system works with an unpressurised reservoir. The micropump self-primes up to 1.3 m below the outlet and does not need a pressure regulator.
Variable liquid conditions
The liquid temperature, viscosity or back pressure varies in the system. The stroke volume of the micropump remains constant regardless of these variations.
20–250 µl per dispense
This is the optimal working range of the Bio-Chem series. The repeatability of ±5% CV is structurally independent of system pressure and easy to validate.
Multiple independent streams
With manifold mounting of multiple 139SP pump heads, each stream is dosed completely independently, without mutual pressure interference from a shared source.
Pharmaceutical or regulated environment
Volumetric independence simplifies the validation protocol. No pressure monitoring is needed as an additional validation parameter. The pump model and cycle count are sufficient.
Self-priming system required
On start-up, the micropump draws in air and liquid by itself. Priming normally takes 30 to 60 seconds and requires no external aid or manual intervention.
When should you choose the solenoid valve?
A Humphrey iDP valve is the better choice when speed, routing, continuous flow or proportional control is the main requirement, and when the system pressure is stable and controllable.
Stable, regulated pressure source available
With a constant inlet pressure and known liquid viscosity, time-controlled dosing with the iDP valve is accurate, reproducible and easy to implement.
Volumes smaller than 20 µl
The 101 Series has an internal volume of only 3.6 µl in the actuated state. With short opening times and low pressure, volumes well below the threshold of the micropump are achievable.
High switching frequency required
The 101 Series achieves 600 cycles per minute with a 15 msec response time, well above the maximum 2 Hz of the micropump. For fast sequences in diagnostic instruments, the valve is the only option.
Controlling flow direction or routing
The 300 Series and 370/371 Series are available as 3-port valves: one valve switches between two outlets (diverter) or two inlets (selector). The micropump structurally cannot do this.
Switching a continuous liquid supply on/off
For opening and closing a liquid stream without precise volume dosing, the valve is simpler and cheaper and delivers a pulse-free continuous flow.
Proportional variable flow control
The MiDP S390/S391 controls the flow continuously via a stepper motor with less than 2% hysteresis. For dosing additives or as a mixing controller, without the need for an external flow meter.
The grey area: where the choice is difficult
In most cases, the application profile makes the choice clear. However, there are situations in which both solutions are technically feasible and the choice depends on system design, budget considerations or the specific requirements of the application.
| Situation | Consideration | Preference |
|---|---|---|
| 20–60 µl at stable pressure | Both are technically feasible. The valve requires calibration for each pressure and temperature combination. The pump is insensitive to pressure and easier to validate. | Pump |
| Aggressive reagents, stable system | Both are available fully inert with comparable material classes. At stable pressure, the valve works well; the pump has the advantage that it needs no pressure source. | Depends on the situation |
| Pharmaceutical, regulated environment | The volumetric independence of the pump simplifies validation. The valve requires pressure monitoring as an additional validation parameter, which increases the documentation burden. | Pump |
| Fast sequences above 2 Hz | The micropump reaches a maximum of 2 Hz. Above that, the iDP valve (up to 10 Hz for the 101 Series) is the only option, even if volumetric accuracy is desired. | Valve |
| Compact OEM, multiple streams | A 139SP manifold combines several pump heads without an external pressure source. With a shared pressure source, a row of 101 Series valves (each 10 mm wide) is at least as compact. | Depends on the situation |
| Dosing and routing in one system | Combine both: the micropump doses the volume volumetrically, the 3-port iDP valve directs the flow to the right measuring cell or outlet. Each component does what it does best. | Combine both |
Micropump and solenoid valve complement each other
In practice, micropump and solenoid valve are not competitors but complements. A widely used architecture in diagnostic instruments combines a Bio-Chem Fluidics micropump for accurate volume dosing of reagents with a Humphrey iDP 3-port valve for directing the flow to the right measuring cell. In addition, iDP 2-port valves open or close separate fluid paths as isolation valves. The pump delivers the volume, the valves handle the routing and isolation. Both components do what they do best.
