How to automatically dispense a fixed amount of liquid with a pneumatic pinch valve

thijs · September 2, 2026, 12:15am

Based on contributions by ThijsvanStrien.

Some projects need a repeatable, accurate amount of liquid dispensed automatically — for example, applying liquid to a roller before it reaches a plate. This describes a simple, pump-free design using gravity feed and a pneumatic gripper that pinches the tube shut, plus how to calibrate it.

What you need

A reservoir (vat) with a tube fitted at the bottom
A pneumatic gripper, sized to fully pinch your tube closed
A solenoid valve and digital output to control the gripper’s air supply
A way to measure dispensed liquid volume, for calibration
Python (or your controller’s usual scripting environment) to control the digital output

How it works

A reservoir feeds a tube from the bottom. Liquid flows out under gravity alone — no pump is needed, which keeps the system simple and energy-efficient.
A pneumatic gripper pinches the tube shut. When closed, the tube is fully blocked and no liquid passes. When the gripper opens, the tube springs back to its original shape and liquid flows again.
The critical requirement: the gripper must fully close the tube with no leakage, and must open consistently each time so the flow rate stays predictable.

Steps

1. Build the mechanical setup

Mount the reservoir above the container to be filled, run a tube from the bottom of the reservoir, and position the pneumatic gripper around the tube so it can pinch it fully shut.

2. Calibrate the dispensed volume

Because flow is gravity-driven, the dispensed amount depends on:

The diameter of the tube
The height of the liquid in the reservoir (affects the pressure driving the flow)
How long the tube is left open

To find the right open time for your target volume:

Open the pneumatic gripper for a fixed amount of time.
Measure the dispensed liquid.
Compare the result against the target amount.

If the measurements aren’t consistent, adjust the design and re-test:

Use a narrower tube to reduce the flow rate.
Stabilize the reservoir’s liquid height, so the driving pressure stays constant over time (a tall, narrow reservoir will vary more per dispense than a short, wide one as it empties).
Fine-tune the timing value in software.

Repeat this until you have a reliable relationship between “gripper open time” and “dispensed volume”.

3. Control the valve from software

Control the pneumatic gripper by sending a digital output signal to the solenoid valve that supplies its air — opening the valve releases the gripper (tube opens, liquid flows), closing it re-engages the gripper (tube pinches shut). See How to control a Festo solenoid valve (VUVG-LK10-M52) from a UR robot for the digital-output wiring and code to drive a Festo solenoid valve, and use your calibrated open time from step 2 as the on-duration for each dose.