Air Pressure Compensation in Coffee Roasting
A Clear Guide to Stable Exhaust and Repeatable Flavour
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Roasters talk a lot about gas, power and drum temperature. Less often do we measure what the beans actually feel; the moving air that carries heat through the drum. If your exhaust drifts, your cup drifts. Rubasse machines read that airflow with an electronic differential pressure manometer, a digital gauge that records live pressure to decimal places and feeds it straight into the control system, so what you set is what the beans feel.
So – lets explore this further – this guide/exercise unpacks differential pressure, why it is the right proxy for exhaust volume, and how an air pressure compensation system keeps your airflow steady even when the duct starts clogging up. It finishes with a simple experiment you can replicate in your own roastery.
The core idea; read pressure to know airflow
You cannot taste percentages on a damper, you can taste the airflow they create. Pressure tells you more truth than a set point.
A differential pressure gauge compares pressure inside the drum against outside. On Rubasse roasters the pressure is measured by a built-in digital manometer and logged to the controller, which means the same number on the screen really does reflect the same exhaust volume every time. That number reflects the true exhaust volume through the system.
Two key rules follow. One, the same exhaust percentage does not guarantee the same exhaust volume. Two, the same differential pressure does indicate the same exhaust volume, which is what the beans experience. If a duct is partially blocked, the same damper opening gives a lower pressure difference, so you must open the exhaust further to restore the original pressure. An automated compensation system measures differential pressure in real time and adjusts the exhaust to keep that pressure steady, so your airflow stays where you intended.
Why compensation matters, stability first, cleaning second
In practice: Cleaner roasts come from stable airflow, not guesswork. Let the gauge settle the argument.
As oil and dust build up on the exhaust path, the system becomes harder to breathe. With the same roaster and the same damper position, your differential pressure number will slide downward over time. That slide means less convective heat and different bean movement, which shifts flavour. A pressure compensation system trims that drift by automatically increasing the exhaust until the measured pressure difference returns to the recorded target. That keeps your airflow consistent, helps you reproduce your reference profile, and reduces the flavour wobble that creeps in between deep cleans.
Extend cleaning intervals without losing the plot
Reality check: Cleaning still matters. The system buys you stability between cleans and tells you when the time has come.
Cleaning is labour and downtime. Many shops also notice they need to re-calibrate a profile after a deep clean because the exhaust suddenly runs freer. A compensation system narrows that gap. It raises exhaust automatically as the duct slowly clogs, then prompts a cleaning reminder when it can no longer hit the expected pressure at a given set point. You maintain flavour day to day and you schedule cleaning on need, not on guesswork.
The system watches the digital pressure trace and prompts a clean when it can no longer reach the mapped value at a given set point.
How the design works, reference a pressure map then correct in real time
Think map and compass: First you map what “normal” looks like, then you steer back to it whenever the wind changes.
When the roaster is clean and exhaust flow is smooth, each exhaust percentage corresponds to a characteristic differential pressure. For example, with exhaust set to 80 percent, the gauge may read around 40 Pa on a clean system. That pairing becomes your reference. As the pipe clogs, you will see the same 80 percent produce a lower pressure, for instance 25 Pa. A compensation system compares the live reading to your reference and opens the exhaust further, even to 100 percent if required, so the measured pressure climbs back to the target. Your beans feel the same airflow you profiled with, despite the blockage. Because the gauge is digital and tied into the software, the roaster compares today’s live reading to the clean-system map and trims the exhaust automatically to hit the target pressure.
The inevitable yet avoidable condition of every roaster’s exhaust pipes
Simulated 66% blockage
What happens during a blockage, step by step
On the controls: You hold the curve steady. The computer quietly opens the air so the gauge returns to target.
With compensation off, a blocked path gives you the same exhaust percentage on the UI but a lower differential pressure on the gauge. In the example, 80 percent exhaust on a clean machine gives about 40 Pa, while 80 percent on a blocked duct might give around 25 Pa. With compensation on, the roaster raises the exhaust position, often close to full open, until the gauge returns to the 40 Pa target. The flavour driver is stable airflow, so you maintain roast behaviour even while the duct is restricted.
And the compensation in action..
And now for a more extreme blockage simulation..
…And the result..
The experiment you can replicate, blocked duct test with and without compensation
No lab coat required: A bit of tape, a reference profile, and a pressure gauge tell you everything you need.
Set up:
- Choose a reference profile you know well.
- Run in fully automatic replication mode so the exhaust and heat follow the recorded curve.
- Simulate a blocked air duct by covering roughly half the duct surface with tape. Rubasse used about 50 percent for this test.
Run 1, compensation off:
Replicate the curve using only exhaust percentage and heat replication. Observe the differential pressure trace. You will likely see large deviation, for example a drop of around 20 Pa compared with the clean reference, even though the exhaust percentage trace looks identical to the original. This confirms the key point, the same exhaust percentage does not equal the same exhaust volume when the path is restricted.
Your roast without Pressure Compensation
Run 2, compensation on:
Enable the air pressure compensation function. Repeat the profile with the same simulated blockage. The system tracks the differential pressure in real time and raises the exhaust to restore the recorded pressure curve. The bean temperature reproduction remains aligned with the original, despite the blockage. This demonstrates that pressure-based airflow control preserves the thermal environment the beans experience.
Your roast with Pressure Compensation
What this means in production, flavour and uptime
Day to day: Your profiles behave, your first crack timing holds, and you do not need to scramble after a deep clean.
Pressure compensation helps you reproduce flavour when ducts are slowly clogging. It reduces the need to tweak a profile after maintenance because the system has been holding the airflow steady in the lead-up. It also gives you a clear signal for when cleaning is due. For busy shops, that combination saves time, stabilises flavour, and keeps capacity available for roasting rather than re-calibration.
Summary
Airflow is a primary driver of heat transfer inside the drum. Measure it with differential pressure, then hold it steady. A pressure compensation system compares live pressure to your reference map and trims the exhaust automatically, so the beans see the same convective environment from batch to batch, even with a partially blocked duct. The result is more reliable flavour and less downtime lost to reactive maintenance.
Specialty Batch is the authorised distributor for Rubasse in the UAE. If you would like a live demo, a sample roast, or a run-through of automation and replication features for your production context, get in touch.
Considered as one of the region’s pioneers in specialty coffee education and Dubai cafe culture development, Ryan Godinho is an Australian entrepreneur who is accredited as the country's first SCAA AST and National Coffee Championships Coordinator. He is a frequent contributor to Forbes and Entrepreneur Magazines and also holds a postgraduate Certificate of Advanced Studies in Coffee Excellence from Zurich University (ZHAW).
