# Building a Small-Batch Tea Roaster

Small-batch tea roasting rewards precise control more than raw heating power. Our benchtop roaster uses a 2 kg stainless-steel drum, a 1.5 kW electric element, and a variable-speed motor. This arrangement handles batches from 150 to 750 grams while keeping the machine compact enough for a production test kitchen.

## Thermal Control

Two K-type thermocouples measure inlet-air and leaf temperatures. A microcontroller samples them at 2 Hz and drives the heating element through a solid-state relay. Leaf temperature is the primary process variable, but inlet-air temperature provides an early warning when the heater is responding too aggressively.

```yaml
batch_grams: 400
drum_rpm: 42
target_c: 112
ramp_c_per_min: 7
hold_minutes: 9
```

A simple PID loop works well during the final hold, but the warm-up phase benefits from ramp limiting. Without it, the element overshoots before heat has propagated through the drum and tea bed. We cap the commanded rise at 7 °C per minute, then reduce heater output as the leaf probe approaches the target.

## Airflow and Drum Speed

Airflow removes moisture and volatile compounds, so it must remain repeatable between batches. A small centrifugal fan pulls air through the drum and into a removable chaff filter. Drum speed is equally important: below 30 RPM, leaves tumble unevenly; above 55 RPM, fragile rolled teas can fracture.

## Logging and Repeatability

Each roast logs temperatures, heater duty cycle, fan setting, and drum speed to a CSV file. Operators also record aroma checkpoints and finished-leaf color. Combining sensor data with sensory notes makes it possible to reproduce successful profiles without pretending that temperature alone describes the roast.

## Maintenance Lessons

Tea dust accumulates quickly around the exhaust path and can distort airflow readings. We clean the filter after every five batches, inspect thermocouple placement weekly, and verify the leaf probe against a reference thermometer monthly. These small checks have improved batch consistency more than any tuning change to the controller.