# Building a Small-Batch Tea Roaster with Repeatable Heat Control

Small-batch tea roasting sits somewhere between cooking and process engineering. A few hundred grams of oolong or green tea can change character within seconds, so the machine needs both gentle heat and predictable airflow. Our prototype uses a 1.5-liter perforated stainless-steel drum, a 500-watt ceramic heater, and a variable-speed centrifugal fan. The drum rotates at 24 RPM—fast enough to keep leaves moving without crushing brittle edges.

## Measuring the Roasting Environment

A thermocouple mounted near the drum inlet measures supply-air temperature, while a second probe tracks exhaust temperature. Neither value is the true leaf temperature, but together they reveal how the batch is absorbing and releasing heat. We sample both sensors at 5 Hz and smooth the readings with a two-second moving average. This reduces control noise while preserving important events, such as the exhaust-temperature plateau that often appears as surface moisture leaves the tea.

## Controlling Heat and Airflow

The heater is driven through a zero-crossing solid-state relay using time-proportional control. A PID loop adjusts heater duty cycle, but its output is limited to prevent aggressive corrections from scorching the leaves. Fan speed is set independently because airflow changes both heat transfer and moisture removal. Early in a roast, moderate airflow carries away humidity; near the finish, lowering airflow can soften the temperature gradient and make the endpoint easier to approach.

```yaml
profile:
  charge_temp_c: 105
  heater_limit_pct: 72
  fan_pct: 45
  finish_exhaust_c: 92
  max_time_min: 14
```

## Designing for Thermal Stability

The main mechanical challenge is thermal inertia. A thick drum stores heat well, but it continues roasting after the heater switches off. We selected a thin 304 stainless-steel drum and insulated the outer chamber with ceramic fiber, concentrating stored energy in the air path rather than the rotating mass. The controller begins tapering power several degrees before the target exhaust temperature, then opens a cooling damper at the endpoint.

## Logging and Repeating Batches

Each roast produces a timestamped log containing temperatures, heater output, fan speed, and operator annotations. Plotting these signals makes it possible to compare batches even when ambient humidity or starting leaf moisture changes. The goal is not perfect automation: aroma and appearance still matter. Instead, the roaster handles repeatable heat delivery so the operator can make deliberate adjustments rather than compensating for unstable equipment.