# Building a Small-Batch Tea Roaster with Repeatable Results

Small-batch tea roasting sits in an awkward space between stovetop craft and industrial process control. A useful roaster must heat gently enough to avoid scorching delicate leaves while still responding quickly to profile changes. For batches between 250 and 500 grams, a compact rotating drum works well: constant agitation reduces hot spots, and adjustable airflow carries away moisture without stripping aroma too aggressively.

## Thermal Design

The prototype uses a 200 mm stainless-steel drum driven at 24 RPM by a geared DC motor. A 1.2 kW ceramic heating element sits beneath the drum, separated by a perforated heat shield that converts direct radiation into more uniform convective heat. Two K-type thermocouples measure inlet air and leaf-mass temperature, while a third sensor near the exhaust helps indicate when evaporative cooling begins to taper off.

## Control System

A small microcontroller samples the sensors twice per second and regulates heater output through a zero-cross solid-state relay. Rather than holding one temperature, the controller follows a staged profile: a moderate drying phase, a gradual ramp for aroma development, and a short finishing phase. The operator can still trim heat and fan speed during a roast, but logging every adjustment makes successful batches reproducible.

```yaml
profile:
  charge_c: 95
  stages:
    - { target_c: 110, duration_s: 300, fan_pct: 35 }
    - { target_c: 135, duration_s: 420, fan_pct: 50 }
    - { target_c: 145, duration_s: 90,  fan_pct: 70 }
```

## Airflow and Safety

Airflow is supplied by a variable-speed centrifugal blower downstream of the drum, keeping the roasting chamber under slight negative pressure. This prevents smoke from leaking around the loading door and makes chaff collection easier. A thermal fuse, independent over-temperature switch, grounded metal enclosure, and normally open heater contactor provide protection if the controller or relay fails.

## Calibration and Operation

Before roasting tea, the empty machine should be tested at several heater and fan settings to map its thermal lag. During production, charge weight, ambient humidity, temperature curves, and tasting notes should be recorded together. The numbers alone do not define a good roast, but they make sensory observations actionable: if one batch develops a cleaner floral character, the operator can reproduce its heat and airflow history instead of relying on memory.