The inside coil is made of copper.
We use copper.
“These days the professionals use stainless because of strength and inertness but for the homebrewer copper and brass are still the cheapest and best choices for wort chillers and fittings”- John Palmer
Thermal conductivity of copper is 20 times greater than the same thickness of Stainless Steel, so the pros have to account for this with larger chillers than if they used copper.
“Copper is an essential nutrient for living organisms, daily intake is 2-5 mg per day only copper salts are recorded as having a lethal dose of 200 mg per kg of body weight” (John Palmer), this is 17000 mg for a 85 kg person.
Our counter flow chillers (G30, G40 and G70) can cool down your wort to within 3-5°C (37.4-41°F) of your groundwater temperature which will take approximately 30 minutes.