27. How many green LED should we have for the water circuit?

The safety is set to switch off the power supply to the furnace, ie at least 4 LEDs (≈ 1.4 l / min or 0.35 l / min per LED) for cryostat structures, 1000-1200 ° C and 1600 -1800 ° C, or at least 5 LEDs (1.8 l / min) for 2400 ° C structures.