Which diameters are available for hotplates? All Hei-PLATE hotplate models are available with a hotplate diameter of 145 mm as standard, or 135 mm on request. 145 mm gives more surface area and fits perfectly with the 145 mm recess on Heat-On blocks and Cool-It Bowls (although these accessories can still be used with 135 mm hotplates). Our Greenhouse, Tornado, Carousel 6 Plus and Carousel 12 Plus reaction stations have a 135 mm recess, so require a 135 mm hotplate.