They now have another option: thick-film heaters from Watlow, St. Louis, Mo. (www.watlow.com). The devices consist of heating films mounted on the outside of quartz tubes. Chemicals and deionized water flow through the tubes. This configuration is said to provide more efficient heat transfer and rapid response to changes in flow and temperature than other heaters. And because the electric heating elements are outside the tube and the tube is made of inert quartz, the devices can be used with aggressive chemicals such as ammonium hydrochloride, hydrochloric acid, phosphoric acid, and ultrapure 18 megaohm deionized water. The modules are half the size of other heaters with similar capacities, and they eliminate the risk of process contamination in event of heating-element failure. Other features include high-watt density, ultrapure quartz construction, long life, a maximum process temperature of 356°F (180°C), maximum operating pressure of 75 psi, and a maximum flow rate of 35 lpm. Heaters can be supplied as stand-alone modules or built into a tool.