Comparison of Temperature Measurements in Fire Test Furnaces Using Aspirated Thermocouples Available to Purchase

There are numerous thermocouple types which are used to measure temperature in experimental fire environments including bare bead, plate, sheathed (grounded and ungrounded) and aspirated thermocouples (suction pyrometers). Furnaces used to test fire resistive construction, as specified in the Standards ANSI/UL 263, ASTM E119 and NFPA 251 employ sheathed, ungrounded thermocouples to measure gas temperatures, while room fire experiments, such as ASTM E603-01, employ bare bead and aspirated thermocouples to measure hot gas temperature layers. Shielded, aspirated thermocouples are quite inexpensive and easy to make, however, the suction needed to create velocities in excess of 10 m/s at the tip is quite a difficult challenge. A commonly used method to create suction includes multiple gas traps to cool and dehumidify the hot gas before passing through a pump rated for high temperatures. This method can be costly, time consuming to maintain and cannot operate for multiple hours at high temperature without damaging the pump. An investigation has been undertaken to determine a cost effective method to measure gas temperatures in fire test furnaces for long durations at high temperatures. A comparison of bear bead, plate and double shielded, aspirated thermocouples will be presented. A low cost, durable aspirated thermocouple will be shown to continuously endure temperatures in excess of 1040°C for multiple hours.