Time-constant of thermocouples and its significance.

Usually, fire testing standards specify the gauge and the type of thermocouple to be used. For example, NFPA 285 specifies 18 and 20-gauge Type K thermocouples for the furnace and the sample respectively. Standards like ASTM E2307 and ASTM E2874 specify that 20 gauge Type K, specifically Bare-wire thermocouples are to be used to measure the unexposed side temperature of the sample. The furnace thermocouple's tip should be of plate type and copper disk-tipped thermocouples should be used to measure the sample temperature in resistance to fire testing standards like ISO 834 and BSEN 1363. Standards like NFPA 252, NFPA 257 and ASTM E119 specify the type of protection tube in which the thermocouples are to be inserted.?Why??

Time constant of the thermocouple is the simple answer. (Time taken for the sensor to measure 63.2% of its final output).

Normally while measuring the temperature of a water bath or a muffle furnace or even the ambient temperature, any type of thermocouple can be used and gives the same value. This is because the thermocouple gets sufficient time to get saturated.?

While measuring an open flame, like in most fire testing standards, things get complicated. Since the temperature will be fluctuating in the varying flame, the thermocouple will not get sufficient time to saturate. The instantaneous temperature measured, which determines the pass/fail, depends on the type of thermocouple used, and hence it is very much important to use the exact type and gauge of the thermocouple.?

I did a small experiment with a few different K-type thermocouple samples and a candle to have better insights.?

The following thermocouples were used for comparison.

1. Bare wire, Ceramic beaded, 20AWG.

2. Open Junction, Mineral Insulated, Inconel 600 Sheath(6mm), 20AWG.

3. Closed Junction, Mineral Insulated, Inconel 600 Sheath(6mm), 20AWG.

4. SS Braided, Multi-strand, 7x32AWG.

5. Teflon Insulated, Multi-strand, 7x32AWG.

6. Teflon Insulated, Single Conductor, 29AWG.

Each thermocouple was exposed to the same area of the candle flame for a fixed period of 15 seconds. The data logging interval was set to 1 second.?

The obtained data from individual thermocouples were combined to plot a graph.

?The peak value of each curve in the graph shows the temperature each thermocouple measured at the end of 15-second fire exposure.?The time taken to get back to the ambient temperature also varies from each other.?

The thermocouple with the smallest conductor diameter was the quickest to measure the maximum temperature(29AWG).?The behaviour of Multi stranded thermocouples (7x32AWG) was almost similar despite the type of insulation used. Out of the three 20AWG thermocouple samples, the bare wire type with ceramic beads had a quick response compared to its metal-sheathed counterparts. Closed Junction Mineral Insulated thermocouple with Inconel 600 sheath had the slowest response time among the 6 samples.

This data indicates the significance of the time constant of the thermocouple. Hence using the thermocouples with the type, gauge, and construction as specified in the test standard is necessary while testing to achieve compliance.?

View the temperature data and the graph at: