The response time of a thermocouple is affected by several factors

The main factors affecting thermocouple response time are thermocouple bead size and the conducting medium including attachment method.

Thermocouples response time is measured as a “time constant.” The time constant is defined as the time required for a thermocouple’s voltage to reach 63.2% of its final value in response to a sudden change in temperature. It takes five time constants for the voltage to approach 100% of the new temperature value.

Thermocouples attached to a heavy mass will respond much slower than one that is left free standing because its value is governed by the temperature of the large mass.  A free standing (exposed or bare wire) thermocouple’s response time is a function of the wire size (or mass of the thermocouple bead) and the conducting medium.  A thermocouple of a given size will react much faster if the conducting medium is water compared to still air.

Here are some typical time constants of various free standing thermocouple bead sizes (bead size is typically 2 times the diameter of the wire) in these conducting mediums:

    Wire (AWG)    Bead Size (inches)      Still Air (sec)        Water (sec)
           42                    0.003                              0.07                      0.003
           40                    0.005                              0.25                       0.02
           36                    0.010                                 1                            0.05
           30                    0.020                                4                            0.17

NOTE: Remember it takes five time constants for a thermocouple to reach 100% of the final temperature value so the above time constants must be multiplied by 5 to get the total time.

So the most common sizes (30 AWG or smaller) of thermocouples used to attach to surfaces or components will have fast enough response time to accuracelly measure the temperatures of reflow solder process which tend to change no faster then 5 degrees/second. If one wishes to measure the air temperature, 36 AWG is common since the air is always moving, and the chart reflects “still air” response times.

The main reason for selecting thermocouples of a specific size is to match the size of the surface or point where the thermocouple is to sense temperature. 36 AWG is a good compromise between cost, size, and strength. Much smaller and it is too easy to break. Much bigger and it may be bigger then the component or attachment point.

One other factor in selecting thermocouples is the heat source. In wave soldering, the heat is typically from the bottom of the assembly and the thermocouples are attached on top. Here the thermocouples will be cooler then bottom of the assembly causing them the sink the heat as it flows from the bottom to the top. Smaller thremocouples will reduce  the heat sinking effect.

In reflow soldering, both the top and the bottom of the assembly are heated at about the same rate causing the thermocouple wire and its bead to heat as fast as the assembly.  Some times the thermocouple can heat faster then the assembly because it is closer to the heat source and can act as a heat source to the component. This is often true where there is exposed thermocouple wire where the insulation has pulled back from the bead more then 0.5 inches. Keeping the insulation closer to the bead prevents this in most cases.