= Cold-junction thermal coupling
\b[Cold-junction thermal coupling] is the heat-transfer relationship between a <thermocouple reference junction>, its temperature sensor and the surrounding apparatus. <Cold-junction compensation> needs the actual terminal-region <temperature>. A sensor with excellent electrical calibration can still give an unsuitable correction if it is at another temperature because of <heat conduction>, air currents, nearby dissipation or finite response time.
An <isothermal terminal block> aims to keep both thermoelement-to-lead transitions and the reference sensor at a common temperature while preserving electrical insulation. The TI design discussion identifies two separate requirements: matching the temperatures of the two terminal transitions and measuring that temperature accurately. Its layout discussion considers metal fill, parasitic heating and airflow.<Precision Thermocouple Measurement with the ADS1118 by Mike Beckman and Luis Chioye>[Section2, printedpp.5–6, isothermal-junction and sensor-placement conditions]{ref} NI's documented terminal assemblies likewise incorporate local temperature sensors rather than inferring the terminal temperature from the raw thermocouple signal.<FP-TC-120 and cFP-TC-120 Operating Instructions by National Instruments>[Printedp.7, Cold-Junction Compensation]{ref}
A simple <lumped thermal model> separates the paths. Let $T_j$ be a prescribed junction temperature, $T_s$ the sensor temperature, $T_a$ ambient temperature, $G_c$ the <thermal conductance> to the junction, $G_a$ the conductance to ambient and $P_s$ sensor dissipation. With sensor <heat capacity> $C_s$,
$$
C_s\frac{dT_s}{dt}=G_c(T_j-T_s)+G_a(T_a-T_s)+P_s.
$$
For fixed inputs, the steady sensor error is
$$
\boxed{T_s-T_j=\frac{G_a(T_a-T_j)+P_s}{G_c+G_a}.}
$$
Better coupling to the junction and weaker parasitic coupling to ambient have different roles; sensor dissipation adds a further term.
For an original example choose $T_j=35\,{}^{\circ}\mathrm C$, $T_a=25\,{}^{\circ}\mathrm C$, $G_c=0.040\,\mathrm{W/K}$, $G_a=0.010\,\mathrm{W/K}$ and $P_s=1\,\mathrm{mW}$. Then
$$
T_s-T_j=\frac{-0.100+0.001}{0.050}\,\mathrm K=-1.98\,\mathrm K.
$$
The sensor settles near $33.02\,{}^{\circ}\mathrm C$ despite the junction being at $35\,{}^{\circ}\mathrm C$. These are stipulated thermal parameters, not specifications of the cited devices.
The same equation gives <thermal time constant> $\tau=C_s/(G_c+G_a)$. With $C_s=0.10\,\mathrm{J/K}$, $\tau=2\,\mathrm s$. Waiting several time constants removes the transient toward the steady sensor temperature, but cannot remove the calculated steady mismatch. A stable display is therefore not proof of thermal agreement.
The <reference-junction temperature sweep in thermocouple testing> can expose both effects. At matched plateaus, compare the reference reading with a local independent thermometer; during ramps, compare lag and sweep-direction dependence. If the two reference terminals themselves have different temperatures, even one perfectly measured sensor temperature cannot satisfy the simple two-junction compensation model. The thermal configuration must be tested as part of the measurement chain.
Back to article page
