Radiometer thermal link ($Q_{\mathrm{link}}\simeq G(T-T_b),\tau =C/G$)

A radiometer thermal link is the intentional path for thermal conduction from receiver to reference block. Near a chosen operating point, write $Q_{\mathrm{link}}\simeq G(T-T_b)$ with local conductance $G$ in $\mathrm{W}{\mathrm{K}}^{-1}$. A weaker link increases the steady temperature rise per added power, $\delta T\simeq \delta P/G$, but also increases the uncontrolled relaxation time $\tau =C/G$ when the link dominates other heat transfer. These are local linear approximations: actual thermal conductivity depends on temperature, and active feedback control introduces its own response. NIST explicitly describes the tradeoff between weaker links and smaller receiver heat capacity.^[ref]