# Empirical temperature problem

## Homework Statement

In almost every thermodynamics book, I see that they define empirical temperature in terms of parameters of a constant volume gas thermometers.
Like: t=(t_3)[(lim P_3 tends to 0) ((P/P_3)_v)].Here 3 corresponds to triple point.Why?If they are to define empirical temperature and they are to refer to triple point then why they do not take thermocouples or resistance thermometers?
Is it because that for constant volume gas thermometers the exposure to the triple point temperature is readily accessible?