Pic related is a respiratory calorimeter. It measures co2 and oxygen levels while a person breathes to determine their metabolic rate.

I've read many older medical textbooks that recommend doctors measure the metabolic rates of patients to determine various things about their condition.

How did they measure these rates in the past? Is there a way someone can measure their own metabolic rate without a respiratory calorimeter? If not can a non medical professional purchase one of these devices?