The Thermal Volume Meter

The thermal volume meter was originally brought out by Professor Thomas, and is frequently called a Thomas meter. The principle is that the temperature of the gas is raised by means of an electrical heater, the change in temperature being accurately measured either by means of resistance thermometers connected in a wheatstone bridge network, or by means of a thermopile. It is believed that the thermocouple method is preferable to resistance thermometers, and most of the paper deals with that construction. The usual construction has walls of wood or micarta, which serve as an insulating support for the heater coils and thermocouples. The general proportions of the volume meter as to section and length are dealt with, the former being determined mainly by the mean velocity of the air, and the latter by the distance needed to give the gas an opportunity to mix after passing the heater. A sufficient number of thermocouples is required to give a good reading of the potentiometer with a comparatively low temperature rise of the gas and also to obtain a good average of the gas temperature. The details of construction are described. The equations that are suitable for calculating the volumes of the gas are then given. Then follows considerable discussion on the possible sources of error in the meter, the principal ones being The non-uniform heating of the gas. Non-uniform distribution of velocities. Heat escapement. Methods are suggested for determining the magnitude of these various conditions experimentally or by calculation.