Abridgment of 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 electric healer, the change in temperature being accurately measured either by means of resistance thermometers connected in a wheastone 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 paper deals with the general proportions of the volume meter as to section and length, 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. Equations 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: a. The non-uniform heating of the gas. b. Non-uniform distribution of velocities. c. Heatescapement. Methods are suggested for determining the magnitude of these experimentally or by calculation. There are several minor sources of error, such as: 1. Time lag of meter. 2. Unsteady source of e. m. f. 3. Instrument errors. 4. Heat flow along thermocouple wires. 5. Loss of pressure in passing through the meter. 6. Variation of specific heat and humidity. Most of these are shown to be of negligible effect. Some notes on design are given, which include means of calculating the proportions of the heater. In Appendix I the advantages and disadvantages - f various methods of measuring gas volumes are included. In Appendix II will be found some notes on comparison of resistance thermometers and thermocouples for temperature measurement. In Appendix III consideration is given to the electrical machine as a thermal volume meter.