INTRODUCTION

This section describes various types of instrumentation used in the practical application of basic gas laws to metering. Included are chart recorders of various types associated with distribution metering, meter-mounted correction devices, pulse transmitters (other than remote reading) and load survey equipment.

A diaphragm, rotary or turbine meter measures gas at the conditions of pressure and temperature existing as the gas flows through the meter. It is this volume that is indicated on the conventional-type index normally found on the meter. Since gas is purchased and sold at certain specific base conditions of pressure and temperature, it is necessary that when measurement is at conditions other than these, a means must be provided for correcting the volume as measured, back to these base conditions.

There are several common methods used to accomplish this. One of these involves the use of chart recorders on which the variation in pressure and/or temperature and time are documented. A visual record is generated from which, by manual or mechanical methods, corrections to the uncorrected volume can be made. A second method involves the use of devices capable of mechanically introducing a correction factor based on Boyle's Law with respect to pressure and/or Charles' Law when temperature variations are involved. Since natural gas is a mixture of several gases, and therefore is not truly a perfect gas, it deviates under pressure somewhat from the composite effects of the Ideal Gas Laws. The deviation is called "Super compressibility". Super compressibility in not significant at the conditions at which most domestic customers are served; however, at the pressure at which pipeline gas and some commercial and industrial loads are measured, it is important that super compressibility be included in the correction applied.

The subject of chart recorders will be covered first, followed by mechanical correction devices and associated peripheral items. Finally, the subject of load Survey equipment and its peripheral accessories will be described.