A. Interpretation of potential measurements. 1. Maximum voltages and track gradients. These measurements, when considered in the light of a full knowledge of all conditions, give valuable data on the condition of the railway track system and the concentration of return current on certain sections of track. They also are valuable when considered in the light of the load on the different lines, as they offer a fairly accurate indication of the track losses and the necessity for the use of additional track feeders. When such potential measurements are taken over relatively short lengths of track, such as 1000 or 2000 feet, the comparison of such measurements on adjacent sections of track will often reveal bad places in the track that are in need of rebonding. 2. Potential difference measurements. Potential difference readings between pipes and railway tracks and between various underground structures are not a quantitative measure of the danger to the affected structures. These readings are valuable in pointing out the general areas in which trouble may be expected to occur and in which more careful search may be made if desired. They are, however, of qualitative significance only. The current leaving a structure for the earth in any locality, which is the real cause of the electrolysis damage, is a function not only of the potential differences but of the resistance of the earth paths. This has been shown to vary throughout extremely wide limits, so that the measurement of potential difference gives no definite quantitative measurement of the extent of the hazard to the pipes. Such measurements are very valuable and have an accurate quantitative significance, however, when used to determine the relative electrolysis conditions under different systems of mitigation. If, for example, under a given set of conditions a considerable number of potential difference measurements are made between the various underground structures and then a change is made in the mitigative system, and the same measurements repeated, the two sets of readings may be used to represent the comparative hazard in the two cases. This is true only if the mitigative measures under test are applied exclusively to the railway return system. High potential differences between gas and oil pipes and other metallic structures with which they may come in contact are objectionable especially in confined spaces, such as basements in which explosive mixtures may be encountered. This is because a transient contact between the two structures may cause an arc which may result in fire or explosion. B. Interpretation of current measurements on underground structures. 1. Relation of stray current to corrosion. The magnitude of the current on an underground structure does not alone afford a measure of the total injury to the structure. If all the current that flows on the pipe is discharged directly, into the earth, then the total corrosion will be approximately proportional to the current flow. Even here, however, the rate of damage to the pipes is not only a function of the total weight of metal corroded away, but of the distribution of such corrosion as a result of pitting or of localized discharge from one system to another where they approach close to each other. Further, if there are metallic connections either known or unknown between portions of the pipe networks and the railway tracks, which carry off a large part of the current on the pipe through metallic paths, the total amount of corrosion cannot. be determined by measurement of the current flow. For this reason current measurements on pipes should likewise be regarded as having only a qualitative significance in so far as any absolute hazard. to the pipes is concerned. If, however, the pipes have no drainage. connections and changes are made in the railway track network, the corresponding changes in the currents on the pipes may, if a sufficient number of readings have been taken, indicate the relative improvement in the electrolysis conditions. 2. Relation of current to fires and explosions. In interpreting the significance of current measurements on gas or oil pipes, due account should be taken of the possibility of fires and explosions due to arcs formed either when pipes are disconnected, or when pipes make transient contact in confined places such as cellars. No definite information is at present available as to what limiting currents on such pipes may be considered safe, but it is generally recognized that the presence of currents on gas and oil lines is more objectionable than in the case of other pipes. C. Interpretation of measurements of current flowing from structures to earth. The only accurate criterion of electrolysis damage is the intensity of current flow to earth at any point on the pipe or cable. If an accurate measure of this current flow from the pipeat any point could be made, it would come nearer giving a true indication of electrolysis conditions than any other measurement. Earth current meter data, obtained and interpreted by an experienced engineer, give quantitative information on the current in the earth at a particular point, and to this extent generally afford an accurate estimate of electrolysis conditions in the small area immediately adjacent to the test electrode. CHAPTER XIX WATER CONSUMPTION Water consumption in American cities is large in practically every instance and frequently it averages more than 100 gallons per capita daily. Instances of average water consumption of 75 gallons are comparatively rare, notwithstanding that this figure is about three times the record shown by representative European cities. Recently obtained statistical data from some of the larger American cities are set forth in table 22. Appreciation of why American communities require so much more water than those in Europe involves an inquiry into a comparison of domestic consumption, industrial consumption and the control of waste. To these questions is related that of metering. Domestic consumption Various estimates have been made as to the volume of water actually used and necessary for domestic consumption. A few years ago such figures ranged from 25 to 30 gallons per capita daily. More recently these figures have been increased by about 5 gallons. It depends upon the type of community, its climate and needs for the use of water outside the house for sprinkling lawns, cleaning automobiles and irrigation of gardens. A pitometer survey at Toledo of three classes of districts was checked by gagings in sewers built in practically impervious clay through which there was substantially no leakage either from or into the sewers. These observations were made during the first week in March when no water was used for lawn irrigation purposes. For the highest type of residential district the water consumption was 48 gallons per capita daily. For two lower classes the consumption was 32 and 15 gallons respectively. The weighted average for three classes of districts was 34 gallons per capita daily. In considering such data as above set forth it is to be borne in mind that there are 5 to 10 gallons per capita daily of water used for public purposes in carefully managed communities. These include water for fire protection, sewer flushing, public buildings |