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1.This document is AE-23, one of a series of the Department of Agricultural and Biological Engineering, Florida Cooperative Ext ension Service, Institute of Food and Agricultural Sciences, University of Florida. Date reviewed: March 1999. Please visit the FAIRS Web site at http://hammock.ifas.ufl.edu .The Institute of Food and A g ricultural Sciences is an equal opportunit y /affirmative action emplo y er authorized to provide research, educational information and other services onl y to individuals and institutions that function without re g ard to race, color, sex, a g e, handicap, or national ori g in. For information on obtainin g other extension publications, contact y our count y Cooperative Extension Service office. Florida Cooperative Extension Service / Institute of Food and A g ricultural Sciences / Universit y of Florida / Christine Ta y lor Waddill, Dean2.James M. Stanley, Visiting Professor, Extension Energy Specialist; Allen G. Smajstrla, Professor, Dalton S. Harrison, Former Professor, Extension Water Management Specialist; and John C. Good, Former Adjunct Extension Agent 1, Department of Agricultural and Biological Engineeri ng, Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida, Gainesville, 32611.Figure 1 Kilowatt-hour meter re g isters ( A ) di g ital or c y clometer t y pe ( B ) pointer t y pe. Both t y pe re g isters show same reedin g .AE-23Reading Electric Meters1 James M. Stanle y Allen G. Sma j strla, Dalton S. Harrison and John C. Good2How to Determine the Amount of Electric Energy and Power RequirementsTo determine the efficiency of electric powered equipment, you need to know the power requirements and amount of energy consumed. This is also helpful in conserving energy. Electric energy is measured in kilowatt-hours, which is the unit used in b illing customers. A kilowatt-hour (kW # h) is the equivalent of 1000 watts of power being used continuously for one hour or of other combination of power and time to obtain this product. A 100 watt lamp operated for 10 hours consumes one kilowatt-hour of energy. The electric power supplier measures the amount of energy consumed by customers with a kilowatt-hour meter. The same meter can be used to determine the power required for a particular operation as will be explained.Kilowatt-Hour MetersKilowatt-hour meters have one of two types of registers that must be read to determine the energy consumption. If the meter has a digital or cyclometer register as shown in Figure 1 (A), the kW # h usage is read just as the odometer in an automobile is read to determine mileage traveled. Some of these kW # h meters, usually those with four numbers on the register, have a notation on the meter face specifying that the reading be multiplied by a constant, usually 10. This means that the register does not indicate individual kW # h values less than the size of the multiplier. A second type of meter register is the pointer type as shown in Figure 1 (B). The reading of this type of meter requires more practice since the pointer or dial hand on any one dial rotates in the opposite direction from the one next to it. The direction of rotation of each dial can be determined by the direction of the increase in number size. This rotation direction must be determined and the dials read from right to left in sequence. Meter readings are recorded by writing the numbers read from each dial in the same order (right to left). Note that both meter registers illustrated in Figure 1 show the same reading. The dial on
Readin g Electic Meters Pa g e 2March 1999 Figure 2 Re g ister readin g 43894 Kilowatt-hours. Figure 3 Re g ister readin g 05049 kilowatt-hours. Figure 4. Combination Kilowatt-Hour and Kilowatt Demand Meters. ( A ) Maximum demand shown b y pointer on outer dial. Note: Readin g to be multiplied b y 10. Thus demand is 6 kilowatts. ( B ) Maximum demand read on pointer t y pe re g ister. The vertical line indicates position of decimal point, thus demand is 41.47 kilowatts.the extreme right must make one complete revolutionrecorded as 3. For another illustration, see Figure 3. Here before the dial on its left moves one number or one-tenththe fourth dial from the right shows the dial hand pointing of a revolution. This sequence is followed across theat 5. In this case the pointer on the dial to the right is register. Some of these meters also have multipliersbetween 0 and 1, therefore, the fourth dial reading s hould designated on the dial.be recorded as 5. The value of each dial reading is determined by the number which the pointer has just passed. If the dial hand is pointing directly at a number, you might have difficulty reading the dial. See the fourth dial from the right in Figure 2 for an illustration. The pointer on this dial appears to be pointing to the 4. Should the 4 or the 3 be recorded? Since the pointer on the dial to the right of the one being considered is pointing between 8 and 9 the number in question should beDemand MetersSome kilowatt-hour meters w ill carry extra registers to designate the maximum kilowatt (kW) demand of the load. Two types of demand meters are shown in Figure 4. In Figure 4 (A) the maximum demand is indicated on the large dial around the outer rim of the meter face. In this case the value is 0.6 x 10 (the multiplier specified on the dial) or 6 kW. In Figure 4 (B) the demand 41.47 kW is read on the lower register. The vertical line between the dials indicates the decimal point location. These meters are used in those cases where the electric rate schedule is based on the maximum kilowatt demand or requirement for a 15 or 30 minute interval during the billing period and are read by meter readers and reset to zero. The reading on this register is not necessary in determining the energy usage.Meter Disc Rev olution Determ ining Power Requir ementIf the power requirement for a particular operation for
Readin g Electic Meters Pa g e 3March 1999 Figure 5. Kilowatt-hour meter showin g meter disc with revolution reference mark and meter constant ( K ) = 1.5.hEquation 1.a short period of time is desired, the same kilowatt-hourpower required to operate the load can be calculated as meter, either register type, has a revolving disc and a meterfollows: constant (K) as shown in Figure 5 that can be used. Forhthe meter shown this constant is 1.5. This means that forPower (watts) = each disc revolution, 1.5 watt-hours of energy have been used. The constants differ for various meters, so always3600 x No. of disc revolutions x Disc constant (K ) .. (1) check the particular meter being used for its constant. TheTime (seconds) meter disc has a mark readily observed on its edge (see Figure 5 for one type of marking) that aids in determiningExample: If 48 seconds of irrigation pump operation are complete revolutions of the disc.required for the disc to make 2 revolutions with a disc With the meter measuring only the energy consumedconstant, K = 57.6 watt-hours per revolution. The power for the particular load in question (other equipment not energized), the time required for a given number of revolutions of the disc should be measured with a stop watch or other method of accurate timing. The number of revolutions to be counted w ill vary with various meters because of different disc speeds and loads being considered. The time period should be ample to permit stab ilizing of the load so that true representation of the power used will be made. For example, when measuring power used by an irrigation pump, make certain that the pressure on the system has stabilized. For meters with high valued constants, (i.e. 57.6) a small number of rotations will be adequate while for meters with low valued constants (i.e. 1.5), more rotations will be n ecessary. From the number of observed disc revolutions and time in seconds, theh h(watts) required is: This is the input power and can be converted to horsepower (hp) by multiplying by 1.34 hp per kW. In the example given, 8.64 kW is equivalent to 11.58 hp. In some cases, especially if the load is heavy, all of the electric current is not passed through the meter for direct measurement. Rather, transformers are used to monitor only a fraction of it and a multiplier must be used with equation (1) to obtain an accurate measurement of the power consumption. The multiplier will be printed on the monthly electric bill. It also is usually shown on the meter face. However, if there is a question concerning it, the electric power supplier personnel can provide the value and assist with using it to determine the power consumption.Determ ining energy consumption and operating costIf the pump in the above example continued to operate for one hour, 8.64 kW # h of energy would have been consumed. This amount mu ltiplied by the unit kW # h cost will provide the operating cost. The energy consumption can also be determined by differences in meter readings obtained by reading the meter at regular intervals of time. This method is beneficial in determining the energy requirements and operating costs for a longer period of time than is normally considered when using the disc revolution method.