Digital readout meters

Chapter Digital readout meters

Teach Yourself Electricity and Electronics Third Edition Book
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Teach Yourself Electricity and Electronics Third Edition Book

  • Reading a pointer type utility meter is a little tricky, because you must think inwhatever direction (clockwise or counterclockwise) the scale goes. An example of apointer type utility meter is shown in Fig. 3-11. Read from left to right. For each littlemeter, take down the number that the pointer has most recently passed. Write downthe rest as you go. The meter in the figure reads 3875 kWh. If you want to be really pre-cise, you can say it reads 3875-1/2 kWh.56 Measuring devices3-11An example of a utility meter. The reading is a little more than 3875 kWh.Digital readout metersIncreasingly, metering devices are being designed so that they provide a direct readout,and there’s no need (or possibility) for interpolation. The number on the meter is the in-dication. It’s that simple. Such a meter is called a digital meter.The advantage of a digital meter is that it’s easy for anybody to read, and there is nochance for interpolation errors. This is ideal for utility meters, clocks, and some kinds ofammeters, voltmeters and wattmeters. It works very well when the value of the quan-tity does not change very often or very fast.But there are some situations in which a digital meter is a disadvantage. One goodexample is the signal-strength indicator in a radio receiver. This meter bounces up anddown as signals fade, or as you tune the radio, or sometimes even as the signal modu-lates. A digital meter would show nothing but a constantly changing, meaningless set ofnumerals. Digital meters require a certain length of time to “lock in” to the current, volt-age, power or other quantity being measured. If this quantity never settles at any onevalue for a long enough time, the meter can never lock in.Meters with a scale and pointer are known as analog meters. Their main advan-tages are that they allow interpolation, they give the operator a sense of the quantityrelative to other possible values, and they follow along when a quantity changes. Someengineers and technicians prefer the “feel” of an analog meter, even in situations wherea digital meter would work just as well.One problem you might have with digital meters is being certain of where the dec-imal point goes. If you’re off by one decimal place, the error will be by a factor of 10.Also, you need to be sure you know what the units are; for example, a frequency indi-cator might be reading out in megahertz, and you might forget and think it is giving youa reading in kilohertz. That’s a mistake by a factor of 1000. Of course this latter type oferror can happen with an analog meter, too.