Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.2 9/18/84; site ut-sally.UUCP Path: utzoo!watmath!clyde!bonnie!akgua!gatech!ut-sally!crandell From: crandell@ut-sally.UUCP (Jim Crandell) Newsgroups: net.consumers,net.analog Subject: Re: power meters Message-ID: <1450@ut-sally.UUCP> Date: Tue, 2-Apr-85 14:12:25 EST Article-I.D.: ut-sally.1450 Posted: Tue Apr 2 14:12:25 1985 Date-Received: Thu, 4-Apr-85 06:24:19 EST References: <500@harvard.ARPA> <973@dual.UUCP> <264@rtech.ARPA> <974@dual.UUCP> <267@rtech.ARPA> Distribution: net Organization: U. Texas CS Dept., Austin, Texas Lines: 66 Xref: watmath net.consumers:2103 net.analog:252 Okay, I readily concede that mentioning common sense on USENET borders shamefully on the oxymoronic, but I'd like to suggest something on this subject. The cheapest device you can find for measuring the cost of operating an appliance -- and that IS the bottom line here, isn't it? -- is so because you don't have to buy it. You already have it. It is (TA-DA!) the exact same device the electric utility company uses for the same purpose. Now, there is one catch for some users: if the meter is mounted about four feet above eye level (I've seen that done) then the activity may prove sufficiently unenjoyable to motivate one to seek another method. But anyway, remember the little wheel someone mentioned? If you get close enough to the meter, viewing it from slightly above its level, you'll see inside it a thin metal disk a few inches in diameter. This disk is (at least in some models) the armature of the motor in the device that does the actual measuring; the theory is that the speed of the motor is directly proportional to the average power over some relatively short interval, and the speed is reduced by gears to drive the indicators. Now the nice feature for our purposes is that the disk is always marked in such a way that its orientation is visually evident at least most of the time. Marking schemes vary, but one feature that seems to have been universally adopted is the big black blob (actually, sometimes its a very neat stripe) at one point near the edge, so you can always count whole revolutions, at the very least. (Okay, it isn't always black, either.) The motor is usually designed so that one of its revolutions represents some nice round fraction of a Watt-hour, but the exact fraction varies by model, so you 'll have to figure out what it is (I've long suspected that there's a universal marking scheme on kWH meters which encodes the value but I've never tried to discover it). The ``calibration'' procedure is quite straightforward, but you may choose to perform it under conditions of daylight and agreeable exterior atmospherics. As simply as possible, you disconnect (unplug or turn off) EVERYTHING connected to your power lines, if possible. Don't worry about the contents of the fridge -- it won't be off long enough to matter. Occasionally there's a load which really can't be easily turned off, such as a doorbell transformer, al- though you can switch off everything at the breaker panel. (If you can't, and if you rent, complain to your landlord at once!) Leave one circuit on, and connect to it only a lamp containing a standard 100-Watt bulb. Of course, you can go get a 144-ohm, 1% wire-wound resistor if you feel that you really must, but we don't really need that kind of precision here. After all, we're looking for relative measurements, aren't we? The rest is pretty obvious: the revolution period under this load (call it T) represents 100 Watts; we then disconnect the lamp and connect instead the device whose consumption is in question, and if it spins the disk once in time X, then the device is partaking of just about 100*T/X joules every second, or at least that's the way the electric company's reasoning goes. > Domestic electricity meters *do* measure true energy. They will even run > backwards if there is a net outflow of energy. True in most cases today. Surely by now everyone's at least heard how various sleazy characters have contrived to rip off the electric companies. NO, I am not recommending or condoning such practices! They are illegal, and for very good reason. But in some areas, sad to say, there are old- style noninstantaneous-integrating meters still in use. However, measuring the power consumption of each appliance on the same instrument the utility company uses to bill you guarantees that you correctly attribute to each item its real share of the bill, assuming you really know how many hours each month each one is using power. If on the other hand, what you really wanted to do was to check the calibration on your service kWh meter, then of course, none of the fore- going would be of much concern to you. -- Jim Crandell, C. S. Dept., The University of Texas at Austin {ihnp4,seismo,ctvax}!ut-sally!crandell