Tip & How-To about Measuring Tools & Sensors
One of the handiest inventions known to man is the voltmeter, which often also comes with an ammeter (current meter), and ohmmeter (the setting that measures continuity, or resistance of components). The voltmeter will let you know the condition of your battery on your car, and also if your alternator's charging the battery with a good output. Always measure from the the positive battery terminal to the frame of the car (or negative post if the frame is unaccessible), unless you're an experienced mechanic. Measuring the alternator output with the output wire disconnected can blow diodes in the alternator. Measuring at the battery posts will tell you all you usually need to know, however measuring from the positive post to the frame of the car will let you know if the negative ground clip is corroded (you will get a lower reading).
Aside from cars, voltmeter's can be set to the AC voltage setting (VAC) to measure outlet voltages in the house. Sockets do get broken and go bad, or wires can oxidize, and vibrate loose over time, and a low voltage reading (approx. 50vac) will tell you that. Setting the voltmeter to VDC (direct current voltage) will let you check computer power supplies, cell phone, ipod, and GPS chargers, and any other device that uses a power supply. Added bonus is that you can measure all batteries, no matter the type or size. Batteries usually show a bit higher than their rating (e.g. 1.5 volt battery might measure 1.56 or higher), and most AA batteries will work down to 1.25 V or maybe 1.17 V, depending on the current "draw" of the device (technically referred to as "load").
The last, but certainly not least function, is the handy ohmmeter. Ohms is a measure of resistance based on the primary formula E=IR (voltage equals current times resistance). It is most often used to measure continuity of cords, wires, connectors, and fuses, and a small resistance reading (maybe 50 ohms, and also probably an erratic reading, can indicate corrosion or contamination, which can usually be cleaned by sprays or slightly abrasive material (sprays or pencil eraser on gold-plated contacts- fine emery cloth is good for resistors, capacitors, and larger wires).
There is no big trick to using voltmeters, but you do have to remember NEVER, EVER measure voltage with the meter set to OHMS. This will usually fry the ohms circuit, or, if equipped, take out a fuse. If you measure AC voltage on a DC setting, or vice versa, you will just get erratic, erroneous readings. Don't use the DC Current or AC Current settings unless you understand what you're doing. They're for more technical analyzation, and usually only technicians and engineers use them.
You can get voltmeters at surplus stores, Walmart, Lowe's, Radio Shack, and online. I like the digital meters myself, they're easier and faster to read than the analog (meter) type. I've fixed everything from problems with my automobile, computer, accutron watch, and a $750 voltage module in a friend's car with just a voltmeter, so to me it was worth the $20 I invested over 12 years ago. These days you can find them for $5 at surplus stores, less than $20 at most places, and they make handy gifts for all around do-it-yourself handymen/handywomen!
Posted by Tom Carson on
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