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Friday, July 6, 2018

Using Multi-Meters


A Communicator Reprise: March 2012

Using Multimeters—Part 2

Last month we looked at multi-meter types. Multi-meters have the ability to measure voltage, current and resistance and more expensive models may add other functions such as temperature. A basic addition to your household tool kit, there are low cost multi-meters available, frequently on sale for less than $10. I’d suggest purchasing a digital rather than analog model as a digital meter is easier to use and will suffice for basic measurements.

Digital models are generally "auto-ranging", a useful feature because you don't need to change the dial to measure different levels. If you think you might be using it in low light, consider getting one with a "back-light."
The first rule for getting the most out of your multi-meter is to read the manual. The manual will have instructions for basic operation of the instrument and safety information about potential dangers. Once you have read the manual, added the batteries, and attached the probes (the wires, which are usually red and black), try some of the example measurements below.


Basic Multi-meter Tests

Resistance Test

Set the multi-meter to read "resistance." Check that the two probes are inserted in the right holes.

What does the readout say when the probes are not touching anything? When the two probes are separated, there is an infinite resistance separating them, since air does not conduct electricity. Make a mental note of your multi-meter's readout for infinite resistance, because it varies with the manufacturer.
Touch the two probes together. Now what does the readout say? When you touch the two probes together, the resistance is close to zero. The metal tips are excellent conductors and the wires offer little resistance to current flow.

Try this…  I first did this as a science fair project. Set the knob to the highest Resistance scale on the meter Dampen two fingers and press one probe to each fingertip. Do you get a reading? With dry fingers you probably won’t get a reading. Dry skin has a resistance of about 1 million ohms, whereas the resistance of moist skin is reduced by a factor of ten or more.
Try it with different liquids including salt water. Did the resistance change? What you are seeing is a Polygraph (lie detector) in its simplest form. As the subject is stressed from telling an untruth, the body produces perspiration which changes the skin’s resistance [scientifically known as Galvanic Skin Response]. Also measured in a professional instrument are blood pressure, pulse and respiration. Once calibrated, a polygraph and trained operator can record and interpret the readings to determine when the subject is truthful or not. For more experimentation check Google or have a look here for a basic kit.

Measure the resistance of some resistors that are not attached to a circuit. For example, test resistors of 100 Ω (ohms), 10,000 Ω, and 1 MΩ (mega-ohm, or 1 million ohms). You can buy these online or at a local supplier. Touch the probes to the wires on either side of the central cylinder. Watch the units: a "k" means kilo-ohms (thousands of ohms), and an "M" means mega-ohms. 
Never measure resistance in a circuit when power is applied. You must also discharge capacitors in a circuit before measuring resistance, because if there is any source of current other than the multi-meter itself, you will get erroneous readings.

Voltage Test

Touch the probes to the terminal ends of a 9-Volt battery [see photo]. You should get a reading of approximately 9 Volts. This one reads 7.57 volts so is obviously spent. The battery has a positive [red] and a negative [black] pole. Note that your multi-meter also has a positive and a negative probe. If you attach the positive probe to the negative side of the battery, it will still read voltage, but it will have a negative sign in front of it.

Current Test

This is probably the trickiest reading to make and one that can damage the meter should you pass too much current through it. Set the multimeter to read "direct current (DC)." Important: Check your multimeter to see where the probe should be plugged in so it reads "current."
Don’t be shy to experiment with your multi-meter.
You will likely find a multitude of uses for it around the house.
In order to measure current, you have to open up the circuit and attach the leads from the multimeter so that the current flows through the multimeter in series. To do this, use jumper wires and wires with alligator clips to add the multimeter to the circuit, as shown.
The current now flows from the 12 volt source through the switch, resistor and the multi-meter because it is part of the circuit.  Because it is connected in series, the meter can be inserted at any point in the circuit shown and show the same reading.



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