Part One of a demonstration of principle functions and features of the Fluke 8808 bench-type multimeter.
Hi and welcome to our 57th Test and Measurement Video. Right now we’ll look at the amazing Fluke 8808A bench-type multimeter. For perspective, here are some far less sophisticated but sometimes useful tools that will measure electrical energy.
This plug-in socket equipped with a durable and rugged appliance bulb is a quick and easy receptacle tester that electricians like for working on branch circuits. To find a defective termination, you can put one of these in each wall receptacle and from the other side of the room they can be monitored, which is helpful in working on intermittents.
The neon test light is great for checking out presence or absence of voltage in entrance panels and motor controllers, just to get a preliminary idea of what is going on. Variations in brightness permit the user to differentiate between 120 and 240 volts.
The solenoid voltage tester, so-called Wiggie, is convenient for quick presence or absence of voltage tests. It makes a visual and audible report, and when you touch the probes to live terminals, it vibrates, so it is great for noisy environments and also when it is not convenient to keep your eyes on the meter. The solenoid voltage tester is a low-impedance instrument, so it is not suitable for sensitive electronic measurements, because it loads the circuit and draws a significant amount of current. It is used for house current and general-purpose electrical work. It should not remain connected for any length of time or it will overheat.
Low-end digital multimeters can be bought in big box stores for as little as $10, but they are neither accurate nor durable, and the continuity beeper quits working after a few uses. A higher-priced hand-held multimeter is useful in many applications. Quality and durability vary widely, but it is actually the workhorse of the industry. For under $500, a superb hand-held multimeter can be purchased and there is no reason that, except for the battery, it should not last a lifetime, maintaining a very reasonable level of accuracy.
Today we’re looking at a far more precise and extraordinarily well-designed instrument. This 8808A bench-type multimeter has the superb built-in features and over-all quality that we are accustomed to seeing in Fluke products. The meter is rugged and over-all a very smart machine.
The dark-colored buttons in the middle row below the display are function keys, similar to just about all multimeters. Here’s a rundown:
Just after the manual range control is DC Volts. Notice that when the probes are not connected to an energy source, there is a fairly low reading that seems to fluctuate. This is called phantom or ghost voltage and it is a consequence of the fact that we are using a very high-impedance meter, meaning that it places a negligible load on the circuit being tested. It constitutes a voltage divider with such high impedance that a minute amount of current that comes from capacitive coupling or a probe acting as an antenna can cause a significant voltage drop. If we shunt this 3000-ohm resistor across the probes, most of the phantom voltage is suppressed. Watch as it drops down to a tenth of a millivolt.
You don’t have to be too concerned over phantom voltage, because as soon as you hook onto a real quantity, the meter’s autorange kicks in and the reading stabilizes. Here’s a 1.5 volt AA cell, which reads 1.6 volts since it is brand new. Notice the very small noise floor that shows up at the fifth decimal place.
Next on the agenda is AC volts, and one good thing about these probes is that unlike many they fit easily into a receptacle or cord connector to give a good reading. The utility voltage is shown at 122 with a little decimal point activity, probably due to loading fluctuations in the neighborhood.
While we are connected to the powerline, we’ll demonstrate two other features of this amazing instrument. The update rate can easily be changed by the user. Pressing the Rate button, the fifth from the left in the row of white buttons at the bottom, the update rate, which corresponds to how frequently new measurements are displayed, can be made slow, medium or fast. Right now it is fast, and the fluctuations are very rapid. And this is what slow looks like.
With the Fluke 8808A multimeter still plugged into the grid that includes all major power plants in Canada and the USA connected in parallel, we’ll move to the right and press Frequency. A very stable 60 Hz is displayed, because frequency does not change except for a maximum phase shift of 90 degrees with a hypothetical purely reactive load.
Next, being sure first to disconnect the probes from the 120-volt utility supply, we press Ohms. Now let’s revisit that 3K resistor. It reads 2.98 kilohm, which is within tolerance. When we touch the probes together, we get 0.070 ohms, the very small resistance, less than a tenth of an ohm, probably due to where the probes contact. My resistance is around 2.25 Megohm, primarily contributed by the dry skin. The resistance of a piece of glass is in the high megohm range.
Very low resistance measurements are notoriously difficult to make where a high level of precision is required. The Fluke 8808A multimeter offers a unique solution that is not seen in hand-held multimeters. It is the four-wire resistance measurement. To see how it works, first we’ll touch the probes together using the conventional two-wire resistance hookup. The reading is .07 ohm. Then, substituting the high-precision resistance probes with gold-plated tips and four-wire adapter and again touching the probes to one another, we see that the reading is shown to the third decimal place and with much greater stability.
The Fluke 8808A multimeter is also capable of making current measurements in the milliamp range and up to 10 amps. Specialized probe jacks, prominently colored red, are used. Great care must be taken not to overload the meter when making current measurements because in this mode the instrument is placed in series with both source and load. The meter now offers very low impedance and the entire current flows through the instrument, as opposed to in the high-impedance volts mode, when very little current flows through it. In the current measuring mode, the probes must never be placed across the power source. In making this measurement, it is necessary to cut open the circuit and later resolder. For high current measurements, the electrician’s clamp-on ammeter works well and for low-current measurements, the oscilloscope clamp-on probe is used to good effect. A clamp-on device measures the magnetic field surrounding a conductor and there is no direct electrical connection.
In Part 2 of this demonstration, we’ll discuss some of the more advanced features of the Fluke 8808A bench-type multimeter, including Dual Display, and we’ll connect it to a Tektronix high-precision programmable power supply to test its accuracy.
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