The purpose of a multimeter is to enable you to test electrical circuits and record resistance, voltage and current measurements for future reference. Recording circuit measurements during alarm commissioning is crucial, otherwise after a false alarm or system malfunction you would not know if any of the readings had changed. But how do you know if the meter is accurate and safe to use?
You must always comply with the current Health & Safety Requirements. Before you go plunging the test probes into potentially dangerous voltages, carry out a visual (and nasal!) inspection of the meter. I’m not joking! It is amazing how many multimeters get blown up by accidental overloads, even by experts. After taking current measurements, it is easy to forget to plug the test leads back to volts, so next time you connect to test mains, there is a big bang. Before attempting to use the meter, sniff around the socket inputs for any noxious smells. This is the first sign of potential danger.
If all seems well, switch the meter on to see if the low battery symbol is flashing. It is incredible the number of multimeters that are returned for re-calibration, simply because the battery needs replacing. While you have got the back off the meter, check the fuses. Are they the correct size and value to protect YOU and the meter, or have they been by-passed with wrapped wire, silver foil, nails or screws? Apart from the obvious danger of electrocuting yourself and blowing up the meter, wrong fuses will seriously impair its accuracy. If they are wrong or blown, replace them immediately.
We can now run through the basic functions. First, have a close look at the LCD. How many digits are there and are any segments missing? Most hand-held multimeters have a 3.50 digit display. A digit represents all numbers up to and including zero, while a .50 digit represents the figure 1. So, a 3.50 digit meter can read up to 1999. Missing segments are often caused by dirt or loose connections between the PCB contacts and the LCD. Replace the meter if the LCD display cannot be fixed. Some multimeters include a sliding ‘bargraph’ scale which moves up and down with digital readout. The bargraph harks back to the days when all multimeters were analogue and had a needle moving across a mirrored display. The advantage of the bargraph is that it enables fluctuations in measurement to be seen much faster than a digital readout.
Continue to check visually the rest of the meter for safety, paying special attention to the test leads. Many multimeters are returned faulty simply because of defective leads. To avoid certain electrocution, never use a meter that is physically damaged, or if the test leads are faulty. Make sure that exposed metal probes are fully insulated to within 2mm of the tip and always carry a spare set of suitable test leads. Now, which sockets to plug into, and what range to use to perform a test? Before you can use your meter you need to understand the basic functions, and test it for accuracy. Most multimeters have either three or four input sockets; COM (usually black) and V Ohm (usually red) to measure volts and Ohms (resistance). To measure AC/DC current, the test leads must be connected between either COM and mA (for milliamps) or 20A (for up to 20Amps). Before connecting the test leads to any live circuit, the meter should be turned on and switched to the correct function and range. Start by testing the leads themselves. Switch to the Ohm symbol and plug the test leads between COM and Ohm input sockets. Is your multimeter ‘Manual’ or ‘Autoranging’? Manual multimeters have a rotating switch which allows you to select a specific range within a function (e.g. 200 Ohm, 2k Ohm, 20k Ohm etc) whereas Autoranging meters have a rotating switch to select functions and a range button which, when pressed, repeatedly changes the range (e.g. switch to the Ohm position and then press the range button repeatedly to select 200 Ohm, 2k Ohm, 20k Ohm etc).
To test the meter leads properly we need to select the 200 Ohm resistance range. Depending on the type of LCD display, the meter should show either OL or a flashing 1 (both mean off limit). Now, short the test probes together to measure the lead resistance. A good set of test leads should normally measure about 00.1 Ohm (that is one tenth of 1 Ohm). With the probes still shorted, waggle the leads about and if the resistance changes significantly, they are faulty.
Most multimeters include an ‘audible buzzer’ continuity range which sounds when measuring very low resistance (usually below 20 Ohm). This enables you to perform audible continuity tests without having to look at the meter. Now that we know the test leads are safe to use, let’s test the meter display on all the resistance ranges. With the test lead probes still shorted, switch to each resistance range in turn and the decimal point should move position as follows: 200 Ohm = 00.1, 2k Ohm = .000, 20k Ohm = 0.00, 200k Ohm = 00.0, 2M Ohm = .000, 20M Ohm = 0.00. (1k Ohm = one thousand Ohms, 1M Ohm = one million Ohms). Before we can use the multimeter to record resistance readings, we first need to check the accuracy of each range against a known resistance value. We can use a ‘powered up’ PIR and an 18k Ohm resistor. Select the 200 Ohm resistance range and connect the test lead probes to the alarm contact terminals of the PIR. Write down the ‘normally closed’ resistance reading obtained and compare it to the resistance specified in the PIR instruction sheet (e.g. 10.0 Ohms). Providing the reading is to within plus or minus 5 per cent, the meter resistance range is accurate. Switch the meter to the 2k Ohm range and record the reading obtained (e.g. 0.10 Ohms).
The meter resolution has changed, but the resistance reading is the same. Check the accuracy of the remaining resistance ranges with an 18k Ohm resistor. Range: 20k Ohm = 18.00, 200k Ohm = 18.0, 2M Ohm = .018 and 20M Ohm = 0.01. To obtain a variety of readings, you can use a wider range of resistors or have the multimeter calibrated.
Did you know that your body resistance changes when you tell lies? Try this out on your children at home. Switch the multimeter to the 20M Ohm range and get them to hold the test lead probes (one in each hand) using light finger pressure. Ask a trick question to catch them out and watch the meter’s reaction! If they lie, the resistance reading will suddenly change. Now moisten your fingers and squeeze the test probes to vary the resistance. The less the resistance, the lower the reading. Everybody has a different level of body resistance, but the last thing you want to do is poke wet fingers into a mains socket.
To understand how a multimeter measures resistance, it needs to be explained simply. The meter sends a small voltage and current (supplied by the battery inside the meter) which passes through the circuit under test and back into the meter. With the test leads shorted together, there is hardly any resistance, so all the current flows back into the meter and the resistance reading calculated is 0. When you connect the test lead probes across a conductive material (e.g. water, metal, cable, skin), the type and amount of conductive material produces a resistance. This resistance reduces the current flow returned into the meter and is calculated and displayed as the measured resistance. A good way to understand resistance more clearly is to take alarm cable and magnetic contacts as an example. If you short out a pair of wires at the end of a 100m roll of ordinary alarm cable and measure the looped resistance with your multimeter, you will get a reading of approximately 10.0 Ohm. So you can work out that a 10m cable should give a loop resistance reading of 01.0 Ohm, which will be confirmed by your meter. The resistance of a new (closed) magnetic contact is 0.1 Ohm). So if you had, say, 50m of cable with five magnetic contacts wired in series, the estimated circuit resistance should be 05.5 Ohm, again verified by your meter.
We now need to check the rest of the multimeter functions. Let’s take volts next. Again, you select the appropriate range by rotating the switch to the desired position, or by repeatedly pressing the range button. Most multimeters include the following AC/DC voltage ranges: 200mv, 2v, 20v, 200v, 750v, 1000v. You can check the basic accuracy of the DC ranges (except for the mv range) with a 1.5 v battery. Before connecting to any live source of supply, make sure that the test leads are connected between COM and V for volts. Select the 2v DC range and connect the test lead probes to the battery terminals; red +, black -. A new 1.5v battery should display a reading slightly over 1.500 v. Next, select the 20v range and the meter reading should change to 1.50 v. Switching to the 200v range should change the reading to 01.5 v. Finally, on the 1000v range, it should change to 001v. Again, they are the same reading, it is just the resolution that has changed. With the accuracy verified to within plus or minus 5 per cent, you can now confidently use the meter to test and record all the DC voltage measurements to PIRs, keypads, LIMs and standby battery. The 20v, 200v and 750v AC ranges can be tested for accuracy by carefully connecting the test lead probes ‘in parallel’ with a control panel incoming mains and transformer output supplies. Make sure your meter is suitable for connection to the mains supply. When uncertain as to the voltage level, always switch the multimeter to its highest AC/DC range to perform an initial test. Once the level of voltage is established, you can switch down one range at a time to obtain the highest resolution. When testing any voltage, always connect the black test probe first and remove it last.
Measuring resistance and voltage have one thing in common! You take measurements by connecting the test lead probes in ‘parallel’ with the circuit under test. However, there is one very important difference. To measure AC/DC voltage, the circuit must be connected to the source. To measure resistance, the circuit must be disconnected from the source. As you now know, when measuring resistance the meter applies a small voltage and current through the circuit which comes back into the meter. If the circuit under test is connected to another voltage source, the resistance reading displayed on the meter will be totally meaningless. To save time when measuring resistance, you only need disconnect one leg of the circuit from the source. If you accidentally forget to do this, the meter has ‘built-in idiot protection’ However, when it comes to measuring AC/DC current, safety issues are very different! Most people hate taking current measurements, because you have to connect the multimeter ‘in series’ with the circuit in order to test; potentially dangerous if you are not careful or have not checked out the current ranges on your multimeter beforehand. Most multimeters include these AC/DC current ranges: 200muA, 20mA, 200mA, 20A. (muA = microamp, mA = milliamp, A = Amp). 1000muA = 1mA, 1000mA = 1Amp.
Danger warnings
To safely measure microamps or milliamps, the test leads must be connected in to the meter sockets marked COM and muA or mA. When measuring Amps, the test leads must be connected between the sockets marked COM and 20A. Before attempting to measure current you should have carried out a visual safety check to ensure that the correct type and value of fuses are fitted to protect YOU and the meter. To prevent injury or electrocution, never connect the test lead probes in parallel across any AC or DC live source with the multimeter switched to muA, mA or Amps. To check the accuracy of the meter DC current ranges you can use a ‘powered up’ PIR. Select the 20mA range on the multimeter and connect the test leads between the sockets marked COM and mA. Next, disconnect the + DC voltage wire from the positive supply terminal (this can be done either at the power supply or PIR). Connect the test lead probes ‘in series’ with the removed + wire and the positive supply terminal (if a minus reading is displayed, reverse the test leads). Allow a few minutes for the PIR to warm up, then record the mA current continuously used by the PIR (e.g. 15.00 mA).
Verify the accuracy of the 20mA meter range by comparing the result displayed against the current specified in the PIR instruction sheet. A plus or minus 5 per cent tolerance is acceptable. Next, confirm the accuracy of the 200mA range (e.g. 15.0 mA), and finally connect the test leads between COM and 20A sockets and switch to the Amps range (00.1 mA).
The reading is the same but the resolution has changed. To check the AC current ranges on the meter you can use the transformer AC output voltage in a control panel or power supply unit as follows: Select the 20Amp AC range on the multimeter and make sure that the test leads are connected between COM and 20A. Disconnect either (but only one) of the AC output voltage wires from the transformer to the control panel PCB terminals. The control panel will now be operating from the standby battery supply. Next, carefully connect the test lead probes ‘in series’ with the removed transformer wire and the PCB terminal. Be prepared for a spark! The reading displayed shows the amount of AC current being used by the alarm system and to charge the battery. The amount of AC current used will vary depending on the size of the alarm system.
thankyou for explaining how to basically check if the multimeter is good to go – it is after all imperative – thanks again
I appreciate how you explained that a multimeter is beneficial because it can enable you to test electrical circuits, record resistance, and record voltage for future reference. My friend needs to test the voltage of some of his equipment for his project. I’m sure he’d really benefit from finding a multimeter provider in his area to help him keep track of the data easily and quickly. Thanks! http://www.pcsmeasure.com.au/electrical