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Electric meters
The electric meter is the best friend of the service tech. You will find that a good meter is the first tool you pick up when troubleshooting. You depend on it for safety, it tells you where the power is and it measures how much of it there is.
Below, we will look at several different meters. There are cheap ones that you may not want to trust your life to. Old analog ones that used a needle and a dial. Most modern meters are digital. This means the readout is in digital numbers. The common meter used in the HVAC industry is the volt-ohm-ammeter. This meter is used to test voltage AC or DC. It can check resistance of components with the ohm feature. It is a clamp meter that can read amperage draw thru a circuit. Another meter is the volt ohm meter. This meter may have the ability to check AC and DC voltage and Ohms resistance. Sometimes this meter has the ability to test very high resistance values. It can also test capacitor strength and diodes. Many of the newer meters have combined all the above features into one meter. Your choice of a meter will depend on what you use it for and how it matches with you. They are a somewhat personal choice. You may try several meters before you get the one that suits you.
Below, we will look at several different meters. There are cheap ones that you may not want to trust your life to. Old analog ones that used a needle and a dial. Most modern meters are digital. This means the readout is in digital numbers. The common meter used in the HVAC industry is the volt-ohm-ammeter. This meter is used to test voltage AC or DC. It can check resistance of components with the ohm feature. It is a clamp meter that can read amperage draw thru a circuit. Another meter is the volt ohm meter. This meter may have the ability to check AC and DC voltage and Ohms resistance. Sometimes this meter has the ability to test very high resistance values. It can also test capacitor strength and diodes. Many of the newer meters have combined all the above features into one meter. Your choice of a meter will depend on what you use it for and how it matches with you. They are a somewhat personal choice. You may try several meters before you get the one that suits you.
The first meters were all analog in design. This means the meter uses a magnetic field to deflect a spring loaded needle. When checking voltage, the amount the needle deflects determines the voltage read. These meters check ohms by using an on board battery to supply voltage thru the component tested. The amount of resistance is shown by the deflection of the needle. These meters use multiple settings to get the user "in range" of the voltage or resistance tested. The dial is moved to a range that is above the expected voltage or resistance but as close as possible. An example is I expect to read 120 volts. On the meter below, I would set the range lower left, at 250 as 1000 would decrease the accuracy and 50 would overload the meter. The same would be done for resistance. These meters are not common in the industry anymore for a number of reasons. The movement is delicate and dropping the meter could damage it. The manual ranging requires setting for each different reading. In addition, if the meter is left in the ohms position and the meter is used to check voltage, the meter will probably be damaged. Most digital meters are auto ranging meaning the meter adjusts itself for each reading. Also most digital meters are protected against misuse and are not damaged when set to the wrong mode. The analog meter still has its uses and are sometimes used when the digital meter will not give a smooth reading of a variable resistance or voltage.
The analog meter has an interesting ability that digital meter does not. Before capacitor testers were commonly available, The analog meter offered a simple way to check the ability of a capacitor to accept a charge. When set to ohms X10K, a capacitor can be tested by this meter. The test is not definitive in that it it will not give the strength of the capacitor. But it will test if the capacitor does work. The battery in the ohmmeter can be used to charge the capacitor. The capacitor is a type of battery that can be charged like any other. When connected to the capacitor, The needle of the meter will show a very low resistance. As the capacitor charges, the resistance will increase just as any battery will resist more charge as it reaches full charge. The needle will move towards high resistance. If the needle does not move from zero to high resistance, the capacitor is shorted. If the resistance stays high, the capacitor is open. Just a simple way to test capacitors if a tester os not available. The demonstration of this use of an analog meter is below.
Some of the digital meters are manual ranging similar to the analog meter. On the meter below you can see the different ranges. On the left under DCV or DC volts, the range can be as much as 1000 or as low as 200 millivolts. As with all manual range meters you set higher than the expected measurement. At the top is AC volts with 200 and 750 ranges. On the right DC amperage from 200 millivolt to 200 microamps measurements can be made. This requires the meter leads to be connected in series with the measured load. Lower right is a 10 amp amperage measurement. This one is connected somewhat differently. The red lead is connected to the top connection that reads 10 amp. It also must be connected in series with the load. Below we have electronic component testing positions. On the lower left is the ohms mode with 200 to 2 million ohms. As with the analog meter these meters are not as common in the industry as earlier.
The clamp meter is a rather unique type of meter. As seen below, the meter has an opening clamp that can be clamped around a wire that is to be tested for amperage draw. This eliminates need to place the meter in series with the load circuit. When using this meter, only one of the wires going to the load should be clamped. This meter is valuable because you may determine if a load is drawing the proper amount of power by simply clamping it. The clamp meter usually has a voltmeter and ohmmeter included. The video below gives a detailed demonstration of how this meter is used.
The multimeter. The description of multimeter usually means a VOM or volt ohm meter. This type of meter can be a standalone meter or part of a clamp meter. Below is a description of the use of a standalone meter. It can be used as an ohmmeter. When testing resistance the meter is positioned to the ohm symbol. The meter can be used to test continuity of components or it can be used to test high resistances to ground or high resistance loads. This is called megohms or millions of ohms. Commonly used for testing the integrity of motor windings. Very low resistance readings will also be noted by a beep if continuity is measured. The continuity measurements must be made with the component or length of wire isolated electrically. Leaving the component wired in may result in inaccurate readings due to back feeding thru the wiring.
This meter can also be used for testing the strength of capacitors used in the HVAC industry. When positioned in the capacitor test mode, the capacitor symbol will be displayed. A capacitor can be connected to the meter with the probes. The result will indicate the strength of the capacitor.
Another use of this meter is for testing microamps. This test is usually for testing flame sense in gas furnaces. When placed in the microamp mode, a rather odd u will be displayed. To test the flame rod microamps, the probes must be placed in series with the wire connected to the flame rod. When flame is present, the reading can be taken.
Voltage readings can be taken by setting the meter is the volt setting. When testing voltage, determine if the voltage is direct or alternating. The squiggle next to the V symbol indicates alternating current. If you are testing direct current, switch to the mode that shows the V symbol with a straight line with a dotted line underneath. If used in the wrong mode, the meter will not indicate a voltage. Be sure you are right here. You could believe no voltage is present when you are in the wrong mode. The video below gives some explanation of the use of this tool.
This meter can also be used for testing the strength of capacitors used in the HVAC industry. When positioned in the capacitor test mode, the capacitor symbol will be displayed. A capacitor can be connected to the meter with the probes. The result will indicate the strength of the capacitor.
Another use of this meter is for testing microamps. This test is usually for testing flame sense in gas furnaces. When placed in the microamp mode, a rather odd u will be displayed. To test the flame rod microamps, the probes must be placed in series with the wire connected to the flame rod. When flame is present, the reading can be taken.
Voltage readings can be taken by setting the meter is the volt setting. When testing voltage, determine if the voltage is direct or alternating. The squiggle next to the V symbol indicates alternating current. If you are testing direct current, switch to the mode that shows the V symbol with a straight line with a dotted line underneath. If used in the wrong mode, the meter will not indicate a voltage. Be sure you are right here. You could believe no voltage is present when you are in the wrong mode. The video below gives some explanation of the use of this tool.
with a smooth sine waveIn recent years, the True RMS meter has gained popularity. So, what is it? Many alternating current loads are 50 to 60 hertz and are easily read by meter that does not have the RMS function. However, many electronic loads have irregular sine waves due to electronic components. These sine waves can make the readings of a non RMS meter inaccurate. In general, the readings of the RMS meter will be higher than of the non RMS meter. To make accurate readings, the newer electronic loads must be read by an RMS meter. Below, is a video demonstrating how this works.
One of the accessories that was common in earlier years was the 10 times wrap for use with clamp meters. Older meters were not capable of reading accurately small amperages. When I was first in this industry, We used to wrap the clamp of our meters with 10 windings of wire then place the wire in series with the load. This would give a reading of 10 times the amperageThis was used for small readings usually of low voltage circuits. A simpler method was to use the device shown in the video below. You could choose either a 1 time wrap or 10 time wrap. The clamp is placed across the hole with the device connected in series with the load. This device worked well for many years.
As the newer meters came into use, they became more accurate. Some techs continued to use the 10 times device. I wondered if the device was still necessary for measuring small amperages. The video below demonstrates the results of my tests.
As the newer meters came into use, they became more accurate. Some techs continued to use the 10 times device. I wondered if the device was still necessary for measuring small amperages. The video below demonstrates the results of my tests.
When we are testing circuits for faults, we often test for faults from the windings to the chassis of the part. Usually a motor. We can use the ohms function of the multimeter if the meter has megohm capacity. However, the voltage used is from a low voltage source. Not all faults will be found using this low voltage. Remember, voltage is force. The higher the force, the easier a fault can be found. For this we use an insulation tester. This tester can sometimes find a fault in the windings that the megohm meter cannot. It uses higher voltages to test the integrity of the windings. Voltages can be 250, 500 and 1000 volts. Below is a comparison of these 2 meters used to test compressor windings.
While I do not recommend the use of inexpensive meters as you sometimes bet your life on it, they do have uses in some situations. The meter shown below, although minimal, can be used to test flame rod microamps on a gas furnace. The video shows how this is done.