Using an Analogue Multimeter
- key hints, tips and precautions for using an analogue or analog multimeter and how to use a test meter to gain the best from it.
Like any instrument an analog or analogue multimeter will perform to its best if it is used in the right way and its limitations are understood.
Analogue multimeters were widely used, and although digital multimeters are far more common nowadays, using an analogue test meter can still produce readings that are more than accurate enough for all but the most exacting requirements.
A few simple hints and tips about how to use an analogue multimeter enable it to produce accurate results, and reduce the possibility of damage from overloads, etc.
Analog meter advantages & disadvantages
One of the key points of knowing how to use an analog multimeter is understanding the relevant advantages and disadvantages.
Like any item of test equipment an analogue multimeter or test meter has its limitations. Knowing what they are and how to overcome them is a key stage in understanding how to use an analogue multimeter to its best.
- Analogue movement: The meter needle gives a continuous movement from which it is very easy to gain a fast idea of the order of magnitude, or of trends for slowly moving changes.
- Availability: Analogue test meters may well be available when digital ones are not.
- Multiple scales: Any multimeter will have a number of different scales and these can cause confusion. They were often a cause of error.
- Lower input resistance: Using analogue technology, analogue multimeters did not provide such a high input impedance as a digital one. Understanding when this may be an issue is a key element of knowing how to use an analogue multimeter.
- Polarities of test leads: Analogue multimeters do not have an auto-polarity function. Therefore it si necessary to correctly connect the test leads, otherwise the meter couldl deflect in a negative direction and quickly hit an end stop.
One cause of errors on an analogue multimeter, or any analog meter for that matter is parallax errors. This is an important concept when using an analogue multimeter.
When viewing the meter, the eye should be at right angles to the plane of the meter back markings. In this way there is no error from viewing the needle at an angle.
Some high end professional meters such as the AVO have a mirror in the scale. In this way it is possible to assess whether the eye is directly in front of the scale - when the eye is viewing correctly, it will not be possible to see the reflection as it is masked out by the needle itself. The offset view below indicates this.
In addition to the mirror, often the needle is made thin in the place of the scale, but much larger in the plane at right angles to the scale. In this way it has sufficient strength and it is also does not take much meter scale area.
Using the correct range
Another concept in knowing how to use an analogue multimeter is that of knowing which range to use.
In terms of the view of the meter, the best accuracy is gained when the meter is towards the full scale deflection, FSD. In this way a given percentage change in the reading gives the maximum and hence most visible change in meter deflection, and accordingly the most accurate reading.
However care has to be taken not to overload the meter by placing it on a range much too low for the reading to be taken. If this occurs the meter can swiftly move to the end-stop, and damage may occur if it is overloaded too much. It is always best to start well below the range expected to give full scale deflection and switch the range when everything has settled.
Analogue meter zero position adjustment
Analogue multimeters, and in fact any analogue moving coil meter will have a zero adjuster.
This zero adjustment should not normally need to be touched, but it may vary slightly with time and temperature.
Adjustment should be made with the meter not in use and it should be gently adjusted with a screwdriver to ensure the meter is properly zeroed. Care should be taken to ensure that the eye is directly above the meter, and any mirror in the meter should be used to ensure that accuracy is maintained.
The adjustment should be undertaken with the meter level as if the meter is placed horizontally, for example, the zero position will change.
When using a multimeter for resistance measurements it is necessary to adjust the zero ohm position. This is required to compensate for a number of variations from small tolerances in the components in the meter to the state of the battery within the meter that is used to supply the current needed for the measurement.
To undertake the ohm-meter zeroing, the test probes for the meter should be shorted together to give a zero ohm resistance between the terminals of the meter, and the small "Ohms Adjustment" control should be used to give full scale deflection on the meter that corresponds to the zero ohms position.
Scale on a typical analogue multimeter
Note that on an analogue meter zero ohms corresponds to full scale deflection, and increasing resistance gives a lower level of deflection. In this way the meter scale is reversed - higher values of resistance are to the left of the scale and lower ones to the right.
Finishing using multimeter
When the measurements using the analogue multimeter have been completed, it is always wise to return he meter to its off position. This prevents the meter being picked up and connected for a voltage measurement, when, for example it has been left set for a current measurement. In this case excess current could flow through the meter causing damage to the meter and also with the possibility of damage to the circuit under test.
If no specific "Off" position is available, after use the multimeter should be switched to the highest voltage range.
Making measurements using an analogue multimeter
When using an analogue multimeter, measurements for voltage, current and resistance need to be made in different ways. To illustrate the way in which these different tests can be made using an analogue multimeter, the simple circuit shown below will be used:
- Voltage measurements: The voltage measurement is the easiest form of measurement to make using an analogue multimeter. The two leads need to be connected across the area of the circuit where the voltage measurement is to be made. Typically the "Common" or "COM" connection on the multimeter is used for the negative voltage end of the measurement and the connection marked "Volts" or similar goes to the positive end of the measurement. For some high or low voltage measurements, there may be a separate connection available and this should be chosen.
Once the relevant connections of the multimeter have been chosen the switch can be turned to the correct range and the measurement taken.
- Current measurements: When using a multimeter to make a current measurement, the meter is placed in series with the circuit where the current measurement is to be made.
- Resistance measurements: For any resistance measurement, the component to be measured should be removed from the circuit as other paths will be present that will distort the reading. Also any power remaining within the circuit will add to the inaccuracies. Never measure resistance when the circuit is powered, as power from the circuit will not only distort the reading, but could damage the meter.
Using analogue test meters can be every bit as easy as using digital multimeters. Their accuracy is normally more than adequate and with a little care and understanding in their use they provide an excellent for of multimeter.
By Ian Poole
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