Analog and Digital Test Instrument Advantages and Disadvantages

In some cases, the function of a test instrument or meter can be an advantage or disadvantage, depending upon the application an electrician is using the meter with. For example, most digital multimeters have an “autoranging” capability. Autoranging allows a meter to automatically adapt to a higher range setting if the predefined range is not high enough for the particular application.

The advantage of auto-ranging is that only the measuring function must be selected (voltage, current, or resistance), and the meter will automatically select the best range (400 mV, 4 V, 40 V, 400 μA, or 4000 mA range). The disadvantage of auto-ranging is that when a meter is set to a selected measurement such as VAC, the meter can display a measurement before being connected to the circuit (ghost voltage), (See Figure 1).

 

Figure 1. Digital test instruments display ghost voltages caused by electromagnetic interference.

 

The test instrument or meter is programmed to search for any voltage, no matter how small, and therefore the meter automatically selects the most sensitive mV range (400mV). The displayed millivolt value may be misinterpreted as an actual voltage measurement. Ghost voltages are also to blame when a meter is connected to a circuit that has no power, and the meter displays readings such as 108 mV, 213 mV, or 455 mV. The display of ghost voltages can be eliminated on some meters by selecting the manual range (RANGE button) mode on the meter. The meter requires that range be manually set to a higher setting, such as the 400 V range or the 4000 V range.

 

Technical Tip

In some situations, digital test instruments are more forgiving of operator error than analog test instruments. When a digital meter shows a negative value, typically, the probes are reversed. Disconnect the probes from the device being tested and reconnect in reverse. An analog test instrument may show no reading when the probes are connected in reverse, this can also cause damage to the test instrument.

 

Analog test instruments and meters also have disadvantages compared to digital meters, such as the following:

• Analog test instrument and meter scales can be misread, especially when a meter has multiple settings (1 V, 10 V, 25 V, 50 V, 250 V) used with the same display scale.

• Analog test instruments and meters are less accurate (for example, typical voltage measurements are within 1% to 5%, depending on meter specifications).

• Analog test instruments and meters offer only basic protection using fuses. There is no protection such as an audible input alert (available with digital meters) that warns when a selector switch position does not match the position of the test leads (for example, the meter is set to measure voltage, but the test leads are plugged into the amperage jacks).

• Analog test instruments and meters cannot offer useful features such as auto-ranging, MIN MAX function, recording function, Measurement Hold function, or Relative function that are available with most digital meters.

Digital test instruments and meters have advantages over analog meters such as the following:

• Digital test instruments and meters display measurements as numerical numbers, not a scale position, and are less likely to be misread unless the prefixes and symbols accompanying numerical values are misapplied.

• Most digital test instruments and meters are auto-ranging. Once a measuring function such as VAC is selected, the meter will automatically select the best meter range for taking the measurement (400 mV, 4 V, 40 V, 400 V, or 4000 V).

• Digital test instruments and meters are more accurate (typical voltage measurements are within 0.01% to 1.5%, depending on meter specifications).

• Digital test instruments and meters offer special functions such as Autoranging, MIN MAX function, recording function, Measurement Hold function, and Relative function.

• Some digital test instruments and meters come with downloading capabilities, which, when used with the software, allow for measurement data recording and manipulation.

Digital test instruments and meters also have disadvantages compared to analog meters, such as the following:

• Digital test instruments and meters can display ghost readings (mV or V readings) when the meter is in the automatic range setting mode.

• Some digital test instruments and meter models offer so many features that the meters can be intimidating to use before the measurement functions are all understood and used a few times.

 

Featured image used courtesy of Wikipedia