Introduction:
In technology, a lot of measuring is done. This page addresses measuring related to automotive technology. In automotive technology, measurements can be taken in various ways, specifically during development, testing, process monitoring, and troubleshooting. Once one knows how to measure, literature (wiring diagrams) is necessary only to determine where to measure.
The most commonly used (electrical) measurement equipment in automotive technology includes:
- The multimeter / analog meter: This is used to measure voltage (U), current (I), and resistance (R). The digital multimeter displays the value on the LCD screen, while the analog meter indicates the measured value on a scale using a needle.
- The oscilloscope: With the oscilloscope, voltages are measured that can be recorded over a timeline. This timeline can be set (number of Volts on the Y-axis and time progression on the X-axis).
Analog Meter:
The analog meter (moving coil meter) consists of a permanent magnet and a moving coil. The current flowing through the moving coil creates a magnetic field. The forces exerted by the magnetic field cause the moving coil (and the pointer mounted on it) to move. The greater the current (and thus the magnetic field), the further the pointer will deflect.
Advantages compared to the digital multimeter:
- Affordable;
- More accurate below 10 Hz (not beyond).
Disadvantages:
- Harder to read;
- Relatively slow due to the moving pointer.
Digital Multimeter:
The digital multimeter is a substitute for the analog meter. The meters are continuously being developed further (in accuracy, speed, and functions). The multimeter contains an A/D converter. The analog signal being measured is processed before being displayed. This processing depends on the chosen function (Volt, Ampere, Ohm, etc.). The digitized signal is then sent to the display. The speed at which this happens is called the “response time,” which can be found in the meter’s specifications. The response time (of the A/D converter) is the time required to register a change in the input signal. The more expensive the meter, the lower this response time will be.
There are digital multimeters with manual and automatic range settings to set the measurement range. The multimeter in the image below does this automatically. The chapter “Range” is described further on this page.
Resolution:
The number of digits displayed by multimeters determines the resolution and thus the reading accuracy of the meter. The resolution is related to the display and not to the measurement range. There are 3½, 3¾, and 4½ digit multimeters. The more digits the multimeter can display, the more numbers are possible (thus a more accurate measurement).
3½ digit:
This is a standard multimeter, which can measure to a maximum accuracy of 0.1 V in the 200 V range. If a measurement is taken where the actual voltage is 22.66 V, the meter would display 22.6 V.
3¾ digit:
This multimeter increases the resolution by a factor of 10 and with the same measurement (22.66 V in the 3½ digit multimeter), it will actually display 22.66 V. That’s one hundredth of a Volt more (and thus more accurate).
4½ digit:
This multimeter has an extra digit available in all ranges. The resolution is again increased by a factor of 10.
Setting the Measurement Range:
The measurement range of the multimeter below can be set manually. This is necessary to achieve the most accurate result in each measurement. When measuring battery voltage, it is best to choose the 20 DCV option. The battery voltage will, for example, be displayed as 12.41. It is best to choose a measurement range that will be less than the maximum measurement result. The battery voltage will never exceed 99 volts. If a greater resolution were chosen (200 DCV), the battery voltage would be displayed as 12.4 (less accurate). This relates to the resolution:
| Range: | Resolution: |
| 200 mV | 0.1 mV |
| 2 V | 0.001V |
| 20 V | 0.01 V |
| 200 V | 0.1 V |
| 2000 V | 1 V |
Examples of this table:
- When measuring a voltage of 100 Volts in the 200 V range, the meter will display 100.1 V. When the same voltage is measured in the 2000 V range, the meter will show 100 V (less accurate).
- When measuring a voltage of 9.188 Volts in the 2 V range, the meter will display 9.188 V. When the same voltage is measured in the 200 V range, the meter will show 9.2 V (rounded, thus less accurate).
The most accurate measurement therefore depends on which measurement range is set and the resolution of the display. With displays that have low resolution, even with a precise measurement range, the most accurate voltage cannot be displayed.
In the multimeter shown, the measurement range can only be adjusted manually. More advanced multimeters have an “Autorange” button, which sets the best range (based on its resolution) automatically. Only simple multimeters allow selection of the Volt, Ampere (etc.) setting, with the range often defaulting to 20 V (thus a resolution of 0.01 V).
Another issue is that there is always some deviation in the meter. When setting a low resolution, this deviation is most pronounced. More about this in the following chapters “Absolute and Relative Error” further on the page.
Calculating the Absolute Error:
Every multimeter has a certain accuracy. This accuracy can be found in the specifications (in the manual). With this data, the deviation of the measurement can be calculated. Two terms can be calculated; the “absolute error” and the “relative error.” The absolute error is the voltage in Volts and the relative error is calculated in percentages.
Example:
Voltage (U) = 12.55 V
7;(0.3% rdg + 1d)
rdg = reading = the displayed value on the display (the measured value)
1d = 1 digit = the resolution (on the 20 V range 1 digit equals 0.01 V and on the 2 V range 0.001 V).
The actual voltage is 12.55 Volts. This is measured on the 20 V range.
0.3% rdg is 0.3% of 12.55 V = 0.038 V.
On the 20 V range 1d = 0.01 V.
The total absolute error is then: the reading + 1 digit = Absolute error. In numbers: 0.038 + 0.01 = 0.048 V
The final answer with the absolute error is:
U = 12.55 7; 0.05 V.
This means that the measurement is somewhere between 12.50 and 12.60 Volts.
Cheap multimeters often have a larger deviation than more expensive ones, so the total absolute error is also greater. This now proves that “cheap multimeters” cannot perform accurate measurements.
Calculating the Relative Error:
When the absolute error is calculated as a percentage of the displayed value, it is called the relative error. This relative error is usually used for comparing meters.
The relative error for the previous multimeter is: total absolute error / (divided by) the actual voltage x (multiplied by) 100% = the relative error.
In numbers: U = 0.038 / 12.55 x 100 = 0.30%.
The final answer with the relative error is:
U = 12.55 7; 0.3%.
12.55 V minus the 0.3% gives an answer of 12.50. Plus 0.3% is then 12.60. This is the same as what was calculated with the absolute error, but expressed in percentage.
Measuring with the Multimeter:
Voltage, current, and resistance are all measured differently. How to measure correctly with the multimeter is explained with examples on the page measuring with the multimeter.
Measuring with the Oscilloscope:
An oscilloscope (abbreviated as scope) is a graphical voltmeter. The voltage is displayed graphically as a function of time. The scope is also very precise. The time can be set so small that signals from sensors like the lambda sensor or actuators like an injector can be perfectly displayed.
How to measure with the scope is explained on the page measuring with the oscilloscope.