For the best electrical and electronic test results, it is necessary to check the accuracy of a Digital Multimeter (DMM), as defined by the manufacturer. This becomes important for test engineers who also need to know how to best interpret results on their DMMs.
The accuracy and precision of modern multimeters are well beyond the requirement for most applications. However, it is recommended that you understand both these parameters in detail for more specialized tests.
Why Is Digital Multimeter Accuracy and Precision Important?
A single faulty connector that had resistance higher than the specifications caused Rocket Lab’s launch vehicle to fail from entering a stable orbit on July 4, 2020. The launch vehicle and satellites fell back to earth before burning up in the atmosphere. As a result, seven commercial satellites were destroyed.
Test and measurement equipment, even DMMs, with higher accuracy are needed for specialized industries such as space technology, electronics, medical equipment, and defense manufacturing and testing. In such cases, high repeatability of test results, high accuracy, and resolution are crucial.
For less critical applications such as day-to-day troubleshooting electronic devices, DMMs with medium accuracy can be used. Such devices are comparatively less expensive.
Every DMM manufacturer displays both accuracy and precision in user manuals but their exact meanings can be confusing for many. Let’s start there.
What Does the Accuracy of a Digital Multimeter Mean?
Accuracy is displayed as a percentage of the largest allowable error. It indicates the variation of the measured value from the actual value under certain operating conditions.
The measured value is then vetted against the industry-standard acceptable for that parameter and Device Under Test (DUT). The higher the accuracy, the closer the result of a DMM would be to the actual value.
Some ways to define DMM accuracy are:
- +/- (ppm of reading + ppm of range).
- (% Reading) + (% Range)
- (% Reading) + Offset
*ppm = parts per million.
A digital multimeter with an accuracy of +/- 2% that is used to measure 100.0 V can display the reading anywhere between 98.0 V and 102.0 V. Some industries could accept this benchmark, others may not.
Accuracy can also be measured by the count of digits added to the basic rating. For example, an accuracy of +/- (2% +2) means that the multimeter may show readings between 97.8 V and 102.2 V for a 100.0 V measurement. Highly accurate DMMs such as those by Fluke can give a basic DC accuracy range between 0.5% and 0.025%.
It may be worth noting that the DC accuracy may be higher than AC accuracy of any given DMM because AC measurements are optimized for 50-60 Hz, making other frequencies less accurate.
How Is Precision Different from Accuracy?
Precision is defined as the ability of a digital multimeter to show the test reading within the defined accuracy limit repeatedly. In simple language, this can be correlated to the bullet holes on a shooting-range target. If the attempts made are close to each other but outside the bullseye, it can be called precise but not accurate. However, if all the holes are in the bullseye, it is both accurate and precise.
In electrical testing terms, a multimeter that can measure a 100.0 V reading and consistently give between 99.5 V and 100.5 V output with +/- 0.5% accuracy would be called highly precise. The DMM you choose should have the right combination of accuracy and precision that best suits your industry applications.
How to Determine Digital Multimeter Accuracy
The first step to determine the accuracy of a DMM is to read the manufacturer’s manual. Next, remember to not assume that multimeter readings are close to the true value. The complete accuracy is often calculated as +/- (% of reading + number of LSD) where:
- Reading = true value of the signal that the DMM measures.
- LSD = Least Significant Digit.
The LSD lets you determine the magnitude of uncertainty caused due to noise, rounding errors, and internal offsets.
Accuracy and range need to be considered independently to avoid gross errors. For example, how should you interpret the true voltage of 1.200 V on a 3½ digit display when the manufacturer specification shows DC volts accuracy of +/- (0.5% + 3)?
- Scenario 1: Set the meter to the 200-V range. The reading will not even be displayed because only one digit after the decimal is shown. The percentage reading is calculated as (1.200)*(0.5)/100 = 0.006 V. When your account for 3 LSDs, the last digit can vary by +/- counts. The meter will display the value between 1.2 +/-0.3 V, or a range of 0.9 V and 1.5 V. This results in almost +/- 25% error which is not acceptable.
- Scenario 2: Set the meter to the 20-V range, which will show a more accurate reading which can be calculated as +/- (1,200)*(0.5)/100 + 0.03 = +/- 0.036 V. Any reading between 1.16 V and 1.23 V is within the specifications with an accuracy of +/- 3%.
- Scenario 3: Set the meter to the 2-V range in which case the complete accuracy can be calculated as +/- (1.200)*(0.5)/100 + 0.003) = +/- 0.009 V. Now the range is between 1.191 V and 1.209 V gives +/- 0.75%, which is highly accurate.
You should strive to select the lowest possible range in which the DMM gives accurate results without showing overload.
Factors That Affect Digital Multimeter Accuracy
While accuracy and precision are important, they are affected by other parameters. Check the manufacturer’s specification sheet for these parameters before you buy a multimeter from a, and compare them with your application and operating environment.
Resolution: It is the smallest change in measurement at the lowest range setting that the DMM can detect and display. DMM with 1/16-inch hatch marks has a higher resolution than the one with 1/4-inch hatch marks.
For example, a DMM with 1 mV resolution in the 3 V range shows variations as little as 0.0001 V in the given range. Clearly, DMMs with higher resolution show higher accuracy.
Range: Range and resolution are related terms. The range is defined as the minimum and maximum reading that a DMM can measure for a parameter before it shows overload (OL). Many advanced DMMs set a range automatically depending upon the magnitude of test measurements.
Higher accuracy is achieved when a DMM can show test results in a narrow range without going in OL state. For example, a 300.0 mV range with 0.1 mV resolution has higher accuracy than a 300 V range with 1 mV resolution.
Display digits and DMM counts: Digits and counts are terms used to define a DMMs resolution. In simple words, a DMM with 3½ digits can show 3 full digits and one-half digit. Three full digits range between 0 and 9, and half digit varies between 0 and 1. In this way, a 4½ digit DMM is more accurate than a 3½ digit device. Benchtop DMMs have a display range between 5½ and 8½ digits.
Modern DMMs are classified on counts rather than digits. A higher count denotes better resolution. For example, a 3½ digit DMM has up to 6000 counts (reading of up to 5999) and a 4½ digit DMM has a count of 20000 or more.
Altitude, temperature, and humidity: Like most other electronic devices, multimeter accuracy can also be affected by temperature variations. The manufacturer usually defines a temperature coefficient in DMMs specifications. The most common way of defining the temperature coefficient is +/- (ppm of reading + ppm of range)/ OC. Humidity and altitude are also defined in a range for most DMMs beyond which they cease to give accurate results.
For example, the Keysight U1252A handheld DMM specifies an operating temperature range of -20OC and +55OC, and a storage temperature range of -40OC and 70OC without a battery. Humidity tolerance is up to 90% for temperature up to 35OC and 70% for temperature up to 55OC. The U1252A model has an operational altitude of 2,000 m.
It is important to check these specifications and compare them with the ambient temperature, humidity, and altitude of the location where the DMM will be used. If you go beyond these specifications, the display may become sluggish, go blank, or show ghost images.
Component tolerance, noise level, and stability of internal references: Finally, the accuracy also depends greatly on the quality of components used inside the DMM. Components with high tolerance to external factors such as shock due to drop and electronic noise can give higher accuracy.
While high-precision benchtop DMMs can be very expensive, they have very specific requirements. In most cases, handheld models can meet regular test needs. An ideal tradeoff between the cost and specifications is important for business, and you can find well-performing digital multimeters online for rent or sale.
The practical tips in this article to understand and check digital multimeter accuracy can help with correct measurements for your application and when you need to buy your next multimeter.