AVO850 True-RMS Digital Multimeter

RMS (Root Mean Square) / TRMS (True Root Mean Square) RMS DMMs use the formula VRMS = VPeak divided by √2 to calculate a reading on a perfect sinewave. An ideal AC waveform should be a perfect sinewave; however nowadays with the abundance of electronic devices either as part of or connected to a circuit the sine wave can now be classed as far from perfect. Non- sinusoidal waveforms with spikes, squares, triangles, and sawtooth patterns can be quite common. A TRMS DMM allows accurate measurements on circuits that contain these waveforms.The TRMS formula is much more complex.                                                       VTRMS = √(V1² + V2² + V3² + V4²....) divided by nThe TRMS of a non-sinusoidal waveform equals the square root of the sum of the squares of a determined number of voltages divided by that number. TRMS DMMs like the AVO415 take multiple voltage readings across the waveform and produces an average final reading. This produces a vastly more accurate measurement on the non-sinusoidal waveform.

Average response DMMs use an average mathematical formula to measure perfect AC waveforms. Although they can be used to measure non- sinusoidal, distorted waveforms the measurement will have dubious accuracy. Depending on the distorted waveform the measurement can be up to 40% lower or 10% higher on the average reading DMM.  On possible distorted waveforms a TRMS DMM will be the preferred instrument. 

Digit rolling or reading rolling is where the display on a DMM may not fully stabilise on certain ranges due to a certain amount of unwanted noise and voltage that is picked up at the input terminals of a DMM.  The DC / AC voltage ranges on most DMMs generally use two techniques, NMRR (Normal Mode Rejection Ratio) and CMRR (Common Mode Rejection Ration) to reject unwanted noise effects and voltage that present on both the COM and VOLTAGE terminals, with respect to ground, that can cause digit / reading rolling or offset in voltage measurements.  NMRR and CMRR is typically specified in terms of dB (decibel). Where neither NMRR nor CMRR specification is specified, a DMM's performance will be uncertain.  Due to the fact the resistance range on a DMM utilises a very low voltage to obtain measurements digit / reading rolling generally appears on the lower and upper ranges on an auto ranging DMM. The amount of fluctuation is shown in digits within the specification. 

Counts are the maximum reading a DMM can display before the range changes. Simply, in most cases the larger the number of counts, the higher the resolution; and the higher a DMM’s resolution, the higher its accuracy. Other design factors come into play, a DMM’s accuracy including the accuracy of the analogue-to-digital converter, noise level, component tolerances, and stability of internal references.  The counts spec tells you the absolute value of the full-scale value that a DMM can display, ignoring the location of the decimal point. Ignoring other issues such as analogue-to-digital converter resolution, noise, etc. Example: On a 4-volt source: 

• 2000 count DMM can display 2 decimal places.
• 6000 count DMM can display 3 decimal places.
• 50,000 count DMM can display 4 decimal places. 

For low count DMMs, the offset accuracy spec (the "digits") generally is a significant fraction of the total measurement accuracy range. So even if the % of range spec is low (e.g., 0.1%), the "digits" can still result in relatively large error. The AVO850 is 50,000 count multimeter which makes it a professional choice.

If your work takes you into wet or dusty environments, learn about water, and dust resistance on your multimeter.  Water and dust resistance standards are defined in IEC 60529, which specifies levels of "ingress protection" (IP) from solids and water. An IP rating has of two digits. The first digit specifies the size of excluded objects. Ingress protection levels for solidsLevelObject SizeEffective Against0Object size No protection 1>50mmAny large surface of the body2>12.5mmFingers or similar objects 3>2.5mmTools, thick wires 4>1mmGranular objects. Most wires, screws etc 5Dust protectedNot entirely prevented but must not interfere with satisfactory operation 6Dust tightNo ingress of dust. DustproofThe second digit of an IP rating specifies the level of protection against water. Ingress protection levels for water LevelProtected AgainstDetail0Not protected 1Dripping waterVertically falling water. No harmful effec2Dripping water, 15 ° tilt Vertically falling water. No harmful effect when unit tilted up to 15 ° from its normal position 3Spraying water Water falling as a spray at up to 60 °. No harmful effect4Splashing waterWater splashing from any direction. No harmful effect 5Water jetsWater projected by a nozzle from any direction. No harmful effect 6Powerful water jetsWater projected in powerful jets by a nozzle from any direction. No harmful effect 7Immersion up to 1mImmersion in water up to 1m for 30 minutes Waterproof to 1m for 30 minutes8Immersion beyond 1m Continuous immersionThe AVO850 has a rating of "IP40." It is designed and tested to be protected from forging objects and to be kept dry.  

Crest factor is the ratio between the value of the peak current or voltage and the RMS value. Crest factor for a pure sinusoidal waveform = 1.414 as the peak value is 1.414 times the RMS value. The illustration shows an example sinusoidal load waveform (blue) and a non- sinusoidal load waveform (red). Both waveforms have a RMS current of 5A. Crest factor for blue waveform = peak current / RMS current = 7.07A / 5A = 1.414. Crest factor for red waveform = 22A / 5A = 4.4 Crest factor is important when selecting an AC source as the power supply must provide the required peak current for a non- sinusoidal load. The specification of a power supply should either state the peak repetitive current or high crest factor to suit non- sinusoidal loads with high peak currents. The AVO850 has a crest rating of ≤3 at full scale up to 300 V, decreasing linearly to ≤1.5 at 600 V.

Digits and Counts are simply two different ways to express the resolution of a DMM.                                                                                Counts: (See DMM Counts) Digits: When digits are stated on a DMM the fraction is taken as the most significant digit. Note example: 3½ digits. The half digit is only a 0 or 1 with 3 full digits, so the DMM has a resolution of 1999 (2000 counts).                                                                                                     To complicate matters there are DMMs with 3¾ digits. The means there are 3 full digits, and the most significant digit can be 0 to 3. Some manufacturers use the 3¾ indication to show the first digit can be up to 2 or 4; so, in that case the DMM can indicate a maximum of 2999 or 4999. 

It can be useful when you want to calculate real power dissipation on the load.  When power source has DC component (DC bias) it leads to additional power dissipation on the load. DMM which measure just AC RMS Voltage don't take DC component into account. The AVO840 has the mode to measure AC+DC voltage considering both components. This means you can see the DC+AC and AC measurement at the same time without needing to take separate measurements saving you time. 

LoZ stands for Low Impedance (Z). This feature presents a low impedance input to the circuit under test. This reduces the possibility of false readings due to ghost voltages and improves accuracy when testing to determine absence or presence of voltage. 

The functions of an advanced loop calibrator allow technicians to troubleshoot on the spot. The 4-20 mA current is commonly used to connect process signals to a controller in industrial applications. The principle of using a 4-20 mA loop is that a process range e.g. 20 mA representing 100% open and 4 mA closed. Readings between the maximum and minimum values indicate that the circuit is controlling the valve. You can measure the 4-20mA process loop on your AVO850 and it will display loop current as a % with 0mA=-25%, 4mA=0%, 20mA=100%, and 24mA=125%.