This why "RMS" measurement instruments are increasingly widely used. the constant error too high, becomes impossible measure very low values accurately. accessories that are perfect condition. "average value" measurement instrument measures the average value sinusoidal current after rectification and filtering and then displays the effective value after applying coefficient 1/0. This not always the case, however, and the manufacturer should indicate clearly, this minimum value also depends the accuracy and, particular, the constant error.
Rated calibre
The calibre instrument the value the quantity measured which corresponds the upper limit the measurement range.
Vrms =
1 T
∫ v(t)
T o
2
In the specific case sinusoidal quantity, application the above equation gives:
v Vpk cos Vrms Vpk v pk cos(ωt)2. Give preference CAT instruments: his means that can withstand higher overvoltages than CAT III product.
Peak value Crest Factor
The crest factor defined follows: Vpeak Vrms This information can used with the RMS value assess signal's distortion qualitative terms. For example, for ammeter, the upper limit described having calibre. general, allows you define the measurement range and the resolution the basis the value chosen for the rated calibre.
Measurement range
This represents the limits within which the digital instrument complies with all its specifications, without the readings being subject level error greater than the maximum error tolerated.), they will longer protect you against possible voltage surges the installation. Otherwise, the measurement error may reach therefore advisable opt for "RMS" measurement instruments which are capable giving correct measurement results whatever the waveform the current voltage.
Safety rules and good practice
- measurement instruments and accessories which are suitable for the appliU cation and the measurement conditions.UNIVERSAL TEST AND MEASUREMENT
Technical reminders
Number counts
This one the fundamental specifications instruments using analogue-digital conversion. Vpeak Vrms
Vrms Vavg For sinusoidal voltage Vpeak Vrms Vaverage 0.dt 2
The amplitude sinusoidal voltage (Vpk) current times its RMS value (Vpk Vrms). defined minimum measurable value and maximum measurable value.9 Vrms t
- uses are protective devices. - t the weakest element which defines your level protection. Any accessory with even the slightest defect must replaced immediately, can longer guarantee your safety. For long time, the linear loads (incandescent lamps, heating) connected the network caused only slight distortion. also the value measurement count unit quantification usually called the "unit". For instrument offering sufficiently linear conversion, may equal the resolution.
Thus, for 230 V/50 network: Vrms 230 Vpeak 325 Vaverage 207 V
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. They are based direct measurement principles: the thermal method (mainly used metrology) and analogue digital calculation methods requiring sophisticated electronic components.111.414 Recommendation: When speak 230 mains network, "effective" RMS value. 1,000 offers protection against electric shocks 12,000 while CAT IV 600 offers protection 8,000 • f lower-CATegory instrument used, you must first check that the instalI lation equipped with protective systems (disconnecting switch, circuit breaker, etc. The RMS value essential for industrial applications because used define current. you use accesI sories whose category voltage lower than your measurement instrument's, you reduce the overall safety level offered your measurement system. they are replaced cheaper models or, even worse, metal element (copper wire, aluminium foil, etc. This indirect measurement method simple and accurate but only gives true value for undistorted sinusoidal currents only tolerates distortion level few per cent.
REsolution
This the smallest difference value that can measured.9 1.
RMS: Root Mean Square value
By definition, the effective RMS value current the current value which would cause the same heating when flowing through resistor.) that operate correctly and are good condition. This often the case… but not always! external installations which may temporary upstream the proF tective systems, CAT instruments are mandatory. For sinusoidal signal, 1.
Minimum measurable value (or threshold)
This the smallest value measurable. The spread non-linear loads (switching power supplies, light dimmers, variable speed drives compact fluorescent lamps) calls this approach into question "pure" sinusoidal mains supplies are increasingly rare. Conventional measurement instruments (calculating the effective value the basis the average value) are definition only accurate with sinusoidal currents
