subject to an error of 3 V, 10% of the reading. Digital meters usually specify accuracy as a percentage of reading plus a percentage of full-scale value, sometimes expressed in counts rather than percentage terms. Quoted accuracy is specified as being that of the lower millivolt (mV) DC range, and is known as the "basic DC volts accuracy" figure. Higher DC voltage ranges, current, resistance, AC and other ranges will usually have a lower accuracy than the basic DC volts figure. AC measurements only meet specified accuracy within a specified range of frequencies. Manufacturers can provide calibration services so that new meters may be purchased with a certificate of calibration indicating the meter has been adjusted to standards traceable to, for example, the US National Institute of Standards and Technology (NIST), or other national standards organization. Test equipment tends to drift out of calibration over time, and the specified accuracy cannot be relied upon indefinitely. For more expensive equipment, manufacturers and third parties provide calibration services so that older equipment may be recalibrated and recertified. The cost of such services is disproportionate for inexpensive equipment; however extreme accuracy is not required for most routine testing. Multimeters used for critical measurements may be part of a metrology program to assure calibration. A multimeter can be assumed to be "average responding" to AC waveforms unless stated as being a "true RMS" type. An average responding multimeter will only meet its specified accuracy on AC volts and amps for purely sinusoidal waveforms. A True RMS responding multimeter on the other hand will meet its specified accuracy on AC volts and current with any waveform type up to a specified crest factor; RMS performance is sometimes claimed for meters which report accurate RMS readings only at certain frequencies (usually low) and with certain waveforms (essentially always sine waves). A meter's AC voltage and current accuracy may have different specifications at different frequencies. Sensitivity and input impedance When used for measuring voltage, the input impedance of the multimeter must be very high compared to the impedance of the circuit being measured; otherwise circuit operation may be affected and the reading will be inaccurate. Meters with electronic amplifiers (all digital multimeters and some analog meters) have a fixed input impedance that is high enough not to disturb most circuits. This is often either one or ten megohms; the standardization of the input resistance allows the use