Ieee std 1159-1995, ieee recommended Practice for Monitoring Electric Power Quality

6.5.2.3 Recording volt/ammeters
The third type of power-line monitor may be referred to as the recording volt/ammeter (rvm). The classical
rvm is the pen and ink chart recorder. There are tens of thousands of these devices in use today and more are
being purchased. They have provided the basic measure of power quality at the service entrance for decades.
They provide essential information as to what is the steady-state condition. The device shall be calibrated to
an external voltage source each time it is used.
Solid-state rvms are also commercially available. They are programmed by a computer, and digitally record
steady-state data at a user-selected sampling or averaging rate. Some sample for two to four cycles and cal-
culate an average value. Others sample once every two to 30 cycles. Some average the data samples over
seconds, minutes, or hours to extend the total length of time they can store data, as data storage is limited by
memory amount.
The data is downloaded to a computer and is displayed graphically or printed. The steady-state values repre-
sent average quantities. Solid-state rvms measure true-rms values, and can be programmed to capture out-of-
limit events, which are stamped with the time and date of occurrence.
Another version of the solid-state rvm is also available. This device is capable of sampling each cycle, has
more memory for increased storage of events, and calculates true-rms values by a digital sampling tech-
nique. The user selects the sampling rate and/or the averaging period. Another enhancement is that the data
stored in memory can either be Òstop when fullÓ mode or ÒwraparoundÓ mode. This feature can be useful to
limit the required memory and only record data that is directly related to a problem.
Users of rvms should understand exactly how their particular device handles sampling technique and rate.
Data storage capacity is an important consideration when specifying the optional memory requirements. It
can be very useful to have the extra detail permitted by higher sampling rates which require extra memory.