Choosing a Voltage and Current Data Logger

A data logger (sometimes written as one word: datalogger) is an electronic recording device that is used to monitor and measure variables that change over time, and that will maintain a record of their changing behavior. Most often the term is applied to devices that measure temperature, humidity, light intensity, as well as power related parameters such as voltage, current, watts, frequency, etc. Thus a data logger for recording power needs to be patched into the electrical system i.e. plugged in or connected to power circuits with probes to sense voltage and current. They may be powered from AC power or run off batteries that need occasional recharging.

The major purpose of a data logger is to make a record of a physical parameter over a specified time period.  A multimeter or clamp-meter will do for a spot checks of voltage and current.  I.e. what is the present reading now?  To understand how the value may change over time,  a few hours or days etc. we need to log.   By adding ‘logging’ capability to a voltmeter – the recording of successive values over time at regular intervals – we can observe any fluctuations in the measured value.  This is usually accomplished in the form of a graph that plots a value over time.

When choosing a voltage or current logger, these are the attributes to look for:

1. Internal memory to record for up to a month (or longer).
2. How often the logger repeats taking measurements.  For general power and energy recording we recommend getting a logger that makes a measurement every second.  For power quality investigations – or more properly should be described as “voltage quality” – we recommend using a logger that makes a measurement every cycle.
3. A safe connection scheme with well-designed safety connectors that a) avoid shocking the user and b) that ensure the connections stay put securely during the logging.  Unfortunately there are cheaply made loggers on the market that use feeble terminal strips that are not only a safety hazard, but do not adequately secure the measurement connections.  Wires can come loose and fall off spoiling the whole recording session because data was missed.
4. An easy to use set-up procedure.
5. Good PC Power Analyis Software to control the logger and download the data from the logger. The software should come with good graphical presentation tools to zoom, expand, export data to Excel, and print graphs and professional-looking final power study reports.
6. High safety rating – such as CAT IV 600V.  This means the logger can be used safely on systems up to 600V RMS.  (Users should note the new safety requirements under NFPA 70E when connecting to dangerous AC voltages.)
7. Small size and weight.  Data loggers are left unattended for long periods of time so small size means that they can be easily installed and tucked away inconspicuously in cabinets, or voids in machines, to avoid tampering or theft.

Buying a data logger for monitoring all aspects of power and energy, or just for voltage or current,  need not be expensive.  The PowerSight PS2500 Power Logger for single- and three phase power monitoring starts at under $1500.  Plus, if you have an infrequent need you can rent a data logger for a week, a month, or more.

Measuring Current with a Power Monitor – be Flexible.

Measuring current with a Power Monitor can be a challenge when you’re confronted with a tight and confined conductor access situation.  The conductors are bunched up too tight, or there are multiple cables, or large bus bars. The standard clamp-on current probe just won’t get around the conductors. The clamp-on is just too rigid or too big.  You find you can’t easily or safely nudge the conductors apart to work it around cables as they just won’t give.   Fortunately new flexible probe technology, Rogowski coils, can solve this vexing problem.

Rogowski coils have been around for many years but the challenge was the manufacturing them. They are like flexible rubber ropes, but with a fine, double-wound induction coil inside. They are slightly energized and an electronic integrator measures the output which will be proportional to the influence of an external magnetic field – as from an AC conductor.