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Insights into product testing - logging and LoZ volts

May 2013

By Chuck Newcombe

During my career at Fluke, I spent many long hours testing early design models and pre-production units to ensure they would meet customer needs. Since I retired, I've been asked to continue this new product testing. I'd like to share with you some of my most recent efforts - testing the recently announced 289 True RMS Logging DMM.

My experiences with users of Fluke's earlier model 189 DMM told me that we could make improvements on the logging function, and I'm happy to say that Fluke engineers have done just that in the 289.

Most notable among the new features is the graphic display, which can now show a strip chart-like display of a logging session. And a cursor function allows you to zero in on a disturbance and determine its value and the time it occurred - while you're still on the job site. These are both common user suggestions.

Then, there's the expanded on-board memory, allowing the user to make several recordings in the field, before having to upload the results to a computer. This is going to be a hit among those who have multiple periodic test sequences that they must run as part of their predictive maintenance programs.

The feature that I'm really excited about though, is something that looks pretty simple - LoZ ac volts - the ability to detect the difference between a solid voltage source and 'ghost voltages' that we encounter regularly in industrial settings. And, although Fluke has long had meters that offered this capability, the new 289 has taken the function to the next level, making it useful for much more than just finding ghost voltages. Part of my evaluation of this new DMM required that I wring out the LoZ function.

If you've been reading my FlukePlus column (www.fluke.com/flukeplus), you may recall that I recently described how to accurately measure the output of an unloaded rectifier circuit, such as an inexpensive battery charger, using a DMM with high input impedance (10 Mohms). I built a test box that represented a 12 V battery charger, and then explained why the measurements would be unreliable until a lower resistance load was placed across the rectifier output.

I used that test box to check out the 289's LoZ function. Not only did the meter display both the ac and dc values at the same time on screen, it provided the needed load as well. In one short test I was able to determine that the rectifier was working properly, without having to go through the complex test procedure I had so recently described.

Then, I built yet another test box, this one with 120 V output, to verify LoZ performance at a higher voltage level, this time with a half-wave rectifier. The ac and dc voltage readings were as you might expect. The function performs just as well, and is amazingly accurate too. Another of the test setups I duplicated was a simulation of an unconnected line in a raceway with live circuits, a common application. LoZ quickly identified the open circuit by reading zero, while a normal DMM with 10 Mohm input would read just a few volts lower than the applied 120 V.

The real surprise though, is in the ability of this new function to conveniently monitor the output of an inexpensive function generator used in engineering labs. In that application, I first selected the normal ac volts function and set the desired output frequency. Then, I found that, even with only 0.1 volt resolution, the LoZ function easily allowed me to quickly set ac voltages and dc offsets to desired levels while changing waveshapes.

Rarely do you have all the testing capabilities you need when you approach a troubleshooting problem in the field. These two new functions of the Fluke 289 have convinced me that, combined with its already powerful feature set, you can probably achieve much more of what you need to do without returning to the shop for additional tools. Never before have I been so confident that this is true.