What does "TRUE RMS" mean?

"P E Schoen"

I found an LEM LT1005-T on eBay for $150:

And the specs:

It appears to be about 0.4% accurate, but I'm not sure how that translates to a 50 amp current.

** The LT1005-T is a +/- 2000 amp device.

The transfer ratio is 5000:1, so 50 amps will provide

10 mA. The offset is typically about 0.4 mA, so that is 4%. Seems like it won't really fit the bill.

** Use hammers as screwdrivers - do you?

The "offset" is due to residual magnetism in the core due to previous use - long time constant AC coupling will eliminate it completely. As your game is mainly massive transient overloads - that should not present a big issue.

LEM and Honeywell current transducers are very cool things.

... Phil

"John Larkin"

** What absolute crap.

1. Point to the specs that prove that claim beyond doubt.

2. 1% accuracy is easily adequate for the purpose P.E. Schoen has indicated.

... Phil

88

That one is only 150A. Here is one of the same series for 600 amps peak:

1798

They do have a 1200A peak LEM for about $60:

But its accuracy is only 1%.

Another one is a LEM, but it's over $500, and only 1%:

p/1002707

This one is a good price, but still not the specs I need:

0a/dp/1898951

Thanks for the suggestions.

Paul

Beyond your doubt? Probably impossible.

But for people who can read numbers, here they are:

Note that they don't use percentages, they use PPMs.

John

"P E Schoen" "Phil Allison"

They do have a 1200A peak LEM for about $60:

But its accuracy is only 1%.

** WTF do you need better than 1% for ???

A real engineering need or some brainless " it's in the rules " reason.

Cos you are sounding real brainless right now.

.... Phil

reason.

The Ortmaster is a test instrument with a stated accuracy of 1%. = Standard=20 rules of calibration require the standards used to certify such an=20 instrument should be at least twice as accurate and ideally three or = four=20 times as accurate. Hence I need an instrument which is about 0.25% = accurate=20 and traceable to NIST. Actually, I will not allow my unit to be shipped=20 unless it is at least within 0.5%, and I like to set the calibration = factors=20 so that it is better than 0.2%, just to be sure, and because I can.

This is probably overkill, as the reclosers that are being tested are=20 generally 10% and even 20% devices, but the recloser manufacturer, = Cooper,=20 has specified that they require the instrumentation to be at least 1%=20 accurate. And they also require at least 2000 samples per second, and = timing=20 within 1 mSec. The Ortmaster, even the original design which was = released in=20

1994, meets this specification, and actually is as accurate as their=20 laboratory test system which cost them well over $20,000. Not bad for an =

instrument that sells for $4000 list, including software, and costs less =

than $400 each to make (not counting years of NRE). And it's rugged = enough=20 that one of them still worked (except for a broken DB25 connector) after =

being run over by a truck:

Usually the calibration factors are very consistently within 0.25% of = each=20 other, which is understandable since I use 1% resistors. But the low = ranges=20

50 and 100 amp are usually set about 1% higher than the others. This may = be=20 partially due to the gain-bandwidth product of the amplifier at the = highest=20 gain settings (since there is also capacitance in the feedback). But I = also=20 suspect it is the accuracy of the DMM which is really insufficiently=20 accurate at 5 mV.

Paul

The last decent oscilloscope HP built was the 130C.

--
Many thanks,

Don Lancaster                          voice phone: (928)428-4073
Synergetics   3860 West First Street   Box 809 Thatcher, AZ 85552
rss: http://www.tinaja.com/whtnu.xml   email: don@tinaja.com

Please visit my GURU's LAIR web site at http://www.tinaja.com

If you're patient, you can get them for less. Probably in the $400 to $450 range. I was lucky on the one I got for my home lab - the guy didn't know how to test it and was unsure if it was working. Got it for $260 and it worked fine.

Yes, schematics are available for the 34401A, unlike the Keithley 2000. I wish I could get my hands on the 2000 schematics!

You're welcome.

I still have my HP 130A, with fast blue phosphor for camera. My father = gave=20 it to me in 1969 after his lab was flooded by a hurricane and it was = sent=20 back by the factory as FUBAR, or perhaps the estimate was unreasonable. = I=20 found that two octal plugs were reversed and when I fixed that, it = worked,=20 but the display was dim. I replaced a HV capacitor and generally cleaned = it=20 up and it worked fine!

It died again in 1983 and I never did fix it, but I kept it. I really = liked=20 the layout of the controls and the overall appearance. Much nicer than=20 Tektronix.

My first scope was a Triumph, which had an optional "wobbulator"! It was =

IIRC only AC coupled and very low bandwidth. I think I gave/sold it to a =

friend who was a Ham.

Ah, yes, the good old days!

My father showed me an early digital meter which used a mechanical means = to=20 adjust the reference voltage through decade dividers and had flipping = metal=20 or plastic numbers, like old clocks. It made a lot of noise especially = when=20 it was hunting for a signal between 0.999 and 1.000 volts. I don't = remember=20 how many digits or how accurate, but it was pretty cool!

Paul=20

For shared office use, the Lexmarks (T654DN B&W and C925 tabloid color) are performing admirably. We've got a narrow (24") carriage HP Designjet as well. For SOHO use, I've been using mostly Brother and Samsung, with good results, and my workhorse HP2015DN is still chugging along despite severe problems with the first one. Perhaps the thought that there is a new Brother equivalent sitting on the shelf, drivers installed, and waiting to replace in an instant it is keeping it going.

Good to hear. I've had bad luck reliability-wise with their SOHO level computers and the small printers are essentially a free box to hold some very expensive proprietary "chipped" cartridges. And I've had major troubles with reliability and grudging support on their mid-range printers. They're up to two strikes and a couple foul balls..

I would definitely still consider their workstations. Some even come standard configured with a professional-level CAD-certified video cards, and I think they've got a pretty good reputation.

Yuk, the 130C wasn't that great.

The only great HP scope was the 185 sampler. I have one, with plugins. It was ugly, in the HP scope tradition, but it was a technical marvel.

My favorite scope is the Tek 547, a thing of beauty. I have a dozen or so, but I don't use them.

John

Was that the smallish Digitec, with the motor-driven 10-turn pot? Those were cool.

John

I chose my Fluke 45 multimeter as the best choice for true RMS reading at the time I bought it about 20 years ago. It has an accuracy of 0.2% + 100 counts from 50 Hz to 10 kHz, but that means that on the 100 mV range, specified for 15,000 to 99,999 counts, it may be an additional 100/15000 or

0.67%. It seems to be actually much better than that, although it has an offset of about 0.240 mV with the input shorted. I have been using this to calibrate my Ortmasters, which are rated at 1% from full scale ranges of 50A to 10,000A, using a 1000A 100 mV shunt. So when I calibrate the 50A range, I am reading about 5.000 mV, which could be as much as 100 counts off, or 2%. The Ortmaster typically shows an offset of 0.10 A on that range, which is only 10 uV. So I may need to get a better standard!

The Fluke 8520A has a 1 year accuracy of 0.15% reading plus 0.05% FS, which in this case is 1.99999 V, so the true error is as much as 1 mV. So its accuracy at 5 mV is only really 20%! No winner here.

The Fluke 289 has a true-RMS accuracy of 0.3% + 25 counts, 45-65 Hz, on the

50.000 mV range, which gives me an effective accuracy on a 5 mV reading of 0.8%. An improvement, but I'd really like 0.5% or better.

The Fluke 8808A is similar to the Fluke 45, but it does not seem to be true RMS.

The Fluke 87 has a true-RMS accuracy of 0.7% +2 digits and 0.1 mV resolution.

The HP 3478 has a true-RMS accuracy of 0.46% + 163 counts for the 300.000 mV range.

I really have not found a meter that is significantly better than the Fluke

1. Any suggestions?

__________________________

How about putting a good low noise instrumentation differential amplifier on the measurement points of the shunt? It should be easy to calibrate it for 20 dB (or 40) of gain and using some good low TC precision resistors should make it very stable. Then you will move the measurement out of the low order least sig digits of the meter.

??

tm

Yes, that would work, but I'd have to build it and add a power supply = and=20 put it in an enclosure, and then it would be another piece of equipment = to=20 calibrate so that everything is traceable to NIST. And calibration = companies=20 may be hesitant to certify customer-built equipment.

In fact, I have essentially that in the Ortmaster devices I calibrate. I =

have an AD620 (or INA129) at the front end with some resistors, = capacitors,=20 and TVS for protection. Then I have an 8-stage amplifier using an LT1112 =

dual precision op-amp and a DG408 and a resistor string in a 1-2-5=20 configuration for gains of 1 to 200.

Much easier to use a 100A 100mV shunt which shifts the readings by a = factor=20 of 10, and it's a very durable, stable, and easily calibrated device = that=20 needs no batteries or external power and is immune to most forms of=20 degradation and failure.

Paul=20