Clamp-On Current Meter Readings

[otrfun]

Got curious with my new clamp-on meter this afternoon. Made the following current readings (one fully charged 24 series battery; converter turned off):

.13 amps - parasitic/residual current with everything turned off
.30 amps - 600 w PSW inverter turned on with no load
70 amps - powering 6.2 amp 120v vacuum cleaner with the 600 w inverter
6 amps - powering all lights in the TT at the same time (just converted to LED); current lighting is about 25% brighter than the OEM bulbs. Never measured current with the OEM bulbs. However, I did replace 14 921 bulbs that drew 1.3 amps each. That's about 18.2 amps total.

Noticed my Progressive Industries Electrical Management System 120 v current readings are a bit optimistic, especially when current readings are less than 7-8 amps (usually 1-2 amps high).

I'm pretty shocked this Power Bright 600 w PSW inverter is able to power this vacuum cleaner! 744 watts! Not to mention the motor's high startup current.

[MEXICOWANDERER]

When you say Tektronix you are in a world different than what the folks on this forum are dealing with.

I have a 20 ampere 50ma 24Kt Gold CLAD (much heavier than plating) N.I.S.T. Manganin shunt that I use for fast calibrating, with my 6-1/2 and 4-1/2 digit bench and handheld DMM's. I was fortunate enough to had bid on a roll of 16 gauge gold clad pure silver wire at the Lockheed Space Shuttle surplus inventory auction. The shunt came with a box of gold clad accessory terminals.

The meters resolve down to the microamp range which is close enough for nuclear exchanges.

Bottom line, DMM inductive ammeters can be accurate at some ranges and horrible in others.

Where tens of milliamps are important like long term battery storage, a VERY accurate discharge amperage resolution is crucial.

[ktmrfs]

MEXICOWANDERER wrote:
Fluke, Triplett, the usual suspects.

I suspect most DC clamp on current probe mfg buy the hall sensors from someone who makes the sensors as a std product. For use in a accurate DC current probe, especially at low current levels there are lots of details around the sensor design and shielding that can have a huge impact.

I know for a Fact that Tektronix always has, and currently does mfg their own hall sensors for the probes they designed and mfg. Lots of patents and trade secrets around the design and mfg of the sensor, including the deposition on sensor material. Tek also has/had some clamp on current probes (not for input to a scope channel) that are OEM'd, and don't have Tek hall devices, but these are generally high current probes and sensors are adequate for high current measurements.

Now that Tek and Fluke are part of the same company (Danaher) it wouldn't suprise me if Fluke is either getting hall sensors from Tek for some products or using Tek methods to make there own.

Tek has DC hall effect current probes that will measure milli and microamps as accessories for the scopes. Very accurate, but also somewhat spendy.

[MEXICOWANDERER]

Fluke, Triplett, the usual suspects.

[dclark1946]

MEXICOWANDERER wrote:
Inductive DC ammeters as Matt Collie pointed out are near useless at low scale. Worse than useless, they are misleading. Horribly so.

For low current measurement, use your DMM, on the 200ma scale if you want to stay out of the land of OZ.

For PRECISION measurement of low value amperage I use a MANGANIN 20 amp shunt and a 4.5 digit DMM (6.5 digit when I get real serious). This would be worthlessly precise for most RV applications.

Measuring very low values of amperage with an inductive meter would be similar to timing a 100-meter dash with a sundial.

I am not sure which inductively meters you have tried but my experience with my Extech has been that it is quite accurate if you zero it before you take a reading. I have also used Tektronix inductive probes in defense electronics engineering work. The Hall effect probes do become erratic if dropped or subjected to excessive overload.

Dick

[MEXICOWANDERER]

Inductive DC ammeters as Matt Collie pointed out are near useless at low scale. Worse than useless, they are misleading. Horribly so.

For low current measurement, use your DMM, on the 200ma scale if you want to stay out of the land of OZ.

For PRECISION measurement of low value amperage I use a MANGANIN 20 amp shunt and a 4.5 digit DMM (6.5 digit when I get real serious). This would be worthlessly precise for most RV applications.

Measuring very low values of amperage with an inductive meter would be similar to timing a 100-meter dash with a sundial.

[camperpaul]

otrfun wrote:
Just purchased a new Mastech MS2108 clamp-on meter which measures true RMS and inrush current (not to be confused with the cheaper, less capable MS2108A).
-- snip --

... the inrush current for the compressor start-up is another story. I measured 61 amps! It dropped to 58 amps with the generator in Economy mode.

-- snip --

For comparison purposes I also measured the current and inrush current on our microwave inside the house. It measured 13.2 amps with 48 amps of inrush current.

These inrush measurements seem very high. Is this possible, or do I have a very inaccurate meter? FWIW, followed the manufacturer's directions closely for measuring inrush current. They're very simple.

Your measurements sound about normal.

[otrfun]

Just purchased a new Mastech MS2108 clamp-on meter which measures true RMS and inrush current (not to be confused with the cheaper, less capable MS2108A).

Measured current and inrush current on my Dometic 13,500 BTU a/c while powering it with my 3100/2800 (23.3 amp) Champion inverter generator. Made the measurements at the generator using a 120 v line splitter (plugged into a short, 15 ft. 10 gauge cord). Wanted to measure closer to the TT, but that wasn't possible because of the L5-30 amp connector on the TT. Measured 12.2 amps (continuous) while the compressor was on which seems like a reasonable reading. However, the inrush current for the compressor start-up is another story. I measured 61 amps! It dropped to 58 amps with the generator in Economy mode. Yes, I was using the 1x position on the line splitter. Guess it kinda makes sense it would drop with the Economy Mode ON since the generator is not able to ramp-up the current as quickly. For comparison purposes I also measured the current and inrush current on our microwave inside the house. It measured 13.2 amps with 48 amps of inrush current. At any rate, if the a/c's inrush current is 61 amps, it's no wonder the generator's overload LED blinks when the compressor kicks on--lol!

These inrush measurements seem very high. Is this possible, or do I have a very inaccurate meter? FWIW, followed the manufacturer's directions closely for measuring inrush current. They're very simple.

[Matt_Colie]

This has been an interesting thread.
Just for reference, I have owned and used more than a few DMM and clamp-DC current probes. My own is a Fluke 87 and a Fluke labeled Bell Current probe. Both were NIST calibrated for a number of years. I trust them, but one has to be ready to back out and re-Zero or else you have garbage at low currents.

Keep it up guys. Remember, any instrument beats not instrument.

Matt

[ktmrfs]

dclark1946 wrote:
I use both an Extech 40/400 A true rms clamp on and a Sears combined DC clamp on /multimeter. The Extech has a zero function and will resolve down to 10 mA and the lower cost Sears unit will not resolve down as low but it also has a zero function. These units are pretty accurate. You are smart to add this type of meter to assist in monitoring maintaining your 12V system.

Dick

X2. And for RV work and many other jobs these meters are more than perfectly adequate. They are accurate enough, rugged enough and low enough cost and will last a long time with any reasonable care.

What we need are "qualitative" readings, that is Is current flowing? Is it close to what I expect? does it make sense. Not necessarily "quantitative" readings. E.g. 3.0004A, exact, accurate, highly repeatable readings.

there area a few cases when DC Volts accuracy is more important, for example determining battery state of charge. There DC accuracy to .1V DC is pretty much necessary. But with today's DMM's that is pretty much achievable at reasonable cost.

I have a NBS traceable certified DMM and AC and DC current probes. They stay in a good case and get pulled out on the very few occasions I need them.

For virtually everything else, I have a very rugged reasonably accurate, reasonably repeatable DMM and AC and DC current meters. In 99% of the cases, they will tell me what I need to know with enough accuracy to determine what I'm looking for.

Only thing else that would be REAL handy on occasion would be a inexpensive 4 terminal ohm meter with long leads. resolution down to .1 milliohm or so, full scale of maybe 10 ohms.

One of my DMM's comes close, 50 ohm scale that you zero by shorting the leads together and it reads to 0.01ohms (10 milliohms) but it isn't all that repeatable for zeroing. but it is handy for finding bad connections.

[dclark1946]

I use both an Extech 40/400 A true rms clamp on and a Sears combined DC clamp on /multimeter. The Extech has a zero function and will resolve down to 10 mA and the lower cost Sears unit will not resolve down as low but it also has a zero function. These units are pretty accurate. You are smart to add this type of meter to assist in monitoring maintaining your 12V system.

Dick

[mena661]

Thanks for that info ktmrfs!!! I'll compare the meter sometime this weekend to my Victron and B&D1093 charger. Maybe I'll post the findings for S&G's.

[ktmrfs]

mena661 wrote:

Matt_Colie wrote:
AZ,

DC current probes are notoriously inaccurate.

Aren't the clamp-ons hall effect? Are those inaccurate too? Also, how inaccurate are they? My meter has an accuracy rating, are those typically incorrect? Sorry for the questions this is the first I've heard of this.

Hall device sensors can be very accurate. Without knowing what brand/model you have it's hard to tell what the accuracy is.

However, if your dc current meter has a hall device rather than a shunt. (Hall are pretty typical today, can find a 250A hall current probe from many sources) a decent one is plenty accurate for RV measurements.

If it is a reasonably quality vendor trust the accuracy readings. Remember with a hall current probe, especially if you are looking at currents under 10A or so, use the autozero function with the probe oriented how you will connect and near the connection point but not around the conductor. This will zero out much of the external magnetic field. On a good meter if you autozero then take a reading without putting it around the conductor, it shouldn't read more than about 0.3A, often less. And even if you don't autozero, the reading should be less than 1A or so.

If your measuring 50, 100, 200A, the need to autozero, isn't near as critical.

How accurate??? again depends on model, but 5% of reading or better is achievable at reasonable cost. And that's probably "good enough" for RV measurements. Repeatability on most is going to be well better than 5% of reading.

menna:

BTW I have a 1000A (yes one thousand) AC/DC clamp on True RMS current probe with peak hold function. When measuring the DC current in my solar charger, or battery charger and comparing it to the trimetric reading the difference is usually in the few 0.1A at currents up to about 40A, at most an amp at 100A charging current. using it on my Vector battery charger also yields readings that are very close to the Vector current reading, within an amp.

[mena661]

Matt_Colie wrote:
AZ,

DC current probes are notoriously inaccurate.

Aren't the clamp-ons hall effect? Are those inaccurate too? Also, how inaccurate are they? My meter has an accuracy rating, are those typically incorrect? Sorry for the questions this is the first I've heard of this.

[ktmrfs]

Matt_Colie wrote:
AZ,

DC current probes are notoriously inaccurate. For starters, they are magnetic field sensitive. I can use mine as a compass and it is one of the best available. They all get real bad at the low ranges.

The reading of 70 amps for a 6.2@120VAC is about right. And, yes, it will toast your little Grp. 24 battery. I am not surprised that inverter put up with it for a while, but that while will be a thermal limit for the hardware.

Matt

I'd say cheap dc current meters may be inaccurate. I have several quality DC/AC true RMS current probes, have designed them as well. A good one needs a "zero" function to null out the surrounding magnetic field field for the hall sensor. and a good amplifier design. Any more a decent accuracy (few %) 150A is obtainable at a reasonable price. And even with 150A + full scale, it's accurate and sensitive enough to make the readings you need down to a few amps or less.

And the good ones are actually pretty insenstive to external fields. Even w/o zeroing, the one I have will only vary at most 0.5A regardless of orientation and surrounding magnetic fields.

but again, it's a case of "you get what you pay for".