Clamp-On Current Meter Readings

otrfun wrote:
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.

thePI EMS is IMHO more than a "little" optimistic. I did a real current measurement and called PI to see if mine was defective. They responded that the current readout was a late addition and is pretty inacurate. I have seen reading of 35A with a 30A feed, and drawing only a little over 20A. And readings of 6-8A when real draw was only a few amps.

your lucky on the parasitic draw, mine is about double yours.

70 amps?

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

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

Yup, this is my first clamp-on meter so I'm learning. Even though I'm beginning to realize it's not all that accurate, it's still very cool to at least get a rough idea of what's flowing (or not) through those wires :)

Ref the inverter, I originally purchased it just to power my laptop and flatscreen. Was gonna buy a 300 w PSW inverter, but this Power Bright 600 w PSW inverter was the same price as the 300 w PSW inverters. With Amazon's return policy and the good reviews it seemed like a good purchase risk. When I discovered it would power my small vaccum cleaner and coffee maker that was just icing on the cake. I've purposely overloaded and shorted this thing out and it keeps coming back for more--very impressed with it. As for toasting my battery, you're right, it might. Actually, I've been loading/discharging/charging this battery lately to see what the limits are for my particular style of use. I'm not fond of tip-toeing around a usage situation hoping it'll do this or do that when I get on the road--rather test hard upfront.

If you charge your battery fully after doing any testing like this it will most likely survive.

Your amp-meter will be accurate enough for your needs after learning to use it. While the meter can be less accurate for low amp readings, the low amp items are not what uses up the batteries.

Using the clamp-meter on your house-battery(s) to note detector usage, turning on a light or two (your normal evening light usage), running the refrigerator on propane, running an exhaust-fan, etc. Noting these different items as they increase the amp reading will give you a good idea of how many amps would be used if these items were each used for a single hour. The amp reading does get more accurate as the current read increases.

Checking your battery voltage at the beginning of camping (to insure fully charged) and then again after 24 hours (let battery rest with only detectors for about 1/2 hour), will allow you to see your current battery voltage (at the battery-terminals) and calculate the battery % charge versus the battery charge chart.

You can see how well your estimated usage compares to reality. Checking the extra amps drawn when using the inverter can be very helpful since these items are likely the real battery killers (high amp usage). They get added to the normal "house" usage).

The amp-meter can give you a very early warning if you check the extra amps being used to run your furnace in cold weather. That amp reading times the hours the furnace may be needed (plus the other usage) when checked against 50% of your total battery-bank can save you from a dead battery before morning.

In order to get by dry-camping you need to know when to cut back on unneeded items early in the camping period. Approaching cold weather may require skipping use of the television and other items.

By the way it's an electric motor running a compressor that requires a much higher current start-up. A motor by itself will start and run but if the required voltage drops (brown-out conditions) the motor will slow and you will see the dreaded "blue smoke" and a belly up motor.

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".

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.

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.

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.

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