Getting weird measurements from a TrueRMS Clamp multimeter - Page 1
Forum section: electronics / beginners

author: axero
author_rank: Regular Contributor
timestamp: August 09, 2016, 03:45:18 pm
content: I just bought a clamp multimeter as per the first attachment but I get very weird readings from it. When I measure frequency directly on the power outlet I get a strange reading of 200Hz, sometimes 250Hz after the multimeter for some reason resets itself upon insertion of probes. When I measure voltage, it doesn't reset itself but reports 230.1V. If I measure the same cable with the clamp the frequency is 50Hz as it should be.I also tried measuring a 12V halogen lamp circuit. The picture of the power supply unit is in attachment 2 and the diagram with measurements is in attachment 3 (Yen symbol means clamp measurement, "-" means probe measurement). The voltage readings are just weird. It measures 6.20V instead of 12V, the current is as high as 60A and that on a circuit with three 35W lamps. The power supply probably uses switching and that may be a problem with some multimeters but this is aTrueRMSmultimeter so it should be able to handle it.I also tried another Halogen lamp circuit that uses a pure toroidal transformer as a power supply. On that circuit the voltage reading is fine, 11.8V, but the amperage readings are just plain wrong; a 35W halogen bulb draws no more than 2A and a 50W draws 3A. This is just plain wrong, a 50W bulb draws 4.17A @12V and a 35W bulb draws 2.92A @ 12V.So what's wrong, is it the multimeter that is at fault or is there something else that I haven't accounted for?PS. I can hear the sound of loose material near the clamp when I shake it, probably a piece of ferrite core that has chipped off, is this normal?

author: MosherIV
author_rank: Super Contributor
timestamp: August 09, 2016, 04:07:22 pm
content: QuoteCheck the specification for the meter.Many DMM do not measure freq above 50V, the strange effects might be the DMM responding to the overload.

author: axero
author_rank: Regular Contributor
timestamp: August 09, 2016, 05:11:55 pm
content: The measuring range is specified to 1-10V RMS for frequency. It's strange to have such a small range, measuring frequency must be a much less delicate procedure than measuring say voltage.But what about the other behaviours of the multimeter? I'm more concerned about the strange ampere readings than I'm over the frequency measurements.

author: bktemp
author_rank: Super Contributor
timestamp: August 09, 2016, 05:23:29 pm
content: Quote from: axero on August 09, 2016, 05:11:55 pmI couldn't find a complete datasheet for the multimeter, but the short spec sheet says 40Hz/400Hz for "Frequency From Clamp". So I assume the current clamp meter it is not suited for higher frequencies.

author: axero
author_rank: Regular Contributor
timestamp: August 09, 2016, 05:33:12 pm
content: The clamp can only measure frequencies up to 400Hz or 40Hz depending on desired resolution. The probes can measure up to 100kHz. But still the frequency measurements are the least of my concern. I bought the meter for the ability of measuring DC and AC-RMS amperage through the clamp up to 1000A. The rest is unimportant for me.

author: bktemp
author_rank: Super Contributor
timestamp: August 09, 2016, 05:39:16 pm
content: Quote from: axero on August 09, 2016, 05:33:12 pmIf it can measure only <400Hz and you are measuring 50kHz or more using the clamp the result is undefined.The high frequency current probably induces a much higher voltage in the current sensor than at 50Hz, so it shows a much higher value.

author: crazyguy
author_rank: Regular Contributor
timestamp: August 09, 2016, 05:44:36 pm
content: Quote from: axero on August 09, 2016, 05:11:55 pmI believe that particular "Frequency" Range is for Digital Logic, it is not intended for high voltage measurementUsually this kind of DMM chipset, in the "Voltage" Range, Press the "Func" button will switch to other measurement Function, such as Frequency/Duty Cycle

author: axero
author_rank: Regular Contributor
timestamp: August 09, 2016, 06:07:43 pm
content: Here's my measurement procedure when I use the clamp. I put the clamp near the area I want to measure. If the "REL" indicator is on, I press and hold zero until it goes out. Then I wait a few seconds and press zero so that the meter shows 0A when there is nothing through it. Then I put it around the cable I want to measure and conduct the measurement.Perhaps this is the wrong way to do it? If I don't "calibrate" it, it will float between 2 and 4 amps even though nothing is going through the clamp.I tried to measure without the calibration procedure: The other lamp uses a pure transformer for power supply, output is 11.89 Volts. There are 3x35W + 3x50W bulbs connected to the PSU, altogether rated for 255W. When I put the "uncalibrated" clamp around the cable as it leaves the power supply I get a measurement of around 20A. That sounds a little more reasonable.But it isn't precise and the TrueRMS module fails to capture correct measurements of the other electrical circuit.

author: crazyguy
author_rank: Regular Contributor
timestamp: August 09, 2016, 06:25:47 pm
content: this clamp meter can measure DC current, you may try playing it with a Power Supply Unit first.