CM3 USB logger

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Ceramicx
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CM3 USB logger

Post by Ceramicx » Mon Jun 13, 2016 12:29 pm

Hi there,

We have recently purchased a CM3 current logger which I am using to log relatively small currents (approx 3.8A). I am trying to log the current in an electrical resistance coil which will heat by simple joule heating. What I am seeing appears to defy metal physics insofar as the current is increasing with respect to time and therefore temperature (i.e as the coil heats, the current increases)! I have double checked the readings with a non-logging clamp meter which shows the current indeed decreasing as one would expect.

There also appears to be a discrepancy between the two clamp metres with the TA138 reading 3.7A and the mainstream metre recording 3.95A. I appreciate that the response time and signal level from the TA138 clamps will influence the readings and response time, however I would expect that it would take less than 100 seconds for the reading to drop from 3.7A to 0.4A.

Perhaps you could shed some light on this

Martyn
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Re: CM3 USB logger

Post by Martyn » Thu Jun 16, 2016 8:12 am

The CM3 has an RC input stage that gives rms reading of the AC current signal, the lower the current, the slower the settling time.
For very low currents it will be necessary to either use more sensitive clamps (10mv/A or even 100mV/A) or to loop the wire through the clamp multiple times to increase the signal. Additionally the clamps may be picking up residual mains hum from nearby wiring. They are designed to respond to 50Hz signals so act as aerials, a 1A current through the probe will only give 1mV of signal, which can be lost in the noise.
Martyn
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Ceramicx
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Re: CM3 USB logger

Post by Ceramicx » Tue Jul 05, 2016 7:10 am

Hi Martyn,
Thanks for the information. We seem to have go over this problem. There's still some residual current, but once my 3.8A is multiplied by 10, it's relatively minor.

Can you tell me what the response time of these clamps is? I've 0.75 seconds in mind, but I can't back this up. Is there a way to get a faster response time, maybe of the order of 0.1 or 0.05 seconds? I appreciate there are two components which need to respond: the clamp and the logger, so I'm not sure which one is the bottleneck so to speak. Perhaps you can advise

Cheers

Martyn
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Re: CM3 USB logger

Post by Martyn » Tue Jul 05, 2016 7:58 am

The conversion time per enabled channel is 720 ms, so your findings are correct.

If you are looking for 0.1 or 0.05 seconds then you would need to use the clamps with an oscilloscope.
Martyn
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marius.fourie
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Re: CM3 USB logger

Post by marius.fourie » Mon Jul 23, 2018 8:49 pm

I just found out about the low pass filter in the CM3 and then read this post explaining it. At 8 amp the 10 - 90% rise time seemed to be 10 seconds and I was not going to wait for the zero Amp reading to settle down.

Is there a way to shorten the rise time, for example using a low input impedance? I am planning on making my own input circuit, but won't bother if the RC filter is that slow.

I also have a PicoScope 2204A. Would that be a better option to measure fast voltage signals (no low pass filter surprises)?
Marius Fourie
www.greenpro.co.za

AndrewA
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Re: CM3 USB logger

Post by AndrewA » Thu Jul 26, 2018 11:13 am

Hi Marius,

Using a low input impedance on the CM3 won't help.
Using a scope like the 2204A would be a lot better to seen current transients. No settling time issues, and faster sampling.
Note the 2204A is currently supported in Picoscope 6 but also PicoLog 5 software.
Regards Andrew
Technical Specialist

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