AM-270 AC Voltage issue? - Page 1
Forum section: electronics / testgear

author: madscientistii
author_rank: Newbie
timestamp: September 02, 2015, 06:32:01 pm
content: Hi Everyone!I recently purchased an AM-270 DMM and out of the box, it takes a while (about a min or so) to come to zero in ACV and ACA modes, with and without any probes installed. I know I can "rel" this out when it settles a bit, but is this Normal for this DMM? Shouldn't it read zero with no probes? It does zero out in DCV modes almost instantly.Thanks for the input.

author: electr_peter
author_rank: Unknown
timestamp: September 03, 2015, 07:45:44 pm
content: Hi,Amprobe AM-270 multimeter is TRMS (true-RMS) type and it has some low noise floor by design. It is designed to operate from 5-10% or even 15% of full AC mode range (see exact details in the manual) - for example, from 30V AC on 300VAC range. Below 5% (or what is specified) it is not as accurate and have some offset/noise that is not possible to rel out.This is a feature on all TRMS type multimeters, it is not a sign of flaw or defect.You cannot rel it out with relative function - it is a wrong thing to do. You have to think in terms of power levels and use root sum of squares to rel it out properly.If you really do care about accuracy, use a proper range (is mVAC range available?), do not try to measure 30 mVAC signal with 30VAC scale. Or use other methods of measurement, namely high end averaging DMMs.

author: madscientistii
author_rank: Newbie
timestamp: September 04, 2015, 03:21:21 am
content: Thank you for the reply. I think my issue is that I don't have any probes attached and I'm not taking a measurement. It seems it should read zero. It does in DCV, but the ACV takes about a min or so to get to zero.I'm just wondering if this is a quirk of this DMM or if there is something I should be concerned with as a manufacturing defect?

author: madscientistii
author_rank: Newbie
timestamp: September 06, 2015, 05:42:37 pm
content: Hi. In addition to my last post...If I attach the probes and only touch the positive probe to a live wire AND I do NOT complete the circuit, ie the com probe not grounded or connected to any part the circuit, I get a reading on my meter.My el cheapo non TRMS meters do not do this.Does anyone have this meter? Did I get a bad meter?I'm working with 120 VAC.Please advise. Thank you.

author: electr_peter
author_rank: Unknown
timestamp: September 06, 2015, 07:55:56 pm
content: Quote from: madscientistii on September 06, 2015, 05:42:37 pmWhat reading do you get?Few factors are involved here - one lead acts as an antenna and picks up all the RF noise in the air; mains is coupled to the meter via capacitive coupling even without any lead attached (stray capacitance); high input impedance of DMM makes this visible. Voltage reading in such cases is meaningless, but it does not show an error.QuoteIt probably has lower bandwidth and lower input impedance, so effects mentioned above are below detection threshold.QuoteSo far you have stated nothing that is out of ordinary to suggest bad equipment. It is more of a PEBCAC than the fault with the DMM.

author: mos6502
author_rank: Frequent Contributor
timestamp: September 06, 2015, 10:39:47 pm
content: What happens when you short the probes together?I can tell you that my UT139C stays at dead zero in ACV with no probes connected. With probes connected, the last digit (or two) fluctuates a bit, depending on how you move it around. But if you set it down and move your hands away from the probes, it immediately drops to zero again.