Are 4 wires Ohm sense necessary in bench top multimeter? - Page 1
Forum section: electronics / projects

author: Lawsen
author_rank: Frequent Contributor
timestamp: June 15, 2011, 11:39:09 pm
content: The Agilent U3401A has a 2 wires Ohm sense and 4.5 digits for a bench top economy multimeter for $450.00 USD. It is not like the classic HP/Agilent 34401A with 2 or 4 wires Ohm sense, RS-232C and GPIB computer interface, and 6.5 digits and price tag around $1,072.00 USD, not affordable. Are 4 wires Ohm sense necessary in bench top multimeter?Lawsen

author: Unknown
author_rank: Unknown
timestamp: June 15, 2011, 11:52:12 pm
content: Depends on the application. 4-wire resistance measurement is designed to prevent variation in lead/contact resistance from causing an error in resistance measurements. Since lead and contact resistance tends to be <1 ohm unless you use really crappy cables or connections, it's mainly significant in the low ranges, especially for low-resolution equipment (at 6.5 digits, .1 ohm may still be significant at 1Mohm). Figure out how much accuracy you need for low resistance measurements, and you should be able to answer the question.

author: Ronnie
author_rank: Regular Contributor
timestamp: June 16, 2011, 02:21:53 am
content: If you will design inductor I think 4-wire resistance measurement is important since you can measure accurately the very low resistance of the coil. Also it is better for relay contact measurement typically in the milliohm range.

author: Bored@Work
author_rank: Super Contributor
timestamp: June 16, 2011, 05:52:03 am
content: Build-in 4 wire resistance measurement is a convenience. You can do the same measurement with two separate meters and a current source. I think Dave even once made a video about it, but I can't find it at the moment.Setting up 4 wire resistance measurement separately is inconvenient, but if you only need to do it once or twice a year you probably wouldn't spend the extra money for a meter with integrated 4 wire resistance measurement. But mind you, if you do it with normal multimeters you need two of them, and they need to be accurate.

author: nukie
author_rank: Frequent Contributor
timestamp: June 16, 2011, 08:44:04 am
content: Fluke bench meters do 4wire measurements with patented 2 wires check it out. Don't know how well it works though.

author: ejeffrey
author_rank: Super Contributor
timestamp: June 16, 2011, 10:00:53 am
content: As above, 4 point (Kelvin -- that guy did everything!) resistance measurements are only necessary for high resolution / low Z measurements where the wire or contact resistance is important.However, good 4-point measurement is (for me) one of the major distinguishing features of a bench-top multimeter, along with improved resolution, accuracy, and computer control capability for automated measurement systems. A 4.5 digit bench meter with only 2 wire ohms would not be compelling to me over a good hand-held unit unless it was going to be a dedicated instrument as part of an automated setup. In that case, get whatever checks the boxes your application requires.

author: saturation
author_rank: Super Contributor
timestamp: June 16, 2011, 12:08:00 pm
content: Its a question of the accuracy you need, and thus will pay for. Per spec sheets, the 3401 is good to 0.1% on 2 wire, and the next model up, 3402, is rated at 0.05% using 4 wire.With a guard terminal you can extend accuracy yet another 10x, to 0.004% which I get on the 30 year old HP 3456a DMM, a feature not included on 3400 series Agilent DMMs.

author: Zero999
author_rank: Super Contributor
timestamp: June 16, 2011, 05:02:00 pm
content: Quote from: Ronnie on June 16, 2011, 02:21:53 amIf you're designing an inductor then you should be able to calculate the ESR value so there's no point in trying to accurately measure it.

author: Unknown
author_rank: Unknown
timestamp: June 16, 2011, 11:31:19 pm
content: Quote from: BoredAtWork on June 16, 2011, 05:52:03 amYou also need a fairly stable current source, otherwise you have to synchronize the acquisition of a reading from both meters at the same time, and verify that voltage and current measurement have a similar transient response. It should be at least as stable (short term) as your required accuracy. Apart from that, this method works fine. It's also useful for very low resistors like shunts, you can just shove a few amps through there, and not have to worry about extremely low signal levels (1mA through 10mOhm is 10uV, 1A through 10mOhm is 10mV).Quote from: nukie on June 16, 2011, 08:44:04 amThey just designed a proprietary connector that has two contacts in the space of one 4mm banana plug (the contacts are split in two). The 2x4W term is just a fancy marketing trick to describe two conductors within one cable and connector, just like a headphone cable. The electrical connection is identical to the normal 4-wire measurement, two sense and two source wires.Quote from: saturation on June 16, 2011, 12:08:00 pmMost high-precision meters can get excellent accuracy with two wire measurement. Only the absolute error increases at the lowest ranges. Often 4 wire measurement doesn't even work for the highest ranges.I think most 6.5 digit meters like the Agilent 344xxA series made today don't have a guard terminal either, they claim to get away with just internal guarding.