AC resistance Bridge -Superconducting measurement - Page 1
Forum section: electronics / testgear

author: mechknoll
author_rank: Contributor
timestamp: March 04, 2014, 07:39:50 pm
content: Hello EEVBLOG forum,So I have a question about an AC resistance bridge. I am about to start using an older unit "Linear Research Model LR-700 AC Resistance Bridge" to measure some superconducting crystals I am making. I know precision resistors will drift over time. Is there anything I should check before I use this instrument for scientific purposes? This unit is key to my research! Thank you!

author: Vgkid
author_rank: Super Contributor
timestamp: March 04, 2014, 08:08:50 pm
content: I assume you have the users manual, if not here it is:http://www.lakeshore.com/ObsoleteAndResearchDocs/LR700_V1_3_%20Manual.pdfTo check the drift pick up some vishay foil resistors of the VHP series(up to .001%, and very low temp co), though you can probally make do with less accurate resistors.After taking a glimpse at the users manual, thermal emf would be a big factor, so try to minimize them.Interesting machine though, wish I could find a service manual.

author: Dr. Frank
author_rank: Super Contributor
timestamp: March 04, 2014, 08:37:27 pm
content: Quote from: mechknoll on March 04, 2014, 07:39:50 pmI just flew over the manual. Nice instrument, good resolution in the nOhm range.What do you want to do with precision resistors?The absolute value of the resistance in such measurements on SC is totally unimportant, especially, as I assume that you have 4 connections to the sample, randomly distributed on the surface on the SC. So there will also be a random value for the resistor.Good shielding of the setup is much more important, although it seems to be a Lock-In type amplifier.Then, you need a real good short to make some tests for zero Ohm, and perhaps a non-inductive resistors, wirewound or metal foil for example, to calibrate the phase of the resistance, i.e. real and imaginary part.Both can also be tested / calibrated when your DUT is in the superconducting state.Needless to say, that the contacts are critical also. Are you using silver epoxy?I'm curious: What kind of SC do you have? High-Tc? Which system?How big are your crystals?Remark: Thermal EMFs do not play a role here, because it's an AC resistance bridge.Frank

author: mechknoll
author_rank: Contributor
timestamp: March 04, 2014, 09:11:12 pm
content: thanks for the replies! It will be a 4-channel measurement using both Lock-in technique as well as the DC Reversal Measurement Method using Keithley nano-volt meter and precision current source. I see the precise values are not important simply the trend, but I would like the instrument to be both precise and accurate. I am using silver epoxy and gold wire for the contacts. We have a PPMS system that is functioning but now Im in the process of building a custom setup with 9 Tesla magnet and closed circuit refrigeration unit capable of sub-kelvin temperatures <1 k.We are making new iridates and perovskites using a floating zone optical furnace to ger mm sized crystals which we verify by XRD and neutron diffraction. The exact crystals are classified at the momentBut we have very promising results!

author: Dr. Frank
author_rank: Super Contributor
timestamp: March 04, 2014, 09:29:25 pm
content: Quote from: mechknoll on March 04, 2014, 09:11:12 pmCool stuff, have fun.I've done such experiments >20 years ago,right from the beginning, when Bednorz & Müller just discovered the Perovskite SC, 1987, I think.Mostly I made magnetic measurements, also in the nV range.Well, the Lake Shore instrument is precise to 0.02%, therefore you need 0.01% wirewound or metal foil resistors to check. Easily obtainable, ww ones most easily and inexpensive.The Keithley nV instruments are also very nice...My precision current source (1µA..100mA) you will find in the forum.. also originates from those experiments.Frank

author: Vgkid
author_rank: Super Contributor
timestamp: March 06, 2014, 12:19:10 am
content: After seeing the users manual, im curious how/if it does the ac to dc conversion on that accuracy level.

author: johansen
author_rank: Super Contributor
timestamp: March 06, 2014, 12:51:49 am
content: Quote from: mechknoll on March 04, 2014, 07:39:50 pmi have a KV divider from 40 years ago and the wirewound resistors on the first decade are still matched to within 2ppm