Calibration of inputs for 0/4-20mA current loop and strain gauge - Page 1
Forum section: electronics / general

author: kim.dd
author_rank: Newbie
timestamp: December 13, 2018, 06:53:50 am
content: Currently we are using a Fluke 8808a for checking/calibrating 0/4-20mA input cards.This meter has the following spec for the 20mA range -> 400PPM of reading + 200PPM of range.To improve the uncertainty on the this range I'm searching for something better, but when looking at other meters this seems already pretty decent?For example a 8 1/2 digit 3458a gives this for the 100mA range -> 30PPM + 5PPM, which is great but this meter is also a factor of 10+ more expensive.When you look at a 7 1/2 digit 34470A for the 100mA range -> 500PPM + 50PPM (the lower range is 10mA and doesn't cover the complete 20mA).The Fluke 8842a is even less good then the 8808a when measuring 0-20mA... or didn't I interpret the specs right?Anyone known something that is less than a 3458a and is able to get sub 100PPM for a 20mA range or it's the only option?Currently we transfer the load cells from a main reference by fixing load cell and input (e.g. the complete chain). So you need to calibrate every load cell with every input you need if can't do an electrical calibration to show that the specific input doesn't effect the reading. When looking at the specs of different meters it looks like any decent 6.5 digit or better should be able to get sub 100PPM of uncertainty when using dual measurement (e.g. measure excitation in 10V range and measure stimulated strain output in 100mV range)? Also recent meters typically do not have a 20/30mV range right?

author: ap
author_rank: Frequent Contributor
timestamp: December 13, 2018, 10:43:38 am
content: The 34470A actually is even worse, 500pm +5x50ppm (50ppm of 100mA range translates into 250ppm at 20mA) in the 100mA range, so 750ppm. A relatively cheap way to go is to use a low TC shunt (1ppm or so, several resistors in parallel or a higher power one to limit self heating, e.g. VPG) with 500 Ohms and a 34465A, characterise that shunt every time with the 34465A prior to a session (to compensate for aging drifts), and measure the shunt voltage. You get the combined voltage and ohms measurement uncertainty, worst case add them up (RSS probably does not work as they are not fully independent of each other) and you end up with less than your targeted 100ppm/1a. need to do some calculation as the shunt will not exactly be 500 ohm (unless you trimm it before every session, which is not too complicated).There may be meters who do it cheap at 100ppm/1a, but I am not aware of any from the top of my head and I would think it is pretty unlikely, other than the expensive ones (3458A, 8508A).

author: TiN
author_rank: Super Contributor
timestamp: December 13, 2018, 11:37:24 am
content: Keithley 2002 has 20mA range and 350ppm/20ppm of the range spec annual. It's cheaper than 3458, but calibration often can be more expensive.

author: bck
author_rank: Contributor
timestamp: December 16, 2018, 01:54:15 am
content: Fluke 707 (~1k€)0.001mA resolutionspec is 0.015% (150ppm) of reading + 2µAthats 250ppm @ 20mAor theBeamex MC6 (not sure about price, i guess something about 5-6k€)0.0001mA resolutionspec is 0.01% (75ppm) of reading + 1µAthat's 150ppm @ 20mA