ΩHere in our plant we have a 1/8 DIN panel mount ohmmeter which is used to measure the resistance of long lengths >30,000ft of wire on spools. measurements are generally <2kΩ. The meter is a Newport Electronics meter and they are one of the only manufacturers I can find of a 1/8 DIN panel mount ohmmeter. The problem is that the measurement on the spools is very jumpy. When measuring a fixed resistor, there is no jumpiness. Handheld DMMs do not have this problem. My first thought was that the spool was acting as a giant inductor, picking up stray noise; but no HV is around the area. I tried countless different RC combinations to try and filter it out, but nothing seemed to work. My second thought was that; with the spool acting as an inductor, the meter would have to build up a magnetic field in the spool in order to get an accurate resistance measurement. I thought maybe the panel meter didn't have enough umph to push through the inductance. The panel meter measures with 4.5v @ 40μA when shorted, working out to .18mW. a Fluke meter measures with 1V @ 1mA = 1mW (5X as much as the panel meter), which is why (I assume) it doesn't have the same problem: seems to support my theory about the low power.
So, I set out to find a 1/8DIN panel mount ohmmeter with at least 1mW output at short. It's alot harder to find than you'd think. Actually, I never found one. So I decided it was time to find another solution. I got a 5W 500Ω potentiometer, a 24V DC power supply & a Red Lion IMP 0-50mA Process Meter. +24V goes through the pot (set to ~480Ω, to limit current to 50mA (24V @ 50mA = 1.2W, 6,667X as powerful as the installed meter)), to the test leads, to the process meter, back to ground. simple. The not-so-simple part is the nonlinear measurement curve created by adding the fixed 480Ω. The meter has 9 scaling points and I plotted the segments along the nonlinear curve and got the accuracy (according to my "ohm-ranger", not a calibrated standard) to within 1.5% from 0-2000ohms. It took me weeks, working in my spare time.
I just now, beaming with pride, went to install it, and saw the calibration sticker next to it. This whole time I thought it wasn't a cal item. All our cal is 3rd party. When the calibration guy comes next month, he's got no way to calibrate the thing. you have to manually change the scaling points which is an all day affair. That, and due to the linear segments falling along the curve, any error is not constant; so simply adjusting a trim pot might fix an error at one reading, but totally mess up the rest of the readings.
This is a terrible solution. a kludge. it cannot be. What can I do?
And to take a step back, is my logic even right? do I really need a high(er) power ohmmeter?
P.S. installing an anolog meter is ruled straight out.
So, I set out to find a 1/8DIN panel mount ohmmeter with at least 1mW output at short. It's alot harder to find than you'd think. Actually, I never found one. So I decided it was time to find another solution. I got a 5W 500Ω potentiometer, a 24V DC power supply & a Red Lion IMP 0-50mA Process Meter. +24V goes through the pot (set to ~480Ω, to limit current to 50mA (24V @ 50mA = 1.2W, 6,667X as powerful as the installed meter)), to the test leads, to the process meter, back to ground. simple. The not-so-simple part is the nonlinear measurement curve created by adding the fixed 480Ω. The meter has 9 scaling points and I plotted the segments along the nonlinear curve and got the accuracy (according to my "ohm-ranger", not a calibrated standard) to within 1.5% from 0-2000ohms. It took me weeks, working in my spare time.
I just now, beaming with pride, went to install it, and saw the calibration sticker next to it. This whole time I thought it wasn't a cal item. All our cal is 3rd party. When the calibration guy comes next month, he's got no way to calibrate the thing. you have to manually change the scaling points which is an all day affair. That, and due to the linear segments falling along the curve, any error is not constant; so simply adjusting a trim pot might fix an error at one reading, but totally mess up the rest of the readings.
This is a terrible solution. a kludge. it cannot be. What can I do?
And to take a step back, is my logic even right? do I really need a high(er) power ohmmeter?
P.S. installing an anolog meter is ruled straight out.