What utter nonsense....I have used, calibrated and repaired 242D systems since 1973! I have consistently been able to calibrate a 242D to within 1 PPM from 100 ohms to 1 Megohm, slightly increasing to 2 PPM towards the top end of 11Megohms and gradually increasing to 5 PPM on the lowest range mainly due to resolution limitations. They are completely traceable to primary standards with low uncertainty. I don't know where you're getting your information but it either incorrect or those 242Ds you've fussed with were not properly calibrated. My system 242D is fully traceable through the SR-104 with uncertainty as low as under 0.5 PPM against it. The uncertainty does increase slightly the farther you go from the 10K range but it is still quite small. A 3458A only comes close to a 242D at a 10K cardinal point if directly calibrated against an SR-104, beyond that, the 3458A is not as accurate as a 242D. It does take skill and experience to achieve such performance.....and don't both quoting the manual to me, those are conservative specifications and the 242D is quite capable of better performance, I've got the years of experience to know so. Each 242D system is a bit unique so quoting just how long a 242D will maintain that 1 PPM accuracy depends on each bridge. If you're worried about it, a quick check against the SR-104 will take care of that.
If you are looking for good accuracy, an older 3456A will give you very good performance at a very good price point, a lot of bang for the buck. Unless you can justify the cost or real need, instruments like the 242D and 3458A tend to be too costly for the hobbyist unless you have deep pockets, calibration is not cheap if you intend on keeping them at peak performance and you will find very few calibration shops that will calibrate an 242D to capability and not just spec, same thing goes for the 3458A.
I've got a 3456A sitting on my bench near the 242D, it is checked against the 242D/SR-104 and my Vref often, so far, it is holding calibration better than the 1 day spec of 10 PPM at 122 days since I calibrated against the Vref.....not too shabby for a 30+year old DVM. I have not tweaked it because I'm curious what the drift will be for 6 months, just to compare it against manual specs.
Hello Edwin,
I hope you are doing well!
Unfortunately I have an issue with your resistors, they are off specification.
In your email dated on Christmas Eve (12/24/2016 18:30) you stated:
...The two 10Ks were about -2 PPM and -6 PPM from nominal....
When measuring them with PTB at the maker fair on 8/27/2017 they were 10,000 441 and 10,000 397 kOhm.
As the 3458A they used this time hadn't been calibrated/adjusted* for some time I waited for a arrival of a friend.
Last Friday we both went to the university department he had worked for.
We had a 3458A in there, 5 years old and never calibrated since.
We measured 10,000 485 kOhm and 10,000 491 kOhm.
By comparing to a Vishay VHP in 10k my friend had measured at work with his annually calibrated 3458A we were able to determine that the 3458A at university was reading roughly 16ppm too high.
But even then, the real value of your resistors would rather measure an adjusted 10,000 325 kOhm and 10,000 331 kOhm.
I just measured them on my 34401A that I adjusted to PTB specs at the maker fair and they were both at 10,000 34 kOhm.
I have to assume that the two candidates have either never exhibited the resistances you indicated or that they drifted roughly 35 ppm in just nine months.
Edwin,initially I did not intend to spread the word about it you but with you bragging about your measurement abilities while simultaneously failing to meet the specs I think others should be aware of my negative purchase experience with you.This is what I wrote to you 2/27/2018 20:19:The following pictures shows one of the two 10k resistors specified 10ppm (your writing!) that I purchased from you at the end of 2016. I just measured it. They are both way off spec.Now I have to read in this thread that you have a 3456A on your bench which you could have used to verify the resistor you produced for me what you obviously not bothered to do for a client order in the 10ppm precision range.Please do not claim high precision. You are probably a good source for PWW resistors in the 0,1% or 0,01% class and are delivering a good service that you do not charge for by producing PWW resistors with resistances as per customer specification.I always enjoyed reading your contributions and still do with a grain of salt but the former confidence is gone.I hope that the bunch of knowledge you acquired will show up in your PWW products sometime in the future.And as opposed to my last email to you where I hinted to you that I ordered and paid 10ppm class and received two classes below:You can continue to ignore it - no problem.Greetings from Germanytry* The 3458A had been calibrated in the voltage ranges shortly before the fair but not in the Ohm ranges.
Why measure resistance? To determine the condition of a circuit or component. The higher the resistance, the lower the current flow, and vice versa.
In general, the resistance of components used to control circuits (such as switches and relay contacts) starts out very low and increases over time due to factors such as wear and dirt. Loads such as motors and solenoids decrease in resistance over time due to insulation breakdown and moisture.
To measure resistance:
1. Turn power to circuit OFF.
2. Turn digital multimeter dial to resistance, or ohms, which often shares a spot on the dial with one or more other test/measurement modes (continuity, capacitance or diode; see illustration below).
3. First insert the black test lead into the COM jack.
4. Then insert the red lead into the VΩ jack.
5. Connect test leads across the component being tested.
Tip: For very low-resistance measurements, use the relative mode (REL; see point 11). It may also be referred to as zero or Delta (Δ) mode. It automatically subtracts test lead resistance—typically 0.2 Ω to 0.5 Ω. Ideally, if test leads touch (are shorted together), the display should show 0 Ω.
Other factors that can affect resistance readings: Foreign substances (dirt, solder flux, oil), body contact with the metal ends of the test leads, or parallel circuit paths. The human body becomes a parallel resistance path, lowering total circuit resistance. Thus, avoid touching metal parts of test leads to avoid errors.
6. Read the measurement on the display.
7. When finished, turn the multimeter OFF to prevent battery drain.
8. Press the RANGE button to select a specific fixed measurement range.
9. Press the HOLD button to capture a stable measurement—it can be viewed later.10. Press the MIN/MAX button to capture the lowest and highest measurement.11. Press the relative (REL) button to set the multimeter to a specific reference value.
The significance of a resistance reading depends on the component being tested. In general, resistance of any one component varies over time and from component to component. Slight resistance changes are usually not critical but may indicate a pattern that should be noted. For example, as the resistance of a heating element rises, the current passing through the element decreases, and vice versa. See diagram below.
When working on a circuit board, it may be necessary to lift one of the leads of the resistor from the board to measure the correct resistance of the resistor. The resistance measurement displayed by a digital multimeter is the total resistance through all possible paths between the test lead probes. Caution is required when measuring resistance across a component that is part of a circuit.
The resistance of all components connected in parallel with a component being tested affects the resistance reading, usually lowering it. Always check the circuit schematic for parallel paths.
Reference: Digital Multimeter Principles by Glen A. Mazur, American Technical Publishers.