Negative Ohms  Readings - Keithley DMM6500  updates November 26, 2018 Page 2 - Page 1
Forum section: electronics / testgear

author: omegaone
author_rank: Contributor
timestamp: November 16, 2018, 11:56:38 am
content: Hello All,When testing lead resistance, I ran into an issue first with FLUKE 289 at selection LoΩ.Lead made 14 AWG, Teflon sleeve 600 V, 200 C.Resistance discrepancy between from 9 mΩ positive reading on one meter, and 9mΩ negative reading on another meter.First time, I ever witnessed the negative ohms territory on a practical test.Since then, I have been trying to get some answers as to why. I have sent the log files to FLUKE product specialist, they are still working on it.If anyone has any suggestions, please give some feedback.Note:I have also received an evaluation unit DMM6500. So I have repeated the test with the leads. To my surprise, DMM6500 read negative 5mΩ in 2 wire mode. ( Agreeing with one of my fluke 289 which read negative 9mΩ)I am also getting Keithley to look into the same.

author: alm
author_rank: Super Contributor
timestamp: November 16, 2018, 12:40:29 pm
content: If it's in two wire resistance mode (the Fluke 289 has no other option) it could just be that the short you are using has a lower resistance than the one used for calibration. If it was calibrated with a 30 mOhm short, anything below 30 mOhm will read as negative. External voltages (e.g. Seebeck effect) can also cause a negative offset.What is the uncertainty Fluke specifies for this range? 1 mOhm resolution is really pushing it without Kelvin sensing or offset voltage compensation in my opinion.

author: commongrounder
author_rank: Frequent Contributor
timestamp: November 16, 2018, 02:50:21 pm
content: There have been other threads on this forum discussing negative readings in 50-ohm mode on the 289. My 289 also does this, and I have found it to be temperature dependent. Measuring resistance at these low values, with only two-wire mode, you have to allow for noise, as well as thermal other reactions due to dissimilar metals. I personally think it does pretty well, considering the applications the meter was designed for.

author: threephase
author_rank: Regular Contributor
timestamp: November 16, 2018, 04:35:15 pm
content: As per the others, you are asking an awful lot from a 2 wire resistance measurement in terms of practicality.You can take a look at the calibration manual for the Fluke 289 at;https://dam-assets.fluke.com/s3fs-public/287_289_cmeng0100.pdfThe basic accuracy on page 7 for the 50 Ohm range is 0.15% +20.If you look at the performance tests for the resistance range on page 19, it doesn't actually give a value for the 0 Ohms test, only that a stable reading is required. For 0.2Ohms applied, there reading should be between 0.18 and 0.22 Ohms. Interestingly, if you scroll back up to page 18, it give a 0 Ohms test applied to the 500 Ohm range with an expected value between -0.01 and +0.01 Ohms.The calibration of the 289 recommends the use of a Fluke 5520a calibrator. Looking at the specifications of that for the 0 to 11 Ohms function, there is a 1mOhm +35ppm uncertainty applied.The DMM6500 looks better as it has a 10 Ohm range, with 0.002% rdg + 0.02% range specified best accuracy. However, reading the small print for 2 wire resistance test, compensation mode has to be applied and even then there is a 100 mOhm uncertainty.For 4 wire measurement on the DMM6500 you can go down to a 1 Ohm range, but the accuracy is at 0.008% rdg + 0.02% range.The above only applies to the DMM6500 if you are within 24 hours of the lat calibration and temperature is within 1 degree C.The problem with the function is the low test current of 10mA, with such low resistances the reading will be susceptible to noise and thermal stability of joints.You are really in the realms of a source measurement unit or a dedicated micro-ohmmeter that can increase the test current and give a more stable reading. The downside being that the increased current affects the measurement of the test item to some extent by heating it.Kind regards

author: omegaone
author_rank: Contributor
timestamp: November 17, 2018, 03:30:15 am
content: "If it was calibrated with a 30 mOhm short, anything below 30 mOhm will read as negative. External voltages (e.g. Seebeck effect) can also cause a negative offset"Very good point. At present, I do not know what was used in calibration.I will look into it further. Thank you.

author: omegaone
author_rank: Contributor
timestamp: November 17, 2018, 03:34:59 am
content: Good to know you noticed similar on your meter.I was comparing two Fluke 289 side by side. As per fluke engineers, I did warm up the meter then made sure lead to meter temperature was consistent before taking the reading close to room temperature.

author: threephase
author_rank: Regular Contributor
timestamp: November 18, 2018, 08:23:25 pm
content: omegaone, it looks like I have the same shunt as you, so I have tried it in my Fluke 8846A, the meter was factory calibrated in April 2018.In 2 wire mode, the reading was 0.00549 Ohms. In 4 wire mode, this dropped down to 0.00389 Ohms. The physical setup was the same, I just switched between the 2 wire and 4 wire mode. Having the short connected directly as per photo 3, the reading was 0.00489 Ohms. I did not get a negative reading at any point, but the ohmic values seem to be similar to yours.Playing around with the instrument, I did get a negative reading in 2 wire, if I compensated and in 4 wire if I reversed the sense connections.I then tested a 1mOhm shunt 0.1% accuracy. Again no negative ohms reading was obtained, although the readings were no where near where they should have been. The 2 wire measurement was high, which is what I would have expected, the 4 wire was low, which is not expected, so there may have been some setup error causing this.A 10mOhm shunt produced a high reading in 2 wire mode, and a much closer reading in 4 wire mode. Which is pretty much what I would have expected, although given the tolerance of the 10mOhm shunt, also 0.1%, the 4 wire reading is too high.Kind regards.

author: threephase
author_rank: Regular Contributor
timestamp: November 18, 2018, 08:25:21 pm
content: Pictures of the 10mOhm shunt being tested, as it got too big for the reply.

author: threephase
author_rank: Regular Contributor
timestamp: November 18, 2018, 08:33:03 pm
content: This is the 1mOhm shunt being tested on a 2450 SMU with different setups being used.Using 4 wire measurement and a 100mA test current, the reading is quite poor, up the test current to 500mA and the reading is better but still not close enough. At 1A, I get a reading within tolerance, but it is still a little high in comparison to a reading from a micro-ohm meter injecting a 10A test current.The last picture shows a 2 wire measurement using the SMU with a really bad result. So even an instrument with this level of accuracy and expense can have its bad moments.Kind regards

author: omegaone
author_rank: Contributor
timestamp: November 21, 2018, 04:33:26 am
content: Thank you for all the pictures and valuable info.I have been bit busy with my regular job and with myself and was unable to go through them all. But I intend to, soon.In the meantime, I am attaching the log files, graph, thermal image, and pictures I sent to Keithley for your review.Note: I have deleted the 4 wire resistance readings from this post due to erroneous connection of leads to keep confusion to a minimum!( Thanks to Threephase and Commongrounder)2 wire connection and negative ohm readings still valid at this point.( 24 November 2018)

author: threephase
author_rank: Regular Contributor
timestamp: November 21, 2018, 10:30:08 am
content: Thanks for the extra information.Not sure why the two wire measurement goes negative, as suggested above, it may be due to the way the meter is calibrated. If I was trying to make that measurement you are doing, I would zero the meter using the shorting plug and then measure the leads. The other question to ponder over, is what resistance value you should actually be getting. That would involve knowing the Ohms/m of the wire from the manufacturer and calculating to the lead length you have.For the 4 wire measurement it looks like the sense leads are swapped, so 'high sense' is connected to 'low force' and 'low sense' is connected to 'high force', that could be responsible for the negative reading on the 4 wire test. You also need to make sure that the sense leads don't short to one another at the connection point.The resistance values you are measuring are very low and also at the point where noise can affect the readings, especially when measuring test leads that are more susceptible to noise rather than the shorting plug.Kind regards.

author: omegaone
author_rank: Contributor
timestamp: November 22, 2018, 01:03:38 pm
content: Thank you for your response.About the leads: My first test was to test the leads independently and if you look at the data, the numbers are very close. The connection was made as per the manual (minus the resistor) see attached picture.And Keithley tech support has repeated my test on request using a common paper clip of approximately .02 Ohms using 4 wires.( Still separate operation from Tektronix )I am also attaching their data here.The Engineer performed the test was a veteran with 40+ years with Keithley just to give you some prospective.I am also attaching a picture of calibration certificate here. It is fresh from factory.About the noise:You may have seen the thermal images I have attached. I took precaution to match the test piece to machine temperature the best I could.Once, the resistance issue is solved, I have more important matter to pursue with microamps and DMM6500. As you know, the creator of this forum has put out his best engineered microcurrent product. I have one of the gold version sitting on my table ready to be tested.Appreciate all your feedback.Samuel

author: threephase
author_rank: Regular Contributor
timestamp: November 22, 2018, 09:17:17 pm
content: Looks like the Keithley engineer produced some good readings. Have you tried to repeat their measurements on your DMM6500?Due to the angle of the photo, I am struggling to see how the leads are shorted together where they are ty-rapped. When I look at the two red leads, they look like they are tied together, but one is going to Sense Lo and the other to Input High?I would be inclined to rearrange the leads to make sure that there is no chance of an accidental short between the exposed metal.Kind regards.

author: omegaone
author_rank: Contributor
timestamp: November 23, 2018, 11:36:00 am
content: I apologize for the not so good pictures, but be assured the leads were connected as per the manual. I do repeat my tests over and over but not have time to take pictures each time.So far my reading went to negative consistently. It also correlated to the AWG. ie. I went from 14 gauge to 12 gauge and the negative value increased as a function of gauge difference.I have requested the Engineer to send me the same paper clip as a joke! May be he will. ( he is on vacation now)In the mean time, I intend to test paper clip and will post results here. I find, I am able to get a better resistance value by using ty-rap. The reason is better pressure applied against the measuring surfaces and not impeded by other material ( ty-rap being and insulator)I am not sure I have mentioned, I have a file with Fluke engineers as well ( Fluke Canada)World famous Paper Clip update: I did not have time to capture data, only posting the pictures with 2 wire vs 4 wire and (Temp, V, A)I do not need to explain, pictures should speak for themselves about the Negative Ohms.They did respond initially with a request to add temperature compensation between the lead and the meter, which I complied with. Now I am waiting for their response as well.Note: I have deleted the 4 wire paper clip resistance readings from this post due to erroneous connection of leads to keep confusion to a minimum!( Thanks to Threephase and Commongrounder)2 wire connection and negative ohm readings still valid at this point.( 24 November 2018)

author: alm
author_rank: Super Contributor
timestamp: November 23, 2018, 07:41:03 pm
content: Is the value of 200 pA really the current through the force leads. Not through the open current input jack? And 300 nV the voltage between the sense leads? If that's the case, then no wonder it's reading negative. At these levels, the Seebeck effect (1 uV/K between copper and copper oxide) and even sneezing at the other end of the room will swamp any signal. You'd need a guarded test setup to get reliable results.I would measure the current through the force leads, and voltage between the sense lead, with other meters.

author: threephase
author_rank: Regular Contributor
timestamp: November 23, 2018, 08:46:55 pm
content: Thanks for the latest set of test pictures.I am not sure what the secondary current and voltage measurements are relating to, as they don't seem to be representative of the resistance value obtained.The thing that is of interest though is that the current is positive, but the voltage negative, I would have expected them both to be positive if the connections are correct.I have tried to look at the test jacks and to me it looks like the top left (+ sense) has a black wire going to it but the top right (+input) has a red wire. But they should both be going to the same side of the paper clip. Or it could be that my old tired eyes are deceiving me yet again.You could try reversing the sense leads and see if that gives you a positive resistance reading.Alternatively, you could disconnect the left hand sense jacks and apply a low battery voltage across them. Battery positive to top left and battery negative to bottom left should give a positive resistance reading, if it is negative it may mean that the sense connections within the meter are reversed somewhere along the line.I have done that test on my 8846A.I have also tested a paperclip on my 8846A and get a comparable reading, although slightly lower, maybe you have a bigger paper clip than meKind regards.

author: commongrounder
author_rank: Frequent Contributor
timestamp: November 23, 2018, 09:37:57 pm
content: Omegaone: I have studied your photos, both the earlier set with the leads assembled with individual banana plugs, and the later set with the leads assembled with dual banana plugs, and observe in both setups that the polarity is reversed between the force leads and the sense leads. You need to connect red force and red sense to one end of the paper clip, and black force and black sense to the other end of the paper clip (as shown in the diagram you posted). Carefully trace out your leads and you will see a red lead in a black jack, and a black lead in a red jack on your meter. This will definitely give you a negative reading. Check it out and let us know if correcting the setup helps.

author: omegaone
author_rank: Contributor
timestamp: November 24, 2018, 03:54:47 am
content: Thank you for your correction. Both were incorrect just as you have pointed out. Relieved to see DMM6500 Does not go into minus in 4 wire connection.I am attaching the new picturesI am also attaching Fluke 289 minus reading. I have used the same leads on DMM6500 and read in minus as well. See pictures attached.Have a look and any feedback would be appreciated.

author: alm
author_rank: Super Contributor
timestamp: November 24, 2018, 09:59:38 am
content: Glad you got the problem with the DMM6500 sorted!As for the Fluke 289, on its 50 Ohm range it is specified for 0.15% of reading + 20 counts (or 20 mOhm), so for a 15 mOhm resistance, any reading between -5 mOhm and +35 mOhm is in spec, so expecting any kind of accuracy for resistances < 100 mOhm is very optimistic. Is the value of these leads (from jack to jack) <= 15 mOhm? You would need four write connections to the banana plugs to figure that out.

author: 2N3055
author_rank: Super Contributor
timestamp: November 24, 2018, 10:13:20 am
content: Don't want to sound disrespectful, but you should really read F289 manual.By specification, zero reading is not even specified for instrument.It would make no sense for a 2W measurements because they cannot know what is resistance of cables used for measurement.It is specifically said that only relative measurement is specified. Which is needed to compensate for cable resistance.Calibration manual doesn't even specify what reading will be when you apply short on terminals before zeroing with REL function.Even when you zero it out with REL it is specified as (as per FLUKE calibration manual):46 LoΩ (289) 50.000 Ω range: 0.2 Ω applied = 0.2 Ω low limit:0.180 high limit:0.22047 LoΩ (289) 50.000 Ω range: 50 Ω applied = 50 Ω low limit:49.905 high limit:50.095Which is in line with specs: range 50Ω, resolution 0.001 Ω, Accuracy 0.15 % + 20. There is no specification for non-compensated zero whatsoever.That reading, no matter how annoying might look is not a problem per se.Real question is: if you zero out cables ( Allow reading to stabilize 5-10 seconds) and then measure known good resistor, is it ok then?If it is, than meter is Ok, and negative reading is just a quirk, but has no practical implications.

author: omegaone
author_rank: Contributor
timestamp: November 24, 2018, 02:11:49 pm
content: You are very respectful and thank you for your feedback.Your practical test sounds very reasonable to do and I have done few with this particular meter. ( 50 W 1 Ohm 5 percent tolerance - precision )There is a reason why I have started with 5 % tolerance and I have tested 10 resistors of similar. Max difference I was able to see was just over 1% with relative function in place.( lead resistance taken out of the equation)Now if you are wondering why such high wattage, I also need to test them on a power environment (6 V, 6A , 1 Ohm config) and 36 W of power dissipation. Then I add time 15 min, 45 min so far. Now another way to see the resistance of these resistors are by temperature dissipation.So is essence, if I have identical resistors they should dissipate equal power when put in parallel and share the load.Before one do all these, it is utterly important, to get the proper resistance measurement. (Because it is a function of temperature )Now so far I have used Fluke 289 and DMM6500.Both start off in negative with 2 wire config.Threephase and Commongrounder are both carefully studies the pictures and pointed out the error in my 4 wire connection. Big thank you for both.

author: commongrounder
author_rank: Frequent Contributor
timestamp: November 24, 2018, 02:14:15 pm
content: When attempting to measure very low values of resistance in 2-wire mode, things like the metals used in the banana plug, and temperature start to come into play. The plating on the banana plug is likely dissimilar to the copper in the wiring, and the metal used in the meter jacks. This can induce voltages that throw off the reading. There are others on this forum that know a great deal more than I do about these effects, but suffice to say, you cannot expect accurate very low value resistance measurements in two-wire mode. You should definitely be in the market for some Kelvin clips for your meter! This will allow you to match resistors with confidence.

author: omegaone
author_rank: Contributor
timestamp: November 24, 2018, 02:25:20 pm
content: You can see, my place is not a fancy lab.So I start off by using the tools I do have.As you have pointed out, the plating on banana plugs, wire, etc plays a crucial role in conjunction with temperature. ( thermoelectric effect )I am doing one test at a time, so far, 2 wire test goes into minus territory 4 wire has not!I intend to post every finding here. So any feedback constructive / destructive all are welcome. Important part is we find FACTS and we all get to share the FACTS and be confident in what we do on our day to day operations.Thank you

author: 2N3055
author_rank: Super Contributor
timestamp: November 24, 2018, 03:23:18 pm
content: Omega,If I was helpful, my pleasure.Thanks for explaining what you're doing.Since you need only relative measurement, you can get very accurate results if you connect several of resistors you want to compare in series and apply external current from PSU.Current doesn't need to be exact only sufficiently stable. For your purpose, normal lab PSU will be fine. Connect and let it heat up and stabilise.Then with a voltmeter measure voltage drop across each resistor. With DMM6500 you can get VERY accurate results if current going through resistors is sufficiently stable during measurements.Regards,

author: omegaone
author_rank: Contributor
timestamp: November 24, 2018, 04:03:15 pm
content: Actually what I have explained is only part of it. It is not what I am doing.Anytime a measurement is made, the reference is vital.So partly why I have been doing resistance measurement.I am also involved in micro Amps. I have purchased Dave's product and very amazed at its capability. Prime example be DMM6500 does not have a dedicated Milli amp port. As a result any measurement one try to take encounters a big resistance at the input fuse. ( design flow) At 10 microAmp selection at DMM6500 the measured resistance is 10.12 killo Ohms. What do you think will happen at this resistance if I try to measure few micro amps with DMM6500?In comparison, Fluke 289 internal resistance at micro amp setting with its dedicated milli amp port is only 100.52 Ohms. I get better micro amp measurements with 289.Dave's Tool gives me eyes to see at higher resolution with just any multi meter. ( Before I came to know about Dave, I encountered this issue, but there were not much research in finding solution for this issue anywhere. So when I did come across his microamp project, I was so thrilled to see some engineer is doing the job right!!!)Burden voltage is a real problem and much more significant problem than the industry has realized.I am really thankful for him to produce a product that can be used in this respect.So the method you have suggested does work but not in my case.I will be back to resistance measurement inaccuracies again in a circle.

author: omegaone
author_rank: Contributor
timestamp: November 26, 2018, 12:21:07 pm
content: Resistance Measurement using Shunt Resistor 10 A 75mV1) 2 Wire at shunt output = (Negative) 1000 micro Ohms2) 2 Wire at shunt input = (Positive) 80 micro Ohms3) 2 Wire at shunt output+input = (Positive) 7548 micro OhmsPlease try and point out any errors you may see on my connections. ( Me being careful not enough, need few sets of eyes) I have also attached additional temperature reference from FlirC3.I have decided to add a poll. Hope members can suggest and participate and offer ideas.Note: Lead ResistanceLead resistance Set #1 = 4110 micro Ohms (Negative)Lead resistance set #2 = 8640 micro Ohms (Negative)Samuel

author: threephase
author_rank: Regular Contributor
timestamp: November 26, 2018, 01:45:01 pm
content: Looking at the third picture of the 4 wire measurement, the value to me is 7.548 milli-ohms not micro-ohms.You have a 10A 75mV shunt, so its nominal resistance will be 7.5 milli-ohms. The manufacturer of the shunt will usually give the nominal resistance value of the shunt at a rated temperature and a correction factor for other temperatures.To evaluate if the shunt is accurate, the resistance value obtained is corrected using the temperature at the time of the reading and the correction factor. Once the calculated resistance is known this can be compared to the manufacturers data to see if it is within specification.At a guess your temperature reading is slightly above 20 degrees C and so is the resistance reading. A lot of manufacturers will specify their shunts at 20 degrees C, so I would say your 4 wire reading is pretty much spot on as the shunt is likely to have a positive temperature coefficient.As I and others have told you, attempting low resistance readings (probably less than 1 ohm) using a non-compensated 2 wire method will always lead to inaccurate results. So I won't comment on your other test readings.Kind regards.

author: macboy
author_rank: Super Contributor
timestamp: November 26, 2018, 05:41:12 pm
content: Samuel,You must understand that a small negative resistance reading on 2 wire measurement is actually completely normal. Upon calibration of the instrument, a "zero" will have been established, but this is dependent on the finite resistance of the shorting plug used, the metals in the shorting plug, temperature of the calibration lab, and other factors. Every difference between that setup and yours will result in measurement error. Even a fraction of a microvolt of difference shows up as several counts on the measured resistance.I see "AZERO" on the display, but not "OCMP" so you are ignoring one of the most important settings of the instrument when performing low resistance measurements (edit: it appears that offset compensation OCMP is only available for 4-wire ohms on this instrument). There is also no indication of a REL being used. The 2-wire method is never used for low resistance measurements without zeroing the reading first. If you want to actually measure the resistance of your test leads with 2-wire connections, then you should short the inputs directly at the meter, zero the reading with REL, then substitute the test leads for the short. You are either trolling us, or you are ignorant of how to make such low level measurements, where multiple error sources all conspire to ruin all but the most skilled attempts at a measurement. I highly suggest that you do some reading:https://www.tek.com/document/handbook/low-level-measurements-handbook

author: omegaone
author_rank: Contributor
timestamp: November 27, 2018, 01:18:31 am
content: Thanks for the correction. 7.58 milli ohms.The shunt did not come with a data sheet or temperature correction chart. I have one ordered and coming soon. I will repeat the test again in similar manner with appropriate charts.I was told fluke engineers are still investigating the 289 data I sent them. Once they respond I will have an answer that is provided by the manufacturer of the instrument.When it comes to reference or calibration one can make any meter read a number based on the reference it was calibrated at. I am not really concerned about that at all.My concern is to do with the flow of the charge across a medium. Since temperature of the medium plays a crucial role in charge carriers, I am doing these exercises to validate certain flow. Mostly repeating the experiments Edwin Hall has performed in the 1800. It is lot of work, many false positives but I believe most of us carry on with what was left to us by the previous generation, no questions asked - mostly.Regards,SamuelSo thank you for your feed back.

author: omegaone
author_rank: Contributor
timestamp: November 27, 2018, 01:34:48 am
content: "You must understand that a small negative resistance reading on 2 wire measurement is actually completely normal'I do and then, I don't ! Here is why:Even small negative resistance will have dire consequences on ohms law. So we must attribute it to measurement errors or instrument errors. In other words, it should be stated as such. All we have to do is label it as such.Let me explain in another way: If you can carry a reference weight that was calibrated to read 1 KG from polar region and happen to take it near the equator it will no longer be in calibrated state because of the distance from the center of the earth is shorter at the poles and longer at the equator due to earth's bulging. Of course it is a minute amount and not really matter in most cases. But the principle stands.About your points on relative function or zeroing:It be good to do those if you are using leads to take measurements. In my case , I am actually trying to validate the leads. Usage of shunt is only an example exercise, sorry I did not make that clear to you.

author: alm
author_rank: Super Contributor
timestamp: November 27, 2018, 07:12:16 pm
content: The resistance may have voltage sources in series (Seebeck effect), and the voltage across the resistance is subsequently sent through a bunch of active components like op-amps that may each add some degree of offset voltage, negative or positive. So I see no fundamental reading why a negative voltage reading on a DMM over a circuit with a constant current applied would violate the laws of physics.About "it should be stated as such", the documented uncertainty includes an offset error of +/- 20 mOhm on the 50 Ohm range even after zeroing the reading. That covers negative values down to -20 mOhm (for a perfect short) as instrument errors.

author: omegaone
author_rank: Contributor
timestamp: November 28, 2018, 05:23:54 am
content: Thank you. Agree on most points.Here is it as simple as I like to put it.What is resistance ? or how do we even know about resistance ?We do need to induce a flow to measure resistance and resistance has no meaning otherwise.In QM ( Quantum Mechanics) there is a concept called Interference free measurement. In other words, to measure something without having to influence it or affect it even in the smallest degree possible. Well we all know, this is impossible. But QM is well defined science today and to learn QM is to be able to grasp a concept and try to understand it with a sixth sense! Even then it is not normal at times.Super conducting is a quality that is beyond Ohms Law. I have great respect for Georg Simon Ohms and he faced his own trouble in his day as his own university did not want to believe in him. So he had to leave and apply his findings in another university.So this is really not about electrons, but life and how we perceive it.We may always do the things we are accustomed to or we may ask questions...When we do and if we are persistent enough, we do get answers. Patience is the key here.By the way, I am not a fan of amplifiers only because I prefer raw signals or the source. Much easier to deal with and simple.FYI: Fluke USA has offered to test the 289 and calibrate it for me free of charge after analyzing the data by their engineers.