Favourite Test Equipment

[squishy36]

That isn't quite true. If you look closely at the image of the Fluke DMM you will see a REL Δ button which is used to zero out the meter. The .1 ohm shown in the image was the lead resistance in the measurement plane. However, pressing the relative button subtracts the lead resistance from the actual reading. So in the case of higher end DMMs it is possible to read low resistances.

Ron's absolute right, of course. However, I was speaking of the typical DMM out there, especially the cheaper ones. This is a good opportunity to present a bit more information.

First, lemme give a good example of what Ron's talking about. My HP bench meter with some leads measures my GR resistance box set to 0.5 ohms resistance as 0.638 ohms. When I short the leads, I get 0.123 ohms. Making the correction, I get the resistance of 0.515 ohms. This is quite close to the 0.5165 ohms I measured using my 1 A current source.

However, low resistance readings with hand-held digital multimeters can have poor accuracy, even when you correct for the lead resistance. For example, my Fluke 83's (a 20 year old meter) accuracy for resistance measurement on the lowest scale is ±(0.4% of reading plus 1 digit). Thus, if I read a resistance of 0.5 ohms (with the lead resistance subtracted out), the accuracy is (essentially) ±0.1 ohms. That's a 20% uncertainty in the reading, so it's something you have to be aware of (and it's due to the digital uncertainty in the last digit). If you were using an Agilent U1252B, which can read to 0.01 ohms, the accuracy spec is ±(0.05% + 10) (this spec also requires you to use the null technique to get rid of lead resistances). To repeat the same example 0.5 ohm measurement, you'd see that the 10 digits part of the spec would result in the same accuracy as the Fluke. Thus, the Agilent meter that looks much better in fact has the same (poor) 20% of reading accuracy for this particular measurement.

If you measure low resistances a lot like I do, it's worth your time to build a current source to make a Kelvin measurement. For example, the CurrentSource.pdf file here gives an example. Another common approach is to use a current meter to set a constant current DC power supply to a particular current, then use it as a constant current source. But building a dedicated current source will save you time. It will also greatly improve the accuracy of your resistance measurements because the accuracy will be based on the DC measurement accuracy of your meter (assuming you calibrate the current source properly). From the above arguments, you can see the accuracy for measuring resistances with a hand-held digital multimeter at an ohm and below will be 10% or substantially larger; measuring the DC voltage drop of the current source will instead let you use the 0.1% or better accuracy of the DC specs of your meter.

There's another issue with resistance measurement to be aware of. When you measure low voltages, you'll often be bedeviled with thermoelectric interference. The same types of voltages can also cause errors when making resistance measurements. They're insidious because, depending on their polarity, they can either add or subtract to the measured resistance. And, if you're not aware of them, they are pure errors in your measurement.

A similar problem can occur when you measure resistances in a circuit. If you've ever measured a negative resistance, then you've experienced what I'm talking about. It's caused by a voltage present in the circuit whose resistance you're trying to measure. Digital multimeters like my HP bench meter (and follow-on Agilent meters) have offset-compensated resistance measurements. Basically, the meter measures the circuit for a voltage, then applies the current source and measures the resistance; the first voltage measurement is used to correct the offset in the second measurement. Naturally, your reading rate halves. Other meters measure the resistance, then interchange the leads and measure again. The measured resistance is then the average of the two readings (this subtracts out the effect of the offset voltage).

This brings up a technique that perhaps not too many folks know. When you're measuring resistances in an unknown circuit, it's wise to flip your leads and measure again. A difference will tell you there's an offset voltage in the circuit. You can average your readings to get the true resistance. This can save you from making a mistake some day. For both this technique and measuring diode drops, I've long wished I had a set of leads that had a small button on one lead to make this lead interchange at the press of the button...

 

[Dean Huster]

That isn't quite true. If you look closely at the image of the Fluke DMM you will see a REL Δ button which is used to zero out the meter. The .1 ohm shown in the image was the lead resistance in the measurement plane. However, pressing the relative button subtracts the lead resistance from the actual reading. So in the case of higher end DMMs it is possible to read low resistances.

And, of course, that isn't quite true, either. The method above somewhat corrects for lead resistance but does not correct for contact resistance of the lead against the resistance to be measured since that will change from measurement to measurement. The 4-terminal Kelvin technique takes care of that. For the "newbie", all this is moot, really, since they're just trying to get a handle on how to use the meter and making "normal" measurements, not measurements at the extremes.

 

[KeepItSimpleStupid]

I think it's called the meter in question doesn;t know how to do leading zero blanking. It costs more money.

Don't look for the number zero for resistance. Pay attention the meter specifications. A simple multimeter cannot easily measure less than an ohm.

A technique that comes in handy sometimes is say trying to estimate the output impeadance of say an amplifier. Measure the output voltage, then put a variable resistor in parallel with the output and adjust it until the voltage is 1/2. Remove the variable resistor and measure it. It will then be the resistive component of the output Z of the amplifier.

It won't work very well for power amps that have a Z around 0.001 ohms or so.

Squishy, these is also a 5-wire ohms technique. I'd have to look up how it works.

 

[kiriakos-gr]

One useful tool for understanding measurements with resistors, its by having around one resistor decade box.
This one is made by me, and I had use high quality parts and the final cost got a bit high.
But you can build a similar with basic quality parts, and you will have also something to remember as long you keep this DIY project,
when you will grow up.
http://hackedgadgets.com/2010/09/04/diy-resistors-decade-box/

 

[Boncuk]

For quick low resistance measurement I usually short the the test leads and note the indicated value which represents DMM inherent errors and resistance of the test leads (including) resistance of the probes.

To minimize probe resistance you might use 1,200 grain sand paper and carefully wipe the probe tips, then dunk them in alcohol and dry them with a paper towel.

After measurement of the DUT subtract the noted "zero" value from the result.

Boncuk

 

[squishy36]

Very nice resistance box, kiriakos-gr! The only catch will be that I would bet those resistors don't have near-zero tempcos...

I first came across the GR resistance boxes in college back in the 60's (we had models going back many decades; many of them were made in wooden boxes), so I've been partial to them even though they tend to be a bit bulky. If you open them up, you see how they were made. GR wound its resistors from Manganin and you can find a picture on the web of one of their lathes used to do the winding. That construction method has stood the test of time (see attached photo).

 

[Reloadron]

And, of course, that isn't quite true, either. The method above somewhat corrects for lead resistance but does not correct for contact resistance of the lead against the resistance to be measured since that will change from measurement to measurement. The 4-terminal Kelvin technique takes care of that. For the "newbie", all this is moot, really, since they're just trying to get a handle on how to use the meter and making "normal" measurements, not measurements at the extremes.

Hi Dean, my experience with kelvin measurements seems to be the measurement is only as good as the actual kelvin clip or lead contact with the surface. Pretty much like a two wire measurement. I strongly agree that for the newbie it is pretty much a moot point. Anyway...........

I took some time and dug into my boxes from days of old (the boxes of really old stuff reminding me of when I was actually young).

Attached are a few images of what I dug out. These are old precision resistors made by L&N. I actually have a set but picked out 4 of them. They are .001Ω, .01Ω, .1Ω, and finally a One Ohm.

Standard resistors like these were submerged in an oil bath of pure mineral oil. The oil was circulated and maintained at a specific temperature. Note the holes in the sides of the resistors. Those holes allowed for oil flow with a goal of temperature uniformity. When we start splitting Ohms into .001Ω temperature is critical. The last image shows how a precision mercury glass thermometer was inserted into each standard resistor when measurements were taken. Today a high accuracy PRT would replace the liquid in glass thermometer.

Additionally resistors like this were useless without their corresponding calibration sheets stating the true resistance. There is no standard .001Ω resistor. It could be .001324Ω and thus such devices as standard capacitors, resistors, thermometers and other such devices were accompanied by calibration certifications stating their true values.

I also have a few old Gen Rad (General Radio) decade boxes buried around here somewhere as well as GR standard caps.

While I have not had a need for the stuff in years it comes in handy every now and then for comparison purposes. So while off topic I thought I would share some really old but cool stuff. Try buying them today, especially mercury thermometers.

Ron

 

[kiriakos-gr]

Very nice resistance box, kiriakos-gr! The only catch will be that I would bet those resistors don't have near-zero tempcos...

I made this box mostly to test DMM and analogue meters, those applications does not stress the resistors,
and so I have the faith that those small DALE can handle the stability with out issues.

 

[Reloadron]

Hi kiriakos-gr

I remember when you first posted that resistance box. That was really a sweet job you did on that. Really, really nice. The stenciling alone reflected workmanship and time.

@Squishy

Try finding and buying just one of those switches used in those boxes today. The switch leaves and landing pads were incredible. When we would clean them the pads got conductive paste laden with pure silver. Really cool stuff.

Ron

 

[squishy36]

These are old precision resistors made by L&N. I actually have a set but picked out 4 of them. They are .001Ω, .01Ω, .1Ω, and finally a One Ohm.

Try buying them today, especially mercury thermometers.

Ron:

I would love to have one of those resistance standards. You can still find L&N and other brands of this style coming up on ebay every so often (they usually go for $75-$150 or so). Alas, I don't have the cash for this, as electronics stuff is just an occasional hobby and SWMBO is pretty tired of the excuses I come up with to buy such things.

30-40 years ago I bought a beautiful little mercury in glass thermometer from Brooklyn Thermometer company. It was in this very nice machined brass case and was quite rugged. It also read in Celsius, which was handy for scientific hobby stuff. Unfortunately, it has disappeared. It's somewhere in this house, but I haven't seen it in a couple of decades or more.

OK, enough -- back to the scheduled topic...

 

[kiriakos-gr]

OK, enough -- back to the scheduled topic...

The scheduled topic is the start of the travel but not the destination.

 

[Dean Huster]

There's no need to get "over-standardized" here. We're hobbyists, not metrologists. Remember that the resistance ranges (and I'll say here "OF ALL") digital multimeters is the least accurate of all the basic functions on the meter, ACV and ACA being second-worst. For my resistance "standard" when I was working at Tektronix where a DM501 was my meter (and resistance was not something a technican had to measure with a high degree of accuracy), I made up a simple switched box with 1% metal film resistors installed, being able to switch among values that represented those nearest the highest values readable on each of the DMM ranges, e.g., 196 ohms, 1.96K ohms, 19.6K, 196K, 1.96M. Then I had our Dallas Service Center use their 18-digit DMM (traceable to NIST) to measure and provide the actual values of these resistors. It made for a plenty-good-enough standard for checking my shop DMM well-beyond its advertized accuracy. I simply had my "standard" rechecked every year or so.

 

[Reloadron]

When were you with Tek Dean?

Ron

 

[Dean Huster]

Ron,

1976-77 at the Dallas Service Center, 1977-1982 at the Oklahoma City Service Center. Couldn't advance past the highest level of electronics technician without moving to Santa Clara, Beaverton or Rockville and moving was impossible because the oil bust kept our house from selling and what idiot can afford Santa Clara on a technician's salary? Anyway, that's when I began teaching electronics (at the Francis Tuttle Vocational-Technical Center in OKC) and did that until 1997 when I moved to SE Missouri. Got a job doing the same thing there in 1997 and then the program was shut down in 2003 when the misguided powers-that-be decided that computer repair was more important than electronics and killed the electronics program. I was also doing adjunct teaching at the community college and that petered out when the economy tanked. So, gotta make the house payments and started a home repair/maintenance business to help make ends meet. Job hunting sucks when you're always "over-qualified" at the age of 54 (back then).

Was that question over-answered?

 

[Reloadron]

Hi Dean

During those years I was pretty involved with the Navy METCAL program out of San Diego and MEC Pomona. I spent some time teaching the repair and use of the old (now really old) Tek 545B with the associated plug in units for it. Finally about 25 years ago I ended up here in Cleveland. I knew you had time with Tek. I remember during those San Diego years every young sailor planned to exit the Navy and go to Beaverton or Palo Alto. Hell, today I just look forward to retirement.

Ron

 

[Dean Huster]

Still off-topic:

During the six years before working for Tektronix, I was a Communications Technician - Maintenance (CTM2) in the Navy. Went through the PMEL program the USAF had at Lowry AFB in Denver and spent my six years repairing and calibrating test equipment. In 1975/76, the Navy (well, at least the Security Group) was just getting in shipments of Tek 465s and 475s and the first thing I did was pull the cover on one to look at this new marvel of engineering. It was the school in Pensacola that was getting the new stuff -- out in the fleet, we were still using the 500-series. That's just not right.

Yeah, Beaverton and Palo Alto were the "meccas", but getting out of the Navy rather than retiringafter 30 years was a mistake. the grass is always greener on the other side of the fence (NOT!). About 10 years ago, a guy I worked with on Okinawa e-mailed me out of the blue. He retired as a CTMCM (E-9) and during his last few years in, was going around the world with a Commander, closing down all the Naval Security Group Activities where we had been stationed as well as the monster Wullenwebber antennas associated with them. The Hanza, Okinawa site where I worked can be seen on page 93 of the June 1997 issue of National Geographic Magazine.

 

[Mikebits]

Hi Dean

During those years I was pretty involved with the Navy METCAL program out of San Diego and MEC Pomona. I spent some time teaching the repair and use of the old (now really old) Tek 545B with the associated plug in units for it. Finally about 25 years ago I ended up here in Cleveland. I knew you had time with Tek. I remember during those San Diego years every young sailor planned to exit the Navy and go to Beaverton or Palo Alto. Hell, today I just look forward to retirement.

Ron

So Ron were you at 32nd Street Navy base? I was stationed there at pier 4. USS New Orleans LPH-11

 

[Reloadron]

Hi Ya Dean & Mike

Cruising down memory lane here. I never caught that the old thread was split. Nice work on the moderators as it is now where it should be I think.

I confess I was a Jar Head. I was in the corps between 68 and 78 with my last 3 years at NAS North Island so was never to 32nd Street unless we count the exchange and commissary. During 75 I was assigned to what was then NAVAIRSYSCOMREPPAC at NAS NORIS. I went through the Air Force PMEL school in Denver in 71 while assigned to MCAS Cherry Point NC (as well as a few other schools). It all began at NAS Millington as it does for most USMC and USN. Anyway, I ended up at NORIS and at that time was the senior enlisted man in the Navy I Level calibration school. Hell, the command was almost all high level GS civilians. Those were some great years and some extensive travel with an audit team. We hit every I level Navy site (including the ships) annually. When I was at NORIS we had the Connie, Ranger and I think one other carrier. I left the corps in 78 and went DoD at the offer of my command. The years in SanDiego had me extensively involved with the Navy METCAL program. When I became a sand crab I went back to school. While at NORIS we trained both NAVAIR and NAVSEA so we had plenty from NAVSTA 32nd street. Brownshoe & Blackshoe. As a Marine I had more Navy than Marine time. Go figure.

It's hard to believe looking back that a few years ago the wife and I attended a 40 year reunion for my old Marine outfit and the Vietnam days.

Yeah Dean those new scopes, Tek 465s and 475s and at the time the Tek 492 spectrum analyzer replacing the old Ailtech 707. I still have a complete YIG from a 707 and some other parts.

Maybe now that nostalgia is over I should make an effort to get the thread on track.

Oh yeah, would you believe I am still involved with the Navy? I support Naval Reactors systems.

Yeah, Beaverton and Palo Alto were the "meccas", but getting out of the Navy rather than retiringafter 30 years was a mistake. the grass is always greener on the other side of the fence (NOT!).

Yeah, if we only had it all to do again. I should have stayed in the Marine Corps but that grass was so green. I also spent time on the rock with visits.

The two best duty stations in the world...The one you just left and the one you are going to.

Ron

 

[cr0sh]

While I have not had a need for the stuff in years it comes in handy every now and then for comparison purposes. So while off topic I thought I would share some really old but cool stuff.

I like these kinds of pics - keep 'em coming, I say!

Try buying them today, especially mercury thermometers.

I know where you can buy a "still in crate, NOS" Ignitron - complete with "corrosive" hazard stickers, mercury hazard stickers, etc. It's not cheap as a surplus item (I posted about it on one of the other forums a while back) - but probably very cheap considering what it is, if you had a need for it...

 

[Reloadron]

Hi Ya Crosh

You have that Mega Surplus place out your way don't you? My wife is going to Phoenix on Thursday for a few weeks. I should give her a shopping list.

Ignitrons are pretty cool for conversation pieces.

Ron