Guildline 9975 Bridge - Fix it or Scrap It?

[Hawaii596]

Greetings Earthlings,

In a recent post, I mentioned that I have a bunch of standard resistors (Fluke 742A's, Tinsley oil filled (Very Nice!!), Guildline 9330 series, an old Thomas 1 Ohm (L&N 4210 - last cal'd in 1997), etc.

Someone asked what I used to cal them.  Well, I just recently started at this position, and trying to put together the electrical standards lab.  One item I pulled out of mothballs was an old Guildline 9975 bridge.  I hooked it up (used a Tinsley 5685 1 Ohm as RS, and a 742A 1 Ohm as RX).  I started tinkering to see what kind of operational condition its in (test current of 3 mA and 10 mA).  I was in process of basically evaluating its operational condition - working with the galvanometer and dials to try and dial in a balance.

After an hour or so, it had been smelling a little (not too bad).  All of a sudden it shut down and had blown a fuse.  I opened it up and it appears one of the small transformers inside near the AC power input (not the main line xfmr) must have have shorted out.  There are two little circuit boards (I'm guessing the current sources for RX and RS).  The transormer on one of them was burning hot, and my line fuse blew pretty hard.

So..... just how good are these old Guildline 9975's?  Is it's uncertainty (when working correctly) worth my while tracking down parts and trying to fix it?  Or am I just as well off doing a substitution method with a new Fluke 8508A, and scrapping the Guildline?  Last time it was used was apparently around 1987.

If (although very old) they are worth it, I'll fix it.  If not, I may sell it along with a pile of other old items (labeled "Needs Repair").

[purpmcrider]

The 9975 is/was a very good bridge.  It could probably give you <0. 25 ppm measurement depending on the range and the standard used.  The parts might be hard to find and the galvanometer suspension is fairly sensitive (meaning breakable).  The 9975 also hit its limits at just above 10 kΩ unless you use the range extender (9923 I think).  Even with the extender the ESI 242D was probably superior at above 10 kΩ.

I would probably work with the 8508A and for the 10:1 transfers look at using some ESI SR-1010 boxes with the shorting bars and compensation networks.  Also instead of just using the ratio function of the 8508A, which is very good, look at the Fluke website for a paper titled something like "a Wheatstone Bridge for the Computer Era" by Les Huntley.  For Lower ranges there was a paper done by Neil Faulkner that might prove helpful.

Don't scrap out the L&N 4210 either.  It works great in oil at 25°C and can hold better than 0. 2 ppm per year.  If you have calibration data for the 4210, plug the data into a control chart on Excel (be sure to account for the 1990 volt/ohm change, about -1. 69 ppm) and you'll be surprised how stable it is.

Good Luck

[Hawaii596]

Sounds like (regarding the 9975) you're saying I may spend my time and money more fruitfully using an 8508A?  Also, I do have a number of the SR1010 boxes (although I'm not sure if I have the full set).  I may send you a PM a little later on regarding some of these details.  I was talking with my two senior techs at lunch today about this, and they also reminded me of the ratio function on the 8508A.  I have a stack of other fascinating projects (getting vacuum system running, quality policy writing, and lots of other neat stuff).

Now, coming back to the 4210 (which is sitting in it's handy case on my desk).  I've been comparing specs among my various resistors to familiarize myself of best potential uncertainties.  I'd mostly been looking at ppm/Deg C, and cal'd uncertainty.  We don't do a lot in high end resistance (YET), so we don't have a running oil bath.  So it was an interesting point regarding the "in-bath" numbers on the 4210.  I believe it is likely in very good shape.  I looked at it 4W on the 8508A and got very good (accurate), stable numbers. 

The thing that kind of led me away from the 4210 was its numbers didn't seem as good as the Tinsley and (in some ways) versus the 742A (since I don't have a proper oil bath for resistors.

One of my senior folks was telling me of a lab he used to work in that had a single stand alone bath for the 4210.  He said it looked kind of like a 1 Gallon soup can stainless steel container.  Any thoughts on that?  And/or any thoughts on any of the other things above?

[MetFan6453]

Just my 2 cents. . .

Using your 742A's and SR1010's as standards, you could probably "get by" using the 8508A (uncertainties will suffer).   while the 8508A is a great meter, it is not a resistance bridge and cannot provide you with the level of uncertainty of the 9975.   The ratio function is really nothing more than a math function.   The 8508A still relies on it's own absolute measurement of both resistors.   Also since it does not have the ability to ratio the current, it is really only usable at a 1:1 ratio.

I've used a 9975 for years and know many people who use them as their primary resistance measurement system.   You should still be able to get it repaired.   While the manufacturer no longer supports it, I know a couple companies that have sent their 9975 to Measurements International for repair with good results (www. mintl. com).   

[scottbp]

I've been communicating with Jeff Willey over at Measurements International because we're looking at upgrading our resistance capabilities. He says that a number of technicians there used to work for Guildline, which is why they can fix the 9975 bridges. They will also accept a 9975 as a trade-in for one of their automated bridges, which is what we're looking to do.

We have a Fluke 8508A, and use it in conjunction with a Fluke 5700A and a Dataproof scanner and Dataproof's OhmRef software to set up a "Poor-man's bridge", and use that along with a SR-104, a set of SR-1010/1050s and a collection of L&N oil bath resistors. The "Poor-man's bridge" is more accurate than the old ESI 242D Kelvin-Varley set, but not quite as good as a 9975.

[Hawaii596]

Hey Scott,

Thanks for that input.  Can you give me some contact info for Measurements International?  I've sent two emails to their cal lab generic address and haven't gotten a reply.  I'm actively trying to see if they'll repair and cal mine.

On a different note, I do have an SR104 and everything else you mentioned.  Do you have any numerical uncertainties (at least ballpark) as to the combined uncertainties of the various options.  That is, how much better is the 9975 with SR104 compared to 8508A/SR104/5700A?

I'm leaning toward fixing this thing, developing some business in that area, then once we have some business to justify it, I can pitch for trading it in on a newer bridge.

[purpmcrider]

The Fluke website has the paper "A Poor Man's Resistance Bridge" that Gary Bennet and Tom Marshall put together and that Gary presented.  I believe that some of the uncertainties that were achieved were given in the paper.

If memory serves the 9975 is capable of < 0. 2 ppm transfers doing 10:1 transfers and a bit better at 1:1.  I'll see if I still have an uncertainty budget for the 9975.  The 9975 doesn't work as well above 10 kΩ even wiith the extenders and that's where other techniques need to be employed.  The method used in the Les Huntley paper I mentioned before was used during a NIST 10kΩ MAP and had an uncertainty of 0. 156 ppm at 10 kΩ.  I believe the paper claimed an uncertainty at the 100 MΩ level of about 20 ppm; it was that good.  The 9975 didn't work quite that well at 100 MΩ and required an external source set to 500 to 1000V.  The technique in Les' paper used a couple 10V sources.

All this being said I really liked the 9975 and the method Les described and have lots of experience with both systems.  I was the "skilled operator" refered to in Les' paper. 

Use which ever method supports your needs and what your management will pay for.  If they let you repair the 9975.  Great, use it.  Its a great tool

I would attach Les' paper, but it's 142 KB.

Good luck

[Hawaii596]

Thanks to some inputs from here, I've been in contact with Meas. Intl's folks in Florida (a couple of phone calls), and am in process of coordinating getting them to repair the unit.  We're looking at having them in for a visit early next year.

Let me throw in a separate little detail I slightly drooled about (in this context).

One of my many projects is replacing our entire freeze point cell system (a big one).  Part of that is enumerating (sorry for the big word) the combined uncertainty of our existing system compared with that of the proposed new system (for our customers).  In that process, I saw an interesting application of the Guildline 9975.  It was used by some national labs (and others) in doing very high accuracy RTD (SPRT) measurements of fixed point cell stability.  I thought --- That is a neat idea!

I'll also take a look on Fluke's website to try and find the paper mentioned.

[Hoopty]

You can download the paper here:

A Poor Man's Resistance Bridge

[jwilley127]

I have a 9975 and a few other great used standards for sale along with some other cool accessories etc.

Check it out: www.jswilley.com

[skipper]

Would you happen to know where I could get a copy of the Les Huntley paper employing the technique you mentioned?

Sincerely,

Skipper