Wheatstone Bridge - Where can one buy it? 2

[eemotor]

Our Wheatstone Bridge that has been in use for a good 40 - 50 years has finally come to a point where it needs to be replaced by a new unit. I searched several places that might possibly sell one but they didn't have any. Can anyone please recommend a place that sells them.

eemotor

 

[VEBill]

Clicky ici:

http://www.google.com/froogle?q=Wheatstone+Bridge

Plus, there's always e-bay.

 

[nbucska]

What are you using it for and what specs does it have
to meeet? The WB is accurate but tedious and time consuming to use -- a modern device may be a more convenient replacement


<nbucska@pcperipherals DOT com> subj: eng-tips
read FAQ240-1032

 

[OperaHouse]

I have a very nice old General Radio bridge with the black front, wood base, and the oscilator/amp. Want to make any offers?

 

[eemotor]

We'll be using the Wheatstone bridge to measure resistance of fractional HP motors. Kelvin bridge would not get the job done as the resistance is too high. I've done some searching around and found some instrumentation that should suit our needs at

http://www.hotektech.com/disbridges.htm

.

 

[ScottyUK]

"Kelvin bridge would not get the job done as the resistance is too high."

Give us a clue: what is the range of resistance that you want to measure, and what accuracy do you require? There are a few applications where the bridge is still a reasonable option, but in honesty this doesn't sound like one.

The likes of Agilent, Fluke and Keithley Instruments all manufacture instruments which go from mid-range bench meters to top-of-the-line reference instruments for the calibration labs. Call their tech support and discuss your application.



----------------------------------

If we learn from our mistakes,
I'm getting a great education!

 

[TurboXS]

I would have to say (like the others have also said) that it sounds like you are looking for a Wheatstone bridge because that's the way it was done in the past.

Knowing your test requirements would help, however there is a wide range of resistance measurement testers such as;

This Agilent Milliohmeter

http://www.home.agilent.com/USeng/nav/-11277.536880954/pd.html

which should cover your resistance range, is accurate but expensive.

HOWEVER by defining your test requirements you could find that a $20 digital multimeter will do the job!

Tell us;

Typical resistance value
Accuracy of resistance measurement
DC or AC measurement (if AC what frequency)
Do you want the measurement at a set current or voltage
Do you also want to measure capacitance or inductance?
Do you want to do an insulation resistance test?

regards
Pete
(TurboXS)

 

[eemotor]

The expected resistance value is going to be approximately 300 ohms. One additional question that I ran into is the accuracy of the apparatus that I was looking into buying. It was .1 to .2% which seems great to me and will be sufficient for our measurements but I don't know how it rates compared to other equipment that's out there. The price that we'll be paying for this Wheatstone bridge is roughly $1700, but price is not much of an issue here, just need an accurate/durable piece of equipment.

 

[VEBill]

A $20 DVOM from your local hardware store will almost do that. Or splurge on a $120 name-brand Fluke DVOM.

 

[TurboXS]

While 0.1 to 0.2% accuracy is great do you REALLY need that level of accuracy?

Accuracy costs money.

10% accuracy can be a very cheap digital multimeter (<$20)
1% accuracy can be a average to good digital multimeter (tens to hundreds of dollars)
0.1% accuracy needs specialised measuring instrument (thousands of dollars)

You should also consider that specialised measuring equipment should be treated carefully.

You also need to consider the cost of calibration. You can find that the yearly cost of calibrating a high accuracy unit could buy you a brand new moderate accuracy unit each year.

regards
Pete
(TurboXS)

 

[ScottyUK]

I seriously doubt you need this performance level for a motor, but to give you some idea of cost, my 6.5 digit Keithley Model 2000 bench meter cost £700. It will measure 100uOhm to 100MOhm to better than 0.01% accuracy. Agilent offer a model which is an almost direct equivalent in performance.

You should ideally look for a four-terminal (or Kelvin) resistance measurement technique as this will avoid the test leads introducing inaccuracy. This would rule out most hand-helds, but it is common even on entry-level bench instruments.

The other thing to consider is that hand-helds are portable and tend to grow legs. A bench instrument is easier to mount in a rack, and not so attractive to the 'borrowers'.



----------------------------------

If we learn from our mistakes,
I'm getting a great education!

 

[VEBill]

I wonder if his 40 - 50 year old Wheatstone Bridge would have been accurate to .1 or .2%... Probably not.

Besides, it's 'just' a motor. I doubt that he needs even 1%.

 

[eemotor]

ScottyUK, thanks for the suggestion about the digital bench meter. I just realized that we have a Fluke 8842A bench multimeter with a 2 and 4 wire input for resistance measurement. It's accuracy is .008% @ 1 year after calibration with just the 2 wires on 2kohm range. Looks like we might not even have to purchase a Wheatstone bridge afterall, but it's always good to know where one might buy it from just in case we do end up buying it in the future. Thanks for the input.