Strain gage question

Can you get accurate, relative or repeatable readings, from a strain gage with a inexpensive digital volt meter?

I would like to build an electronic scale using a strain gage, but I could care less about the units on the display. I just want to be able to accurately measure say, 400 pounds of product, again and again for a batching operation.

Could I load the scale with a 400 pound weight, observe the reading on the volt meter, remove the 400 pound weight, and load the scale with product until the same meter reading is observed?

Would battery life cause the need for frequent recalibration? If so, would a low buck 9v power supply eliminate this?

Reply to
Maxwell
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Self contradicting sentence as you don't include the permissible errors of readings.

Every part of the system has effect on readings so all of them have to be accurate and stable during the "life time" cycle. Or include a calibrating known deflection of reading (on strain gauge bridge a stable resistor in parralel with one leg of the bridge) to give you the starting point.

Been there, done it!

HTH

Stanislaw

Reply to
Stanislaw Flatto

I should have mentioned that, my current method is probably no more than 5% accurate. So anything less than that would be a plus.

Well I have a good bit, but the fine points are beyond me. I understand strain gages change very little in resistance throughout there usable range, and usually require bridges and very high quality power supplies for ultimate use, but then they yeild results as accurate as .03% in many cases on a 5000 lb load cell. That's why I'm posing the question here, and framing it as I have. One percent accuracy should be more than adaquate for my needs. So will most ohm meters costing less than $100 have the ability to measure say 600 steps in the useful range of a strain gage?

Reply to
Maxwell

Permissiable error of one percent would be very nice. As for calabration, we can just load the scale with a know quantity most any time. That's what we do now. But now we are using a 1.125 dia, oil filled cylinder with a 1000 lb. pressure gage. But I don't think we are getting closer than 5%, and it seems especially suseptable to temperature change.

Reply to
Maxwell

A typical full-bridge load cell will give 5 millivolts of output per volt of excitation at full load. So 9 volts of excitation gives 45 millivolts of signal. Some DVMs have enough sensitivity to be useful with such signals.

The output will droop as the battery dies, so a regulated power supply would be a better idea.

Buy a load cell that's rated at or just a bit over your expected load.

You can buy a complete load cell excitation/display box fairly cheap; they're a commodity.

You will generally have to adjust for zero offset ("tare") and full-scale (using a known test weight) calibration, as load cells aren't always very accurate. They usually are extremely linear and stable.

John

Reply to
John Larkin

Now you _are_ talking sense!

5% is NOT in my vocabulary as measuring range. In my lifetime in testing anything greater than 1% was a failure static and/or dynamic. So now the temperature creeped in, next will come something else, so do yourself a favor, do it once but do it as perfect as possible to eliminate insecurities, it will pay in the long run. Are you measuring now by reading pressure on circular gauge or having pressure transducer and getting the readings from that? And just a stupid querry, how time consuming is this activity, if it is daily routine for hours it costs in people's time plus errors in results. As you carry the calibrating quantity initially why not to use it on a lever as counterweight and watch for microswitch opening as indication of equality?

Just talking

Stanislaw

Reply to
Stanislaw Flatto

Isn't a load cell and excitation/display box about $700? If not, how cheap and where.

Reply to
Maxwell

--
The viability of the whole thing depends on the granularity of the
measurement and what kind of repeatability/stability you\'re looking
for.

I suggest you Google "Strain gage tutorial" to find out what you\'re
up against.
Reply to
John Fields

Measuring now by mechanical gage, but suspect 5% errro is primarily caused by friction in the cylinder.

Measuring operation is done at least 30 times an 8 hour day, and increasing.

Space constraints eliminate the possibility of a counter balance.

Reply to
Maxwell

Omega has indicator boxes for around $250, and I'd expect there is cheaper stuff around.

If you want really cheap, try ebay.

Have you heard of google?

John

Reply to
John Larkin

Don't you _?know?_. So what improvement you want to apply?

As mentioned before, do it once and do it properly. If the hydraulic system has some faults replace it with load cell, most reasonable quality load cells are self temperature compensated (one of your complains), many include precise calibrating resistor inside to give indication of part scale reading of the full range. So a three point mechanical suport with one point beying a load cell gives you a weghing possibility. One point attachment can be used by fully hanging the load on a load cell. Not electronics, mechanics;^)

Have fun

Stanislaw.

Reply to
Stanislaw Flatto

When given the possible 5% error you can either gain or cheat one and a half units per day ( maximum taken as sample), so make some calculations how much it will cost you over the continuation of this activity, against some investment in reducing the errors.

Have fun

Stanislaw.

Reply to
Stanislaw Flatto

Thanks for you help.

Reply to
Maxwell

If you'll be satisfied with 1% accuracy use a mechanical balance.

Bye. Jasen

Reply to
jasen

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