# Strain gage question

Discussion in 'Electronic Basics' started by Maxwell, Jan 22, 2007.

1. ### MaxwellGuest

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?

2. ### Stanislaw FlattoGuest

Self contradicting sentence as you don't include the permissible errors
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

3. ### MaxwellGuest

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?

4. ### MaxwellGuest

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.

5. ### John LarkinGuest

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.

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

6. ### Stanislaw FlattoGuest

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

7. ### MaxwellGuest

If not, how cheap and where.

8. ### MaxwellGuest

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.

9. ### John FieldsGuest

---
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.

10. ### John LarkinGuest

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

If you want really cheap, try ebay.

John

11. ### Stanislaw FlattoGuest

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

Have fun

Stanislaw.

12. ### Stanislaw FlattoGuest

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.

13. ### MaxwellGuest

Thanks for you help.

14. ### jasenGuest

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

Bye.
Jasen