Strain Gage resistance
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Posts: 46,084
Im trying to get my bs2 to read a 350 ohm strain gage glued to a flexible flat
bar. My problem is that when I ohm the gage, I read 348-349 ohms no matter how
much the gage flexs. Am I doing something wrong or is my meter not capable of
reading such a small change in resistance. Sorry I have never delt with strain
gages and any advice would be greatly appreciated.
Thanks,
Jim Gorbet
Do You Yahoo!?
LAUNCH - Your Yahoo! Music Experience
[noparse][[/noparse]Non-text portions of this message have been removed]
bar. My problem is that when I ohm the gage, I read 348-349 ohms no matter how
much the gage flexs. Am I doing something wrong or is my meter not capable of
reading such a small change in resistance. Sorry I have never delt with strain
gages and any advice would be greatly appreciated.
Thanks,
Jim Gorbet
Do You Yahoo!?
LAUNCH - Your Yahoo! Music Experience
[noparse][[/noparse]Non-text portions of this message have been removed]
Comments
into a whetstone bridge type circuit. This allows you to detect a much
smaller change in resistance than a meter will accurately show.
http://ceaspub.eas.asu.edu/imtl/HTML/Manuals/DemoCart_intro.html
Scroll down about halfway to see an example.
Original Message
> Im trying to get my bs2 to read a 350 ohm strain gage glued to a flexible
flat bar. My problem is that when I ohm the gage, I read 348-349 ohms no
matter how much the gage flexs. Am I doing something wrong or is my meter
not capable of reading such a small change in resistance. Sorry I have never
delt with strain gages and any advice would be greatly appreciated.
>Im trying to get my bs2 to read a 350 ohm strain gage glued to a flexible
>flat bar. My problem is that when I ohm the gage, I read 348-349 ohms no
>matter how much the gage flexs. Am I doing something wrong or is my meter
>not capable of reading such a small change in resistance. Sorry I have
>never delt with strain gages and any advice would be greatly appreciated.
Several possibilities.
1. You may not have the strain gage bonded to the metal bar in the correct
orientation.
2. You may not have the gage bonded with the proper material so it may not
follow the changes in shape of the bar.
3. Most strain gages I've ever used are set up as at least one element in
a whetstone bridge, driven with a constant current, and then the voltage
generated by imbalance of the bridge is read - amplified before reading if
necessary. In this configuration most strain gages put out values in the
millivolts, before amplification. This means the change in resistance is
quite small.
You probably need to look at the specs for your gage and see how much
resistance change you can expect and then look at the resolution the Stamp
is capable of providing in whatever undefined configuration you're
using. I think you'll probably find you need to set up as in 3. above.
Also, assuming you can visualize a bridge configuration, look at your gage
with a magnifier and determine if the orientation is correct to produce an
unbalanced bridge that will give a maximum reading.
Jim H
I read with interest your post from the instrument company about strain
gauges and bridge circuits. I was looking for information about the
degree of change of resistance. Are we talking ohms here, or milli ohms?
Say a load cell is to measure 100-150 lb. Any ideas on the amount of
change I should expect to see? Also any ideas on the realm in which a
Strain Gage weight measure bar is stressed? Is the compression measured
with a strain gauge as the bar flexes end to end or is it as the bar is
flexed -- side to side?
thanks for your help,
Leroy
Jim Higgins wrote:
>
> At 21:17 05/12/02, Jim Gorbet wrote:
>
> >Im trying to get my bs2 to read a 350 ohm strain gage glued to a flexible
> >flat bar. My problem is that when I ohm the gage, I read 348-349 ohms no
> >matter how much the gage flexs. Am I doing something wrong or is my meter
> >not capable of reading such a small change in resistance. Sorry I have
> >never delt with strain gages and any advice would be greatly appreciated.
>
> Several possibilities.
>
> 1. You may not have the strain gage bonded to the metal bar in the correct
> orientation.
>
> 2. You may not have the gage bonded with the proper material so it may not
> follow the changes in shape of the bar.
>
> 3. Most strain gages I've ever used are set up as at least one element in
> a whetstone bridge, driven with a constant current, and then the voltage
> generated by imbalance of the bridge is read - amplified before reading if
> necessary. In this configuration most strain gages put out values in the
> millivolts, before amplification. This means the change in resistance is
> quite small.
>
> You probably need to look at the specs for your gage and see how much
> resistance change you can expect and then look at the resolution the Stamp
> is capable of providing in whatever undefined configuration you're
> using. I think you'll probably find you need to set up as in 3. above.
>
> Also, assuming you can visualize a bridge configuration, look at your gage
> with a magnifier and determine if the orientation is correct to produce an
> unbalanced bridge that will give a maximum reading.
>
> Jim H
>
> To UNSUBSCRIBE, just send mail to:
> basicstamps-unsubscribe@yahoogroups.com
> from the same email address that you subscribed. Text in the Subject and Body
of the message will be ignored.
>
>
> Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/
in words:
delta R over R = k times Epsilon
In english:
the change in resistance divided by the nominal resistance is equal to the
gauge factor of the gage times the strain experienced by the gauge.
So, for a 350 ohm gauge, probably with a gauge factor of 2 (that is common)
on a bar, depending on the thickness of the bar, you might get 2000 micro
strain, so you could expect a change in resistance of:
kER
= 2 * .002 * 350
=1.4 ohms
that's why you don't see it on your meter.
If you want to discuss it further, email me off list - i'd be happy to help
you out.
Regards,
Kyle
R. Kyle Schmidt, P. Eng.
Section Head, Control Systems and Instrumentation
Messier-Dowty Inc. "The Landing Gear Company"
kyle.schmidt@m...
>
Original Message
> From: Leroy Hall [noparse]/noparse]SMTP:[url=http://forums.parallaxinc.com/group/basicstamps/post?postID=_fAK4eM8qk38vl3HzQ33XHMtvckdQLcGt2NU9Ry9Zs6zkTiKSYHVbHBLh42tPWHVuoITb4JCPDqX]leroy@f...[/url
> Sent: Monday, May 13, 2002 12:06 AM
> To: basicstamps@yahoogroups.com; HigginsJ@s...
> Subject: Re: [noparse][[/noparse]basicstamps] Strain Gage resistance
>
> Daer Jim,
>
> I read with interest your post from the instrument company about strain
> gauges and bridge circuits. I was looking for information about the
> degree of change of resistance. Are we talking ohms here, or milli ohms?
> Say a load cell is to measure 100-150 lb. Any ideas on the amount of
> change I should expect to see? Also any ideas on the realm in which a
> Strain Gage weight measure bar is stressed? Is the compression measured
> with a strain gauge as the bar flexes end to end or is it as the bar is
> flexed -- side to side?
>
> thanks for your help,
>
> Leroy
>
> Jim Higgins wrote:
> >
> > At 21:17 05/12/02, Jim Gorbet wrote:
> >
> > >Im trying to get my bs2 to read a 350 ohm strain gage glued to a
> flexible
> > >flat bar. My problem is that when I ohm the gage, I read 348-349 ohms
> no
> > >matter how much the gage flexs. Am I doing something wrong or is my
> meter
> > >not capable of reading such a small change in resistance. Sorry I have
> > >never delt with strain gages and any advice would be greatly
> appreciated.
> >
> > Several possibilities.
> >
> > 1. You may not have the strain gage bonded to the metal bar in the
> correct
> > orientation.
> >
> > 2. You may not have the gage bonded with the proper material so it may
> not
> > follow the changes in shape of the bar.
> >
> > 3. Most strain gages I've ever used are set up as at least one element
> in
> > a whetstone bridge, driven with a constant current, and then the voltage
> > generated by imbalance of the bridge is read - amplified before reading
> if
> > necessary. In this configuration most strain gages put out values in
> the
> > millivolts, before amplification. This means the change in resistance
> is
> > quite small.
> >
> > You probably need to look at the specs for your gage and see how much
> > resistance change you can expect and then look at the resolution the
> Stamp
> > is capable of providing in whatever undefined configuration you're
> > using. I think you'll probably find you need to set up as in 3. above.
> >
> > Also, assuming you can visualize a bridge configuration, look at your
> gage
> > with a magnifier and determine if the orientation is correct to produce
> an
> > unbalanced bridge that will give a maximum reading.
> >
> > Jim H
> >
> > To UNSUBSCRIBE, just send mail to:
> > basicstamps-unsubscribe@yahoogroups.com
> > from the same email address that you subscribed. Text in the Subject
> and Body of the message will be ignored.
> >
> >
> > Your use of Yahoo! Groups is subject to
> http://docs.yahoo.com/info/terms/
>
>
> To UNSUBSCRIBE, just send mail to:
> basicstamps-unsubscribe@yahoogroups.com
> from the same email address that you subscribed. Text in the Subject and
> Body of the message will be ignored.
>
>
> Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/
>
>Jim Higgins wrote:
> >
> > At 21:17 05/12/02, Jim Gorbet wrote:
> >
> > >Im trying to get my bs2 to read a 350 ohm strain gage glued to a flexible
> > >flat bar. My problem is that when I ohm the gage, I read 348-349 ohms no
> > >matter how much the gage flexs. Am I doing something wrong or is my meter
> > >not capable of reading such a small change in resistance. Sorry I have
> > >never delt with strain gages and any advice would be greatly appreciated.
> >
> > Several possibilities.
> >
> > 1. You may not have the strain gage bonded to the metal bar in the correct
> > orientation.
> >
> > 2. You may not have the gage bonded with the proper material so it may not
> > follow the changes in shape of the bar.
> >
> > 3. Most strain gages I've ever used are set up as at least one element in
> > a whetstone bridge, driven with a constant current, and then the voltage
> > generated by imbalance of the bridge is read - amplified before reading if
> > necessary. In this configuration most strain gages put out values in the
> > millivolts, before amplification. This means the change in resistance is
> > quite small.
> >
> > You probably need to look at the specs for your gage and see how much
> > resistance change you can expect and then look at the resolution the Stamp
> > is capable of providing in whatever undefined configuration you're
> > using. I think you'll probably find you need to set up as in 3. above.
> >
> > Also, assuming you can visualize a bridge configuration, look at your gage
> > with a magnifier and determine if the orientation is correct to produce an
> > unbalanced bridge that will give a maximum reading.
> >
> > Jim H
>
>Daer Jim,
>I read with interest your post from the instrument company about strain
>gauges and bridge circuits. I was looking for information about the
>degree of change of resistance. Are we talking ohms here, or milli ohms?
>Say a load cell is to measure 100-150 lb. Any ideas on the amount of
>change I should expect to see? Also any ideas on the realm in which a
>Strain Gage weight measure bar is stressed? Is the compression measured
>with a strain gauge as the bar flexes end to end or is it as the bar is
>flexed -- side to side?
On just this last part - the dimension in which the gage measures depends
on the orientation of the strain gage, whether it is aligned with the
length of the bar, where it would measure flexure, or aligned more with the
circumference of the bar, where it might measure torsion - assuming any
were present. For measuring flexure it could be on the side of the bar
that's in tension, or the side that's in compression. How you mount the
bar is very important in assuring a reliable result. If you have the
manual that came with any of the digital scales that are probably in your
school's chem lab, they may cover this point in the section that brags on
why they're the best you can buy. ;-)
I've seen a demonstrator consisting of a strain gage bonded to the center
of a 2" diameter steel bar maybe 18" long. When this bar was held in both
hands and "bent" or "twisted", the indicating meter shot nearly off
scale. As you can imagine, there was a *LOT* of amplification involved for
the purposes of this demo, since you can't twist a 2" steel bar appreciably
by hand. It didn't measure in any specific units; it was just super
amplified to impress potential customers for custom built bonded strain
gages. A buddy of mine used to be in this business.
>thanks for your help,
>Leroy
Leroy,
Once I get past a general overview I have to defer to the experts. My
approach is to use products from well known manufacturers so I can
generally ignore all the details such as how the gage is bonded, what
orientation, etc. I just mount it as directed. (And that can be a trick
in itself because you want to avoid introducing off-axis stresses due to
the mounting system.)
There is more general overview type information than you can shake a stick
at at:
http://www.omega.com/techref/
particularly at
http://www.omega.com/techref/strain-gage.html
and more specific information about assorted products elsewhere on the
Omega site.
More here also:
http://www.daytronic.com/products/trans/t-load.htm
http://www.efunda.com/designstandards/sensors/strain_gages/
If you absorb as much as 5% of the theoretical stuff mentioned here you'll
leave me in the dust. It's been 30 years since I've had to use
differential equations and I don't plan to start again now. ;-)
From memory - a typical load cell output might be 2 - 3 mv per volt of
excitation, though some give as much as 15 mv/V. From this you can see the
resistance change in a typical 350 ohm load cell is rather small - smaller
than you're going to reliably resolve by measuring resistance with a Stamp
into say several hundred or a thousand parts for high accuracy over the
range of the gage.
Something I didn't give as much importance earlier as I should have - the
original Jim mentioned his strain gage was "glued" to the bar. If glue
means something like model airplane cement, it isn't going to work
well. You need to use epoxy, special epoxy for best results (and I don't
know the name of it or specs for it). You don't want to get any on the
upper surface of the gage, and you want as thin a film as possible between
the gage and the surface to be strained. In actual practice the epoxy
would be applied very carefully then the gage would be clamped in place
under a fairly high pressure that does not place the surface under lateral
strain. I've seen the manufacturing process that bonds strain gages to
assorted surfaces to make a strain transducer and it is rather exacting if
you want a final result that is sensitive and linear.
One last thing about playing with strain gages. Whatever you bond them to
must not be flexed so hard that the strain exceeds the elastic limit of the
material the gage is bonded to. Otherwise it will deform
permanently. This shifts the zero point of the gage and plays hell with
linearity of measurements - not to mention the whole system eventually will
break or bend catastrophically.
Jim H
bridge (12vdc is fine) and Then amplify the output x1000. I have jumped
over many issues here. The excitation voltage matters, the orientation of
the gages matters, the signal conditioning matters. However, you ought to be
able to read voltage swings as you twist bar.
richard
Original Message
From: "Jim Higgins" <HigginsJ@s...>
To: "Basic Stamps List" <basicstamps@yahoogroups.com>
Cc: "Leroy Hall" <leroy@f...>
Sent: Monday, May 13, 2002 1:23 PM
Subject: Re: [noparse][[/noparse]basicstamps] Strain Gage resistance
> At 00:05 05/13/02, Leroy Hall wrote:
>
> >Jim Higgins wrote:
> > >
> > > At 21:17 05/12/02, Jim Gorbet wrote:
> > >
> > > >Im trying to get my bs2 to read a 350 ohm strain gage glued to a
flexible
> > > >flat bar. My problem is that when I ohm the gage, I read 348-349 ohms
no
> > > >matter how much the gage flexs. Am I doing something wrong or is my
meter
> > > >not capable of reading such a small change in resistance. Sorry I
have
> > > >never delt with strain gages and any advice would be greatly
appreciated.
> > >
> > > Several possibilities.
> > >
> > > 1. You may not have the strain gage bonded to the metal bar in the
correct
> > > orientation.
> > >
> > > 2. You may not have the gage bonded with the proper material so it
may not
> > > follow the changes in shape of the bar.
> > >
> > > 3. Most strain gages I've ever used are set up as at least one
element in
> > > a whetstone bridge, driven with a constant current, and then the
voltage
> > > generated by imbalance of the bridge is read - amplified before
reading if
> > > necessary. In this configuration most strain gages put out values in
the
> > > millivolts, before amplification. This means the change in resistance
is
> > > quite small.
> > >
> > > You probably need to look at the specs for your gage and see how much
> > > resistance change you can expect and then look at the resolution the
Stamp
> > > is capable of providing in whatever undefined configuration you're
> > > using. I think you'll probably find you need to set up as in 3.
above.
> > >
> > > Also, assuming you can visualize a bridge configuration, look at your
gage
> > > with a magnifier and determine if the orientation is correct to
produce an
> > > unbalanced bridge that will give a maximum reading.
> > >
> > > Jim H
> >
> >Daer Jim,
> >I read with interest your post from the instrument company about strain
> >gauges and bridge circuits. I was looking for information about the
> >degree of change of resistance. Are we talking ohms here, or milli ohms?
> >Say a load cell is to measure 100-150 lb. Any ideas on the amount of
> >change I should expect to see? Also any ideas on the realm in which a
> >Strain Gage weight measure bar is stressed? Is the compression measured
> >with a strain gauge as the bar flexes end to end or is it as the bar is
> >flexed -- side to side?
>
> On just this last part - the dimension in which the gage measures depends
> on the orientation of the strain gage, whether it is aligned with the
> length of the bar, where it would measure flexure, or aligned more with
the
> circumference of the bar, where it might measure torsion - assuming any
> were present. For measuring flexure it could be on the side of the bar
> that's in tension, or the side that's in compression. How you mount the
> bar is very important in assuring a reliable result. If you have the
> manual that came with any of the digital scales that are probably in your
> school's chem lab, they may cover this point in the section that brags on
> why they're the best you can buy. ;-)
>
> I've seen a demonstrator consisting of a strain gage bonded to the center
> of a 2" diameter steel bar maybe 18" long. When this bar was held in both
> hands and "bent" or "twisted", the indicating meter shot nearly off
> scale. As you can imagine, there was a *LOT* of amplification involved
for
> the purposes of this demo, since you can't twist a 2" steel bar
appreciably
> by hand. It didn't measure in any specific units; it was just super
> amplified to impress potential customers for custom built bonded strain
> gages. A buddy of mine used to be in this business.
>
> >thanks for your help,
> >Leroy
>
> Leroy,
>
> Once I get past a general overview I have to defer to the experts. My
> approach is to use products from well known manufacturers so I can
> generally ignore all the details such as how the gage is bonded, what
> orientation, etc. I just mount it as directed. (And that can be a trick
> in itself because you want to avoid introducing off-axis stresses due to
> the mounting system.)
>
> There is more general overview type information than you can shake a stick
> at at:
> http://www.omega.com/techref/
> particularly at
> http://www.omega.com/techref/strain-gage.html
> and more specific information about assorted products elsewhere on the
> Omega site.
>
> More here also:
> http://www.daytronic.com/products/trans/t-load.htm
> http://www.efunda.com/designstandards/sensors/strain_gages/
>
> If you absorb as much as 5% of the theoretical stuff mentioned here you'll
> leave me in the dust. It's been 30 years since I've had to use
> differential equations and I don't plan to start again now. ;-)
>
> From memory - a typical load cell output might be 2 - 3 mv per volt of
> excitation, though some give as much as 15 mv/V. From this you can see
the
> resistance change in a typical 350 ohm load cell is rather small - smaller
> than you're going to reliably resolve by measuring resistance with a Stamp
> into say several hundred or a thousand parts for high accuracy over the
> range of the gage.
>
> Something I didn't give as much importance earlier as I should have - the
> original Jim mentioned his strain gage was "glued" to the bar. If glue
> means something like model airplane cement, it isn't going to work
> well. You need to use epoxy, special epoxy for best results (and I don't
> know the name of it or specs for it). You don't want to get any on the
> upper surface of the gage, and you want as thin a film as possible between
> the gage and the surface to be strained. In actual practice the epoxy
> would be applied very carefully then the gage would be clamped in place
> under a fairly high pressure that does not place the surface under lateral
> strain. I've seen the manufacturing process that bonds strain gages to
> assorted surfaces to make a strain transducer and it is rather exacting if
> you want a final result that is sensitive and linear.
>
> One last thing about playing with strain gages. Whatever you bond them to
> must not be flexed so hard that the strain exceeds the elastic limit of
the
> material the gage is bonded to. Otherwise it will deform
> permanently. This shifts the zero point of the gage and plays hell with
> linearity of measurements - not to mention the whole system eventually
will
> break or bend catastrophically.
>
>
> Jim H
>
>
> To UNSUBSCRIBE, just send mail to:
> basicstamps-unsubscribe@yahoogroups.com
> from the same email address that you subscribed. Text in the Subject and
Body of the message will be ignored.
>
>
> Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/
>
>