Load Cell Signal

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Shaneyj

Senior Member
Location
Katy, Texas
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Project Engineer
Can anyone enlighten me as to why I can not measure a load cell signal with my Fluke 87, but when the same signal is routed straight to load cell gauge it displays mA reading?
I was measuring in line- directly before load cell signal enters a signal splitter. Red lead on load cell wire, and black lead into signal splitter terminal. No reading with leads swapped.
Thank you
 

iceworm

Curmudgeon still using printed IEEE Color Books
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North of the 65 parallel
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EE (Field - as little design as possible)
Can anyone enlighten me as to why I can not measure a load cell signal with my Fluke 87, but when the same signal is routed straight to load cell gauge it displays mA reading?
I was measuring in line- directly before load cell signal enters a signal splitter. Red lead on load cell wire, and black lead into signal splitter terminal. No reading with leads swapped.
Thank you

Be good to have some context. There are a lot of loadcells out there. Some require excitation. Do you have a model number?
 

GeorgeB

ElectroHydraulics engineer (retired)
Location
Greenville SC
Occupation
Retired
In my very limited experience, a load cell is a 4 resistor bridge and when excited (personally, always with less than 15 vdc) and unloaded, half of the supply occurs across the "output" terminals. Without excitation, there isn't much you can do PRACTICALLY.

Output is commonly in the range of 1 to 3 mV per volt of excitation, so with 10V of excitation, and a 3 mV/V device, (assuming tension/compression) you will see -30 mV in one mode, and +30 mV in the other. These are in the range of a Fluke 87.

A load cell system might have a signal conditioner to give some industrial level output, perhaps +/- 20 mA or +/- 10V. It is certainly possible to have 4-20 mA, but it is necessary there to recognize that 12 mA is unloaded.

As iceworm said, we need a little context.
 

Shaneyj

Senior Member
Location
Katy, Texas
Occupation
Project Engineer
Be good to have some context. There are a lot of loadcells out there. Some require excitation. Do you have a model number?

This load cell comes from Interface and the part number is... 3432 FFN. It reads 70k lbs in compression and -35K lbs in tension.

Another potentially important piece of info; the load cell is running through a barrier.
 

iceworm

Curmudgeon still using printed IEEE Color Books
Location
North of the 65 parallel
Occupation
EE (Field - as little design as possible)
This load cell comes from Interface and the part number is... 3432 FFN. It reads 70k lbs in compression and -35K lbs in tension. ...
Did a google search on "interface force 3432 ffn" - nothing.

... when the same signal is routed straight to load cell gauge it displays mA reading? ...
What is the display? This makes it sound like the readout is displaying 4-20ma.

... Another potentially important piece of info; the load cell is running through a barrier.
As in an "intrinsically safe? barrier? Or, as in "signal conditioner"?

Without knowing what you got - I can't help.
 

Shaneyj

Senior Member
Location
Katy, Texas
Occupation
Project Engineer
Did a google search on "interface force 3432 ffn" - nothing.


What is the display? This makes it sound like the readout is displaying 4-20ma.


As in an "intrinsically safe? barrier? Or, as in "signal conditioner"?

Without knowing what you got - I can't help.
Display is a "contact" brand weight indicator. It reads weight in lbs, but has a setting that will display mA.
I'll get details about barrier in the morning.
I'll see if I can link the data sheet from the load cell.

Sent from my SM-G965U using Tapatalk
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
181101-0914 EDT

Shaneyj:

To troubleshoot a circuit you need to know how the components of the circuit work.

There are many possible ways to build a loadcell. Since your loadcell in made by Interface it is most likely a 4 wire resistive bridge circuit for the transducing mechanism.

This will require an excitation source, and produce an output signal. Can be AC or DC for excitation, usually in the 5 to 20 V range.

In a 4 wire bridge circuit the output signal, of necessity, is floating relative to the excitation, and for loadcells is often 2 mV/V output at rated full scale. Thus, a 1000 # full scale loadcell with 10 V of excitation would produce 20 mV output with a 1000 # load. You need to study and know how unbalanced bridge circuits work.

A loadcell may have built in signal conditioning, then the signals can be quite different. Since you mention mA, then if you look at the Interface options you see that they do make a current loop output unit. Likely that is what you have. So study current loop circuits to see how they work.

.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
181101-0914 EDT

Shaneyj:

To troubleshoot a circuit you need to know how the components of the circuit work.

There are many possible ways to build a loadcell. Since your loadcell in made by Interface it is most likely a 4 wire resistive bridge circuit for the transducing mechanism.

This will require an excitation source, and produce an output signal. Can be AC or DC for excitation, usually in the 5 to 20 V range.

In a 4 wire bridge circuit the output signal, of necessity, is floating relative to the excitation, and for loadcells is often 2 mV/V output at rated full scale. Thus, a 1000 # full scale loadcell with 10 V of excitation would produce 20 mV output with a 1000 # load. You need to study and know how unbalanced bridge circuits work.

A loadcell may have built in signal conditioning, then the signals can be quite different. Since you mention mA, then if you look at the Interface options you see that they do make a current loop output unit. Likely that is what you have. So study current loop circuits to see how they work.

:thumbsup:
Among other things, if the interface unit is loop-powered on a 20ma loop, then you will measure nothing when you connect just to the interface. And the "splitter" is likely to put the two receivers in series rather than in parallel, or else be a powered "current mirror" that duplicates the same current to both receivers.
 
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