Load Cell Measurement mV/V ??

zmanvortex

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Apr 2002
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Nebraska
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I am experimenting with load cells and I was just wondering what the term mV/V means.

The excitation voltage of the load cell is 10V with a 2 mV/V output rating. I am assuming the maximum output should be 20mV.

The selectable ranges of my analog input card are :
0-39.0625mV, +/-39.0625mV, +/-78.125mV, 0-156mV, +/-156mV, and 0-1.25V.

The maximum excitation voltage on the load cell is 15V so I was hoping to use a 12VDC power supply, the analog card in the 0-39.0625mV range, and the load cell.

Will this work? With a 12VDC excitation voltage I should get a maximum output of 24mV.

The input impedence of the analog card is 5Mohms.

The input resistance of the load cell is 350ohms min. and the output resistance is 350ohms.
 
Yes, the 2mV/V is just as you surmise. If you use 12VDC supply for excitation you should get 24mV at the full output of the load cell when maximum load is applied to it. Looks like your analog card will accept that voltage range so you should have few problems, depending on how you need to scale the value once you have it in the PLC.
 
Thanks Randylud. I know I won't have the full range of the analog card (39mv), but I should have enough range with 0 - 24mv for the application.
 
zmanvortex said:
The excitation voltage of the load cell is 10V with a 2 mV/V output rating. I am assuming the maximum output should be 20mV.
Correct.

Will this work? With a 12VDC excitation voltage I should get a maximum output of 24mV.
Also correct. I recommend a precison excitation voltage source.



The input impedence of the analog card is 5Mohms.

The input resistance of the load cell is 350ohms min. and the output resistance is 350ohms.
Are you dealing with a load cell or a strain gage?

If you have a load cell then 350 ohms probably refers to the bridge as 350 ohms is a common bridge spec. A load cell consists of a strain gage already inserted into a bridge. The bridge will output a voltage that you measure with a high impedence device, such as your 5mohm input.

If you actually have a strain gage then the strain gage must be inserted into a user supplied bridge and so it has a bridge specification, again, 350 ohms being a common bridge spec.
 
Thanks guys. Well I just found a load cell which takes a 24VDC input and outputs .5 - 9.5VDC.

We already have 24VDC available and the 0-10VDC analog card cost less so I think I will go this route.

I did not realize load cell companies were making models that are so easy to interface to plc's.

I figured you would have to purchase an expensive load cell controller which would output an appropriate signal for plc use.

Thanks again.
 
Last edited:
zmanvortex said:
Thanks guys. Well I just found a load cell which takes a 24VDC input and outputs .5 - 9.5VDC.

We already have 24VDC available and the 0-10VDC analog card cost less so I think I will go this route.

I did not realize load cell companies were making models that are so easy to interface to plc's.

I figured you would have to purchase an expensive load cell controller which would output an appropriate signal for plc use.

Thanks again.

I am going to assume that you are using a standard 24VDC power supply to the load cell. This may not be a correct assumption, but I wanted to stir up some discussion. As Alaric said:

Also correct. I recommend a precison excitation voltage source.
What differentiaties a "precision excitation voltage source" from say a Sola 24VDC SDN switching power supply?

Also, you may want to look into signal conditioning based upon how accurate you want the Voltage to Loading conversion to be. Things like temperature and other factors can bring non-linearities to the curve. This can lead to measurement error without a signal conditioner that is calibrated across multiple load values.
 
To clairfy: If you are using an internally amplified load cell then usually any good power supply will do.

If you are not using an amplifier or signal conditoner which provides the excitation, then you want a very stable excitation voltage source because even small changes or ripple in the excitation voltage could lead you to believe the load is changing in ways it is not changing.

The internally amplified load cell is definitely a better way to go. Check out these guys also: http://www.transducertechniques.com/
 
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Alaric- how can I determine if the load cells in a particular

application are internally amplified or not? Most of the ones I run into are the Cleveland Machine Controls brand. If these are not internally amplified, would a Sola switching or linear be provide a stable enough voltage source?
 
To clairfy: If you are using an internally amplified load cell then usually any good power supply will do.

This is indeed what I am planning to use. It explicity states that it is an amplified load cell.

Here is the link:

http://www.omega.com/ppt/pptsc.asp?ref=LC509

I tossed around the idea of a signal conditioner but I have a very tight budget on this project. I will add a signal conditioner if it is warranted, but for now I will try the stand alone load cell.

I will keep you guys updated on the progress and results of this load cell. Thanks a ton.
 
The stand alone load cell with an integrated amplifier is indeed a better choice - especially if you have some distance between the load cell and the cabinent in which you would place a signal conditoner. The reason for choosing an external amplifier would be if you anticipate replacing the load cell frequently. I have one app where the load cell is single use. In this case we use a straing gage with an external bridge and external amplifier, thus we only need to replace the inexpensive strain gage.
 
Something else to consider is the resolution of the analog input card. It will determine the final value measured.

For example, if your card is a 12 bit card with 0-39 mV, and your full scale load cell equals 24 mV, then your resolution will be from 0 - 630 (0.038 mV per division). If you want the measured value to be in 100ths of a pound (0.00) the best you could hope for is 6.30 lb. Of course, this assumes you could even find a load cell with 6.30 lb full scale. Most of them I see are in kg (1, 2, 5, 10, 20, etc).

Now, if we go with a 5 kg load cell, then the mass per division approximately equals 7.94 grams. This means that you would not be able to use this setup to measure grams. You would be restricted to 10 gram increments. With the right signal conditioner though, getting 1 gram increments is definitely possible.

This is meant to give you something else to be aware of when using analog input cards with load cells.
 

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