Minimum Load cell readability

What's kind of load cell? Piezo, strain guage, pneumo?

If strian gauge with mV output, sensitivity of load cell could be defined as transfer factor K=Uout/Uex, where Uout --- maximum output voltage in mV, Uex - excitation voltage in V.

For example if k=2 and you've applied 10V excitation voltage you can wait Uout=2*10=20mV for maximum load. In your case 20 mV for 500lb.

"Readability" is not a predefined parametre for load cell. There is a repeatability for instance. Though it's reasonable to consider it[readability] for instrumental system as a whole device. For example if you use ADC with 5V input and resolution in code 2¹²=4096. You have to apply Op-Amp with properly calculated and implemented gain, so your full load 500lb will be converted to 4096 as code. It's easy to devide and get weigh of code unit as 500/4096=0,1221 [lb/unit of code]. You have to keep in mind that as rule any applied load cell should endure significant overloads so it wouldn't be a bad idea for given load cell don't exceed load more than 500/2 either 500/1,5. It worsens overal resolution, of course, in 2 or 1,5 times.

Some of load cell's brands could be eqipped an embedded normilizer, device which converts mV-input into [4-20/0-20 mA] output. Way of calculation remains the same.

Overal quality of your system [repeatability,sensetivity,creep, accuracy, etc] depends on how you could in your design cope with any kind of noise and adverse factors.

I'm suggesting you should google this and other subjects yourself.

Good luck.

Here's thumb's rule as well.

Take an accuracy of your system measured in % than divide it on 5. It'll be the least quantity what your system should discriminate as a quite enough.

For instance: if your accuracy is 0.01 [1%] so your system shoud discriminate 500*0.01/5=1 lb. It does not mean that it'll be impossible to measure 0.5 lb or less having applied your load cell, simply it make no sense for given accuracy.

I am afraid I don't agree caramba.

The overall accuracy is specified for the complete range 0 to 500lb. if the range is reduced to a tiny fraction of the full scale range accuracy will improve up to a point, at this point the resolution of the strain gauges is essentially infinite, but the resolution will only be limited by the noise in the system, hysteresis, temperature changes etc. These effects are not related to the overall accuracy of the full range.

Using synchronous demodulation techniques noise and many other effects can be minimised, so the answer is that tiny measurements may be measured quite accurately and repeatably down to many decades less than the full scale range. With careful design!

thanx for the reply.

it is a strain guage based load cell having a capacity of 500lb. its model number is LCJA-500 make of OMEGA (American company www.omega.com)

the logger i am using is also make of omega.

its model number is DP41-S

i am in trouble how to read the min readability of the system.

can u help me by viewing the data sheet of above mentioned load system n logger?

i will be grateful

I mentioned a noise as crucial adverse factor in my posts and thumb's rule that I offered to ours cr4mate is just thumb's one. Relative error converted to plausible strain gauge output and devided on 5 is a quite enough level design resolution[discrimination]. Of course, as developer, you can divide this one on 10 or even 100, but is it senseful? Unless you have more chances to measure noise instead relevant data. In my practice I came accross the scale systems [for truck/lorry weighing] which could sense 1 kg, but it was so hard to make them stable and accurate due to its bad design as a whole. Metller Tolledo scale, as example of a very good design,has for 5 tonn range mobile platform the resolution 20 kg which corresponding to accuracy class 0.5%. It's pretty enough for given relative error and it's working fine.

Using synchronous demodulation techniques noise and many other effects can be minimised, so the answer is that tiny measurements may be measured quite accurately and repeatably down to many decades less than the full scale range. With careful design!

There are a lot of techniques for noise mininizing, and we're enjoying its. Mentioned you careful design can be applied not only to electronic/digital parts but to "iron" things as well. On other hand to make load receipt[LR] devices more accurate means to make its much more expensive. If you hadn't improved LR,in most cases any advanced DSP techniques believed to be useless.

Yes , resolution can be theoretically endless, but engineer has to choose design resolution as well grounded and as cost effective.

Hi

Omega is NOT a manufacturer, but a distributor. however, they do have a good customer service, Why not just ask them rather than guess?

Wangito.

As a rule of thumb usually force transducers are calibrated so that they guarnty the specified dispesion down to 10%FS. In fact with accepting a hygher unceratinty one can go a lot lower. What you call readability is not used as a quality parameter for a force measuring device but one could accept the limit of 10% with safe results and 5% FS with a higher risk. This is valid for usual conditioners as was mentionned a better logger and a calibration WITH the logger would bring more.