Using a weight scale to calculate flow rate

Hello,

I realize this is a PLC forum but thought I would get some good feedback for components by posting this question!

We are using corrosive liquids in our processes and normal flow meters do not work well. The reason is because some of the liquids cannot be mixed together without them turning into a gel like mess. Mass flow meters are probably best for the application but due to the expense I would like to try a weight scale that has an analog output to the PLC. We would zero the scale after filling our material bottles (500 ml bottles) and then I would calculate the flow rate based on the weight. Does anyone have experience with a good weight scale for this purpose that can be used to calculate flow rate?

Thanks for your help!

Pete

Look up look in weight
SHort version is calculate the time it takes to change the weight. That becomes your flow rate either filling or emptying

Hey Jeff, Thanks for your message. I was looking more for people who have experience with a good weight scale and who could recommend one that works accurately for this type of project. But thanks for your answer!

Pete

I have used a dozen plus scales. You can get chemical grade scales from just about any manufacture. I have been using Rice Lake systems lately. They seem to be the middle of the road.
I did a job with GWT now (Sartorius)and they had great technical support. The only problem was they where on the other side of the pond from me so parts where an issue.

Hi Pete

Mettler toledo panther make a scale unit that panel mounts and depending on what you order and are working with control wise will have the output you need, the rest is pretty much straight forward to do... do you intend to weigh your liquid then put it into a bottle ? or place the bottle, then tare and fill ?

Regards

John

As others have said Rate of change in weight over time is your basic flow rate. THe down side is accuracy. This is not something I would bet the farm on.

We build systems simlar to this a lot. We use Mettler Toledo IND131 weight controllers. They can do this calculation themselves and it is reasonably accurate. We have our own code we built into a Red Lion G3 HMI to do this which we find is easier to use.

That said if you just looking for the flow rate as a reference and not for precision control then you should have no issues.

The IND131 series is the next generation from the Panther another mentioned so if this is a new install I would recomend it.

Clay,

We have a customer who needs a rate of about 0.3 lbs/hour. Would the Toledo IND131 be accurate for such low weights?

Also, what PLC do you use (or recommend) with the Red Lion G3?

for this slow rate use a hose pump with stepper motor like the ones used in hospitals for your blood and infusions.
for corrosive filling use a force cell going to your plc or anything.
To speed up filling use a big pipe to fill to 90% and use a small one to top it up.
Have the filling in of the bulk on one station before the endfilling, this way the speed will be about 8 times higher as on one single station.

Long winded reply

Lancie1 said:

Clay,

We have a customer who needs a rate of about 0.3 lbs/hour. Would the Toledo IND131 be accurate for such low weights?

Also, what PLC do you use (or recommend) with the Red Lion G3?

Click to expand...

Depends on what you mean by accurate. The IND131 is rated to +/- .01% accuracy at 5000 divisions. So if your measuring 0 to 50 lbs then your going to have an possible accuracy of +/- of 0.005 lbs. Notice I said the word possible. While you can measure at this accuracy is it possible with your process or even pratical.

This high accuracy is based on "static" weighing. In the real world we rarely do this. A more reasonable accuracy is .1% to .5%. This is what we call "semi" static. Without getting into alot of neerdy details basically your taking measuring a weight you are giving some time to settle.

Since your asking about a flow rate I would assume you do not plan to stop. This is called "dynamic" weighing. In dynamic weighting the best you are going to do is +/- 1% to 5%. It may even be worse depending on the process.

And that really takes us to our most important part. What exacly are you trying to do.

Think of it this way. A weight controller is like that meter in your tool bag. It is just measuring mVolts. If a weight controller (any brand) has NTEP aproval then it is the "most" accurate on the market. The resolution/accuracy is really set by the load cells. In the above example if you used 1000 lb load cells then your resolution is .2 lbs (maxium load cell weight divided by number of divisions). Now the accuracy standard is .01% static is going to work out to .1 lbs. So that said if you add up .1 to .2 you get .3 lbs error as the BEST you can do. Reality is more like +/- 3 lbs with a 1000 lbs load cell.



So what makes one controller better than another. Well in my opinion (notice I said my opinion) is how it utilizes this measurement. How fast can you get this data to your process control and at what resolution can you get this data (think analog here). This is where I really like the IND131. I have several comm options I can use with this weight controller. In the Comms I use (ASCII serial, ethernet/ip, or profinet) setup is easy and Comms are consistant. They are also fast.

I apologize for the long winded answer but a simple yes, may not be true. I will say this: The IND131 is as accurate as any other brand and better than some other brands. The same applies for the Comms on the IND131

As for my standard setup, I use the IND131 to a Red LIon G3 to a Siemens S7-200. We also use Omiron and Allen Bradley when requested. We use the S7-200 mainly beacuse of cost, it really does not have a major advantage over a Micrologix.

Now for the curve ball. I have built systems that would measure and consistantly fill a bag to +/- 100 grams on a 400 kg bag. To get this took over $20K in hardware alone. Not to mention a large amount of design and testing. But this was a perfect storm of physics and controls. If the material had not had certain physical charistics or the controls would not have had the speed they did then this would not have been possible. I did however use a IND131 in this system.

I probably created more questions than I answered but I hope I helped you find the right questions.

Thanks, Clay, and Shooter.

I have not seen the setup yet, but the mechanical engineer told me it would be a pot with total weight of about 5 pounds, with 1 pound of liquid material, hanging on a load cell, to feed out at 0.3 pounds per hour. The material is a fertilizer additive (more I do not know).

(Hose pump and stepper motor is not something I had considered, but will look into that). Shooter, how do you control your stepper motor?