Is my math right?

kurt.brinker

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If i am flowing a fluid at 300 GPM with a density of 8lb per gallon and an receiving one pulse per lb, i would be getting 1 pulse every 25ms correct?

I am about to put a full load on my slc and hopefully the scan time doesn't go past 20ms. Is this still cutting it too close?


Our normal range is around 260GPM @ 7.5Lb per gallon but i went high to make sure we had a little headroom.
 
25 msec is what I get.

Remember you need to see the off state also. If you want to pick up a 25 msec pulse, assuming exactly 50% duty cycle, you need a scan time no greater than 12.5 msec. At 20 msec there will be several scans where you won't see a transition.

Keith
 
kurt.brinker said:
If i am flowing a fluid at 300 GPM with a density of 8lb per gallon and an receiving one pulse per lb, i would be getting 1 pulse every 25ms correct?

I am about to put a full load on my slc and hopefully the scan time doesn't go past 20ms. Is this still cutting it too close?


Our normal range is around 260GPM @ 7.5Lb per gallon but i went high to make sure we had a little headroom.
Click to expand...

Your 25ms per pulse is correct. How long are the pulses?
You still might have trouble with catching the pulses and counting them fast enough. You should probably look at a 1746-HSC (High speed counter) card.
 
wow, i didn't even think about the off state. when i go online the "last 1ms scan time S:34 is 13-14"


So i am missing pulses for sure. I guess the only thing i can do is slow down the pumps if they want to continue the way they are. Or get a high speed counter card. (which i would have to put an expansion rack in for)
 
The key is the pulse width. It has to be on long enough to be picked up by the PLC's input update. It also needs to be off long enough. It looks like you're close to needing a High Speed Counter module. Either that or see if you can set up the sensor to output one pulse per kilogram (2.2 Lbs).
 
jimbo3123 said:
Your 25ms per pulse is correct. How long are the pulses?
You still might have trouble with catching the pulses and counting them fast enough. You should probably look at a 1746-HSC (High speed counter) card.
Click to expand...

we posted at the same time.

the pulse width is at 18ms right now. should i set it at 20 or 25?

i'm guessing minimum 25 the way it is right now.
 
Last edited:
Steve Bailey said:
The key is the pulse width. It has to be on long enough to be picked up by the PLC's input update. It also needs to be off long enough. It looks like you're close to needing a High Speed Counter module. Either that or see if you can set up the sensor to output one pulse per kilogram (2.2 Lbs).
Click to expand...

that might be the ticket right there. Its the promass 83 and it has tons of options. I am sure i can set it to 1 pulse per kg. I'll double check to make sure but that should work.
 
Originally posted by kurt.brinker:

the pulse width is at 18ms right now. should i set it at 20 or 25?
Click to expand...

I would set the pulse width to half the time between pulses at your maximum flow rate. This will give you a 50% duty cycle at full flow. Anything higher and you run the risk of missing the on state.

You may be a candidate for an interrupt input module. This would allow you to pick up the pulses and you would be able to use the other input points for general purpose stuff.

Keith
 
its easier when i type it out.
RIght now they can choose to load in either gallons or lbs. In lbs the amount needed is the setpoint and when the pulses reach that number, everything shuts off. If they choose gallons, that number is multiplied by an operator defined density and then placed in the same setpoint as before.


If i set the meter to 1 pulse per Kg, i would need to take the lb input and divide it by 2.2 and place that as the setpoint. Then for gallons i take that input, multiply it times (variable density / 2.2) and make that my setpoint for the pulses.



is that right?
 
kamenges said:
I would set the pulse width to half the time between pulses at your maximum flow rate.

Keith
Click to expand...

assuming the flowrate is one pulse every 25ms. You would set the width to 12ms? Just making sure i understand correctly.
 
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If your flowmeter settings don't provide for a solution, and before you plunk down several hundred dollars and a bunch of development time for a HSC, you may look into an STI routine. It is free (if you already own the RSLogix and a processor!). You specify the scna time of a particular ladder file and put your pulse counter logic in that file. You can set the STI interval for say 10ms and not have to worry about missing pulses as long as they are not faster than the hardware on the input card you are using. There are considerations when using STI, but as long as there is very little code in the routine there should be no trouble...
 
i am currently researching the meter but we are using the 1746-IB16 with a signal delay (max) of 8ms on and 8ms off.

EDIT: The meter has a pulse value setting that comes up as 1.000 lb/p. The book isn;t much help but i am assuming this means 1 pulse per pound. If i set that to 3, i am hoping it will give me 1 pulse per 3 pounds (and not 3 pulses per pound). that will give me more than twice the resolution of metering by gallons and yet still leave plenty of scan time headroom.
 
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You might also want to look at doing an IIM (Immediate Input w/Mask instruction) on the address for the card slot. This output instruction allows you to update data prior to the normal input scan. When the IIM instruction is enabled, the program scan is interrupted. Data from a specified I/O slot is transferred through a mask to the input data file. This makes the data available to instructions following the IIM instruction in the ladder program.


Rockwell Software 2005
 

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