Pulsed Analog

[John_R]

Hey All,

The boss has made arrangements for a "try before we buy" test of a Mass Flow Meter to monitor liquid nitrogen usage on one of our cryogenic freezers (if the test goes well, there is a possibility that we install these on eight freezers at which point I'll build something to accommodate). At this point though, I plan on doing some quick and dirty logic in the existing V570 I have on this freezer.

The "sales engineer" sent me a blurb sheet that tells me this device can output an analog 4-20 signal, or a pulsed output via HART or MODBUS RTU (note the attached screenshot of the blurb sheet "output signals").

I figured for this test (since the flow meter will be 10 feet from my V570) that I would just take his 4-20 into an unused analog input and scale (linearize) it into whatever they want it to be.

A couple weeks went by and the "sales engineer" called me to verify something else for him, so while I had him on the phone I questioned him as to whether they had a starting point for the scale factor (based on pipe size or something) or would we just have to do some trial and error to get it close. He responded that the signal would be  "direct reading" (and as he is saying this my head is saying "no, I'm feeding this into a 14 bit analog input and 4-20 is going to give me a range of 3277-16384"), and then he said something about it was "pulsed analog", so I just agreed with him at that point to regroup my thoughts.

Now, I have heard the term "pulsed analog" before, but I was thinking that it had something to do with digital audio (pulsed digital simulation of analog audio signals).

I can see the pulsed output via HART or MODBUS, but pulsed analog clearly has me stumped.

Can anyone out there clarify this for me?
Am I missing something obvious?

Regards,
JohnR

[Flex727]

Never heard of "Pulsed Analog" (doesn't mean it doesn't exist), but this looks like the sensor hardware offers a choice between multiple analog outputs or a digital pulse output. I would think the choice on your part is whether you're looking to obtain rate or volume of the flow.

[John_R]

Yep Flex, that's the way I'm looking at it, in my 30+ years of industrial control I've not run across that term either, and like you say "doesn't mean it doesn't exist".....

Somehow I get the feeling the guy I'm talking to is more "sales" than "engineer".......

I'll figure it out when I get the device in my dirty little hands, just thought perhaps I was being challenged with some new technology

[Flex727]

Please report back. Can't wait to hear what you find.

[Ausman]

"Pulsed analog" might refer to the way the meter works and reads in the first place.  My understanding of them is that they are essentially measuring distortion, so perhaps that would be an analogue reading done every pulsed time.  This is then interpreted internally to give you whatever user output you want to utilise.

For precise checking of the flow rates, though, I'd log all your readings and try to tally this against consumption deduced from other indicators.  ie  changing tank levels with just this one is in use.

I'm always wary of salesman/woman gobbledeegook.  Many haven't got a clue.  Time and again I've had to figure something out that is nothing like what it was meant to be.  And to make matters worse, I've often found manuals just the same, with incorrect info...often from big name makers, too.

cheers, Aus

[Gabriel Franco]

I´ve used mass flowmeters with Unitronics and other PLC brands. Usually I use analog 4...20 mA to measure flowrate and pulses to count/totalize or communications whenever is possible.

Some of them have active outputs and others passive like any other instrument.

[Joe Tauser]

Many, many moons ago when I sold Red Lion they used a form of serial communication called a "pulsed 20 mA current loop".  It was not analog signal, but the reference to 20 mA made people think it was.

HART rides on top of a 4/20 signal so they're probably talking about using a resistor at the receiving end to generate a loop for it to exist.  It is rather weird.

If you get the Modbus option this would probably be helpful-

http://www.bb-elec.com/Learning-Center/All-White-Papers/Current-Loop/How-do-I-connect-the-Current-Loop-Converter-to-my.aspx 

You'll know more when you have it in hand.  Empirical data is the best data.

Joe T.

[Ausman]

Joe, that is a great page at B+B, and a great section it is in. 

In all the time I've occasionally used their products I've never even noticed the "learning center".

cheers,

Aus

[John_R]

Well, we finally got this device in hand, and after dealing with a couple other projects higher up on the priority list I got it installed.

The analog 4-20ma output was simply that, a value calibrated to a specific range, 4ma = 0gpm to 20ma = 67gpm (odd range, but that's what it is). And as we all suspected, the "pulsed" output was something completely different.

Hooked it up to my V570/V200-18-E3XB, and in short order I had a scaled value on screen that mirrored the display on the massflow meter.

Now I am toying with the possibility that we install more of these devices, at which point I plan to make a dedicated PLC and MODBUS them all together. I have started to mock that up with a V560 I had sitting on the bench, I have managed to sort through their comms protocol, and got my V560 talking to the device (which is half the battle), now I'm a bit confused with their data layout. I'm getting values from different registers but they are not the numbers I'm expecting to see.

They have a setting for MODBUS Order, I am assuming that their default would match the Vision protocol.

Can anyone confirm or refute this?

Regards, JohnR

[John_R]

Update....

After a little experimentation, I realized that the MODBUS data was correct.

Just had to get my head out of my back pocket and think about what I was looking at. The data registers I was accessing were Float, and I was trying to stuff them into MI's. which obviously didn't work.

JohnR