What pressure transducer to use?

[JustinCochran]

I'm looking to get a v120-22-r1 and I need to use an analog input to hook up a 0-5 psi pressure transducer. What I'm doing is making a plastic bottle leak detector. Using compressed air, I'll fill the bottle and the pressure transducer will see if there are any leaks or not. I'm pretty new to this PLC stuff and really have no clue here. Anyone have any suggestions on what type of transducer to use for this application? Do I need anything else to make this work properly, as far as the analog input goes? I was looking at a honeywell transducer at this webpage. Would something like this work for my application? Or do I need to go a different route?

 

http://www.alliedelec.com/search/productdetail.aspx?SKU=70120546&mkwid=s7QconsaU&pcrid=23475462857&pkw=honeywell%20asdx005g24r&pmt=b&pdv=c&gclid=COT76-vssrkCFY9AMgod6EsAhQ

 

We had someone make a leak detector like this using a different plc program and all we know is that they used the above transducer and a couple small capacitors were tied in with the 5 VDC that powers the transducer.

 

Thanks in advance.

[Joe Tauser]

That transducer will be fine if you're comfortable interfacing with a DIP package.  I don't like component level stuff in a panel and usually stick to something more industrial-

http://www.dwyer-inst.com/Product/Pressure/SinglePressure/Transmitters/Series626-628

 

How do you define your leak test?

 

Joe T.

[JustinCochran]

I haven't wrote a program for the leak test yet. I haven't tried or know exactly how I plan to define my leak test. I do like the industrial type you referenced to me. Think I'll go that way. Now that wouldn't need it's very own power supply does it? Can I come off the 24 VDC power supply that would power my v120-22-r1 too? I see that it has a 4-20mA output, so that would take care of my problem with the hardware. The program part though, might give me a hiccup too. I'll find soon enough. Do you have any recommendations? 

[Joe Tauser]

1.  Don't write the PLC program until you've defined your leak test.  This is putting the cart before the horse.

2.  The Dwyer sensors are two-wire devices - you can power them from the same 24V you're using to power the PLC.

3.  What are you doing with a V120?  Did you have it laying around or did you buy it recently for this project?  This has a 10 bit A/D converter so your resolution will be limited to about 800 counts - do you need more resolution than 0.01 PSI for your test?

4.  Just out of curiosity, where are you located?  Do you have a distributor you can lean on?

 

Joe T. 

[JustinCochran]

I haven't bought the v120 yet. Want to make sure I am going to get the proper hardware before I commit to getting anything. I am nearby Kansas City, MO and we have a contact, haven't been able to here anything back from him though, which is why I came here. Not entirely sure how much air would leak from a tiny pin hole in a plastic bottle, let's say the size of a squirt bottle, with 5 psi put into it. I'm guessing that's what you meant by my resolution. Like I said in my first post, I'm not sure on what I'm doing really, this is my first attempt at using any plc or making my own leak detector. Still have a bunch to learn, so forgive me if I don't understand all that much as of now. Not sure by how to define my leak test, not sure what you mean. Only thing I can say to that is 5 psi compressed air is forced into a bottle and after a few seconds it measures the loss of the pressure since the start of the test to the end of the test. If it's too much of a loss it rejects, if it is suitable it passes. Anythng as small as a needle hole is no good.

[JustinCochran]

Also I would have a setpoint of a known good bottle as the point to make or break the test.

[Simon]

Hi,

 

Since you haven't yet purchased the PLC, I would suggest looking at the V130.  They are a newer generation unit and should compare well on price with the V120.  For example the V130-33-TR20 is equivalent to the V120-22-R1.  If you want the higher resolution of 12 or 14-bit on the analogue signal, you can go to the V130-33-TRA22.

 

Even a pinhole should lose pressure at 5psi, its just the timeframe that will vary depending on hole size.  A higher resolution will let you detect smaller leaks in a shorter time.  Take a look at price difference between the TR20 and the TRA22 and buy the higher resolution unit if you can.

If you want to hedge, I would suggest possibly purchasing the transducer first and using an analogue meter to measure the pressure drop for different sized holes.

 

As for defining the test, write out the logic in steps, like:

* check bottle in position

* extend plunger to bottle mouth

* open air valve,

* when bottle at test pressure close air valve

* measure starting pressure

* start timer

* when timer expires measure ending pressure

* calculate pressure loss

* determine pass/fail.

*... and so on....

 

By defining the sequence well at the start, you have the best chance of setting up the program structure to suit the application.  If you start out with the wrong program structure, it can be difficult to "bend" it into the final shape you want.  However, even with all the best planning, and especially if you are starting out, you may find that once you get the machine working, it is best to rewrite the program from scratch, to include all the lessons learnt by doing it the first time.

[JustinCochran]

As far as defining my leak test, I basically have all the steps I want it to do. I know exactly what I want it to do. I have been writing and changing the logic for a couple days. Everytime I re read it, I find a small problem I over looked. I think I just about got it this last time. It's a good learning process.

 

Instead of making a new topic, I might as well ask this here too. I wanted to put a counter on the main screen showing the total bottles tested. My question is, is there a way to reset the counter value once it reaches the max of 99999? Does it start over once it maxes out? I couldn't find my answer in help documents. Thought about just making it a preset value by using the keypad and resetting it manually. I wanted to avoid that if possible.

[Simon]

Sounds like you are heading in the right direction.

 

As for counters, an MI integer value will just keep going to 32767 (or 32768 - I can't remeber without looking it up), then will wrap to the -32767 and count back to zero.

 

So you won't actually get to 99999 using an MI, that is just the placeholder that the system uses within the HMI editor.  If you want to go beyond 32768 then use an ML value.

 

To prevent the counter going past your upper limit, got to the ladder program and use a Compare function (Greater Than or Greater Than or Equal To) to detect when the count has exceeded your limit.  Then use a store function to reset the value to your starting point.  If your starting point is 0 then you can use the "Reset Numeric" function  ( ---[R]--- ) instead of the store.

[JustinCochran]

I ended up using a counter that is reset each time the power is turned off and on again, or when it reaches 9999. That way they could reset it if they wanted to by just powering it up again. Using spray type bottles I was able to get 27 bottles a minute tested. I might be able to get it even quicker but that exceeds expectations. The V120-22-R1 I looked at before did the job perfectly. The dwyer pressure transducer for 0-5 psi did the job great too. Thanks for the help you guys gave me, made it much simplier on my part. Of course this was a rather simple project compared to others, but it's a start in the right direction.

[cantcliff]

I highly recommend using some sort of a calibrated orifice if this is for any type of production application to understand the leak rate combined with volume.

Depending on the test volume, how you're stabilizing your pressures, and you're test resolution you may be passing bad parts.  

We use a much higher tolerance for leak testing parts, typically <1 sccm @ 45 psi and cycle times run around 15 seconds total for a single tester to run a reliable leak test.  The total test volume is fairly small compared to a spray type bottle.