PLC analogue voltage range selection?

hi everyone
pretty new at this PLC stuff so really need some help.
i have a pressure transducer that has a range of 0-5bars, and a siemens 314ifm PLC with integrated analogue inputs(12 bits)
the problem being that i need a high resolution, and the only pressures i am interested in is between 4-5bars, the rest i do not need.
now my pressure transducer gives me a current of 20mA at 5 bars and 4 mA at 0bars,and 16mA at 4 bars. i need only the 4-5bar pressure(16-20mA) to be spread over the 12 bits to get a higher resolution.
is this possible and if yes please tell me how?the range between0-4 bars is of no use to me
can i do this without changing my transducer. maybe by varying the range in the plc

I dont think you can set up your input card for 16-20mA, generally it would be 4-20mA or 0-20mA. You can definetly set up your program to only be affected at whichever pressure your looking for. Before doing this I would verify to see if you can calibrate your pressure transmitter to output 4-20mA with a range of 4-5bar. This would give you better accuracy in your wanted range rather then "wasting" 75% of your range.

@sboudreau

yes i understand that i can get my program to work at any pressure, but that would still mean that 4-16mA would be consuming bits leaving me with the same resolution.what i need is that 16-20mA range be divided into as many steps as possible.
the transducers(smart transducers) that can be calibrated are smart transducers and cost a lot over $1000, the one ill be using is going to be a normal one.
so there isnt any way, a gain function or something available in the PLC?

I think you may need a pressure transducer with an analog out range of 4-20ma that corresponds to 4 to 5 bars. Many of the digital pressure switches such the SMC's ISE40A offer 4-20 ma out and I think you can program them to output over your desired pressure range. They are generally it the $100 to $200 range. You may want to download the manual for this pressure switch to see if I'm correct about the programmable pressure range. At any rate it may be a cost effective solution.

I second ddnnis. Many transmitters have an adjustable output. By using the offset and range adjustments you can have them provide 4 mA at 4 bar and 20 mA at 5 bars.

A few reality checks to consider:

A 12 bit analog input gives you a resolution of one part in 4095, or about 0.02%. Is your transmitter really more accurate than that, even at its existing range? Remember, resolution is not the same as accuracy!

Many transmitters have accuracy specs based on a percent of maximum full scale. Make sure that the accuracy of the new range is going to accomplish your objective.

Are you sure you don't ever expect to have less than 4 bar or more than 5 bar prssure? I usually want a little range above and below my expected readings to cover unforseen occurrences.

I didn't realize transducers with this type of adjustable output where readily available. Everything I've been involved with is set range. I was thinking the digital pressure switch with a programmable analog out might be a cheap solution. I'm still not sure if the one I directed him to is adjustable. It sure is good to have guys like you available in these situations, a guy could spend a lifetime trying to figure some of this stuff out.

just another query

thank you for all your help,
i have looked up some smart transducers which are of reasonable cost($400-$1000) heres a link to one of them: http://www.ueonline.com/bulletins/tx200-b.pdf

just another question
so basically this is what i'm doing, designing a leak detection system(by measuring the pressure drop), so basically ill be charging a test piece to a pressure of 5 Bars and sealing it, and then measure the pressure drop, which i calculate not to be more than a few hundred pascals.
so heres my question, i now have a smart pressure transducer which is capable of measuring btw 0-5 bars, i have programmed it so that it gives me an output of 4mA at 4 bars, and 20mA at 5 bars. now while the piece is charging my transducer will be exposed to the 0-4 bar range, my question being while the transducer is exposed to the 0-4 bar range what will its output be, or will there be no output, will this range inflict any damage on my transducer?
you might ask why the 0-4Bar range isn't important , its because it has nothing to do with the part testing, and data aquisition.

When you change the scaling on the transmitter you are just changing the way the amplifier in the transmitter converts the mV or similar output from the primary sensor to the high level 4-20 mA signal. The sensor and its low level output remain unchanged. You won't damage the transmitter at less than 4 bar if that is your concern. Below 4 bar your transmitter may lock in at 4 mA, or it may give you a specific under range signal of 0 mA or other. Contact your transmitter supplier about that.

I have worked with clever pressure transmitters, temperature transmitters etc, that have been controlled via Profibus.

During different stages of the process we are concerned about different ranges that the transmitter will provide.

So when we want to read 1 Bar - 2 Bar we write the parameters to the instrument that 1 Bar = 4mA and 2 Bar = 20mA and do the scaling accordingly.

Then when we are interested in the process at 5 Bar - 10 Bar we write those parameters to the device that 5 Bar = 4mA and 10 Bar= 20mA and do the scaling accordingly in the PLC.

I don't know if your device has this feature but IMHO if you need the function in your process it is well worth the little extra money you pay for the device.

Mark

I think you need to look at the leak tester function.
you mention 5 Bar - If that is the actual test pressure
you are now at the maximum so no room for a higher pressure
It is possible the supply Pressure could be 6 bar
as that value is above the analogue test range
it may leak 1 Bar further than a few hundred pascal prior to your actual testing.
Then as testing is done by time it could pass - even if it should be a FAIL.
as mentioned 0.02% - 1 Bar = 100000 Pascal = +/- 200 Pascal error
(5-4 bar)

I have programmed leak testers for many vehicle devices -
Your may need to look a bit closer.

@iant

thanks everyone again,
iant you mention the resolution which between 4-5bars comes out to be +/-20,
is there a reason why you mention it to be +\- 200.
i did read in an article someplace that a margin of x10 is needed, but the cost contraints simply arent allowing me to increase the resolution higher than 12 bits, the range could be lowered though?
really interested in your insight , what kind of resolution do you normally work with?
our main concern with this system is to eliminate operator dependance, as the current testing system is a water dunk testing (the same that is used to find a puncture in tires), so im aiming for a sensivity of 10^-2mbar.l/sec?
do you think this can be achieved with a 12 bit resolution?

The resolution minimum is 12 bit -
14 bit next higher value.
QUote" A 12 bit analog input gives you a resolution of one part in 4095, or about 0.02%.
you asked for a couple of hundred pascals
the 0.02% was calculated @ 100000 (1 Bar) Not 4 bit

0 - 5 Bar (4~20 mA) = 5000/4095 = 1.221 per bit
4 Bar = 4*1000*1.221= 4884 +/- 0.02%
Measurement factor 5000 - 4884 = 116 bits for 1 bar measurement
116 / 100000 pascals (1 Bar) = 0.00116

this is all the math - what accuracey do you need?

the no' of bits are not shared in the range like i think you were hinting at

I have a further question for you

Is this for a job or a schooling tssk?

resolution

iant if im using a smart transducer, which gives me 4ma at 4 bars and 20mA at 5 bars,will this resolution not be spread over the 12 bits?
my resolution should come out to be 100000/4095=24 pascal
@iant i was aiming for a sensitivity of 10^-2 sccs which i calculate to cause a drop of pressure of 170pascal.


this is both a schooling task and a company project for a local refrigeration company, its going to be my final year project.

that is correct
but pay attention to the over pressure and under pressure factors.
Most air pressure - (any type of Pressure or Vacuum) - systems
do not run at 5 bar exactly - they may run over preasure.
you need to be able to see this when doing leak testing.
A higher pressure will only be read as 5 bar or an analogue error
depending on the card setup
this will not work with leak testing