Resolution of Transducer

[basheera 2]

Hi every one I am a student of Manufacturing Engineering working on a Project named "PLC based Leak Detection System" for a refrigeration company.We will be measuring pressure drop over a period of time by a transducer to find out whether a leak is present or not.First we will be pressurizing the test piece at 5 bar. I am new to this electronic Stuff.That is why i getting a lot of problems.One of which is regarding Resolution that i will be getting. if we find a transducer(simple one) that has a fixed range of 0-5 bar.We carried out calculations regarding the resolution of the transducer and it came to be Range/no of steps=500,000pa/4096=122 pa (in case of 12 bit Analog Channel of PLC) we want some better resolution ,let 50 Pa or less than that. Some one told us that we can get our desired resolution by Scaling(in PLC). If we are using a 4-20 mA transducer having 0-5 bar range than it means at 4mA=0 bar 20mA=5bar Now if want to decrease our range(as rest of it is of no use for us) to 4-5 bar ,Someone said that we will just have to scale it from, let say 16.8mA to 20mA.Now i want to know ,will it solve my problem and will it increase my resolution.The thing that is confusing me is that will this shortened range will be divided into whole no of steps (4096 incase of 12 bit) or the no of steps will also get shortened.

[Ken Moore 23]

There are analog PLC cards with higher resolution than 12 bits, many vendors now have 16 bit resolution. 500,000pa / 32767 =15.26 or a 15 bit card will give you around 31 still less than 50. Are you using existing hardware, or are you able to purchase new? If you get an transducer with a range of 4-5 bar, it will still have an output of 4-20 mA, 4mA = 4 bar 20mA = 5 bar So you would end up with 100,000 / 4096 = 24.4

[JRoss 84]

As Ken suggests, resolution is tied to the Analog to Digital (A/D) conversion on the PLC end.The A/D converter takes the 4-20mA and translates it based on the numbers of bits of resolution. In your situation with a 12bit converter, you have 4096 divisions across the 16mA (20mA - 4mA) range of your signal. This is fixed by the hardware and cannot be changed. So if you ignore the bottom end of the analog siganl, and only use the top 3.2mA (20mA - 16.8mA) of the range, then you are likewise ignoring the bottom end of the A/D conversion, and only using the top 819 divisions of the range (3.2/16 = 0.2; 4096*0.2 = 819.2). So your resolution has not changed. Two options: 1) Get an analog input card with higher resolution. 2) Rescale the analog signal so that it represents a smaller range. This may require a different transducer/transmitter, but some transmitters allow you to adjust the zero and span. Either way, decreasing the range of the analog signal will increase the resolution of the A/D conversion. Ken shows the calculations for that.

[DanW 41]

1) If resolution is of concern, you should get a 'smart' industrial pressure transmitter whose input range can be configured, and configure it to 4.0-5.0 bar. That way the entire range D/A is used for 4.0 - 5.0 bar, not just 20% of the D/A range. 2) The pressure of a closed systems reflects temperature changes as a well as pressure changes. When a filled system is static (not running) the static pressure will increase as the ambient temperature increases and decrease as the ambient temperature decreases. Telling the difference between a temperature change and true pressure change can be difficult.