# Thread: A little help? Anyone??

1. ## A little help? Anyone??

A friend of mine is doing an electrical/electronics course which I plan on taking next year also, he has kindly sent me some of the questions from his assignments for me to have a go at as I would like to go into the course with as much knowledge as possible. My main problem is my maths SUCKS. On the subject of PLC's (programmable logic controllers) I came across this question;

A temperature transmitter is measuring the outlet flue gas temperature of a boiler. It is configured to give an output of 4-20 mA for 100-200 Deg C. The flue gas temperature is 162 Deg C.

A)Determine the value of the mA signal from the temperature transmitter.
B)Determine the resolution of the system i.e. the minimum temperature change which can be detected by the PLC.

Ive tried figuring it out, only problem is I am not sure if I am getting the right answer or not. Any takers??

2. Hey! Something that I can help with! I actually teach an intro to PLC course along with other Instrumentation courses.

There is a process to these problems that have a "floating zero" or "live zero" where your lowest value is not zero, but in this case 4 mA or 100 Deg C.

The trick is to convert the value to a percentage of the span first, staying in the given units, then multiply that percentage by the span of the new units and add the live zero back in.

So for this problem we would say our temperature span is 200 Deg C - 100 Deg C, for a total span of 100 Deg C. If you have 162 Deg C we could find the percentage of that span by subtracting the live zero and dividing by the span in that unit range:

(Given Value - Live Zero) / (Span) ==> (162 Deg C - 100 Deg C) / (100 Deg C) ==> 0.62 or 62% of our total span.

Now, if we want to relate that to a span of a different set of units we just have to work in reverse!

To find the Span of our new unit range (4 mA - 20 mA) we just have to take the largest value, and subtract the live zero.

Span = 20 mA - 4 mA = 16 mA

Now to relate our percentage of span from the original units of Deg C to our new unit range in mA:

(% of Span * New Span) + (Live Zero) ==> (62% * 16 mA ) + (4 mA) = 13.92 mA

Hope this helps!

3. Thank you so much! that clears a lot up, I wasnt too far off in the end! It also asks to describe how the 'word length' used here has impacted the upon the resolution of your measurement. I may have to slow down as I am getting a little too far ahead of myself. I believe the word length is the 16bit binary value?

4. Yep! If I'm remembering correctly (we do PLCs in Summer) the word length is simply however many bits are used to define a value, so if you have a 16 bit register for the input value your word length would be 16 bits.

I just realized there was a second part to your question regarding resolution as well. Let's say that you have a 16 bit input register, that means that the number of steps that you would have for that register (a step being the change from 0 - 1, 1 - 2, 2 - 3, etc.) would be 2^16 - 1. We have to subtract one from that value because there will be one less step than you have numbers represented by the bits themselves.

If you're looking for the resolution you then compare the number of steps you have to the voltage input. So lets say you have 16 bits for the input register and an input of 0 - 10 volts. You'd have 2^16 - 1 steps => 65,535 Steps that are divided across those 10 volts, so 152.59 micro Volts per division. Meaning it takes a changes of 152.59 micro Volts for the PLC to sense a change at that input.

Often times there are examples of these problems in the manual, and that is a really great place to learn the specifics of your PLC, but hopefully those examples will clear up enough to get you started!

5. Good stuff.
To mess with your teacher, tell them they forgot to specify whether the transmitter had a negative or positive temp. coefficient.

6. great help, thank you!

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