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How to tune the zero and span

Thread Starter: how come Started: 7/10/2009 4:51 AM Replies: 8

7/10/2009, 4:51 AM

how come

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Dear all,

I bought an isolator which has input for 4-20mA and output 4-20mA. I notice there is a zero and span adjustment. The manual didn't indicate how to go about adjusting the zero and span. Does anyone know how do it? Appreciate someone could give me some simple procedure or examples of explaining the tuning of zero and span.

Thanks!

with regards,
how come?

=== Edited by how come? @ 7/10/2009 4:53 AM [GMT ] ===

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7/10/2009, 5:15 AM

Lucky123

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how come? wrote:

Thanks!

with regards,
how come?

Hi how come,

Although I am not sure which Brand of Isolator you are using, however irrespective of that I may give you some information which might be useful for you.

AS you mentioned there are two types of adjustment.

So lets see what does it mean!

Zero Adjustment pot:

You would need to adjust this pot for adjusting your zero point, For example when you are supplying 4mA as an input, you would need to make sure that you would get 4mA as an output,

If the output is lesser or more than 4 mA then you would need to adjust the zero pot so that you can get the 4 mA output at 4 mA input.

Span is when you give 20 mA Input to Isolator then at output you should get 20mA, if not then you would have to adjust the span pot in order to get that.

I hope that clarifies you.

If not then wait for other members to post their views.

thanks

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7/10/2009, 9:40 AM

how come

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Dear lalit,

Thank you for your information! so am i right to say that:-

1] if the input is 4mA and the output is e.g 3.95mA or 4.02mA, i have to use the "zero" to tune the output to 4mA?

2] after that at input 20mA and the output is e.g 19.98mA or 20.04mA, i will have to use the "span" to tune the output to 20mA instead of using the the "zero" tune?

I have one more question, in the little note that comes with the isolator there is a formula:-

Calculation of the F.S adjustment point A = (MV2 * constant)/(MV2-MV1)
Span pot = F.S adjustment point A +/- adjustment tolerance
Zero pot = output upper range (full scale) +/- tolerance

where:-
MV1 = measured output with lower range input value
MV2 = meaasure output with upper range (f.s) value
constant = 16mA
output upper range value = 20mA
adjustment tolerance = +/-1uA

Does anyone know how this formula was derived :-A = (MV2 * constant)/(MV2-MV1)

Thanks!

with regards,
how come?

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7/12/2009, 8:14 PM

Ayman Elotaify

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how come? wrote:

I have one more question, in the little note that comes with the isolator there is a formula:-

Calculation of the F.S adjustment point A = (MV2 * constant)/(MV2-MV1)
Span pot = F.S adjustment point A +/- adjustment tolerance
Zero pot = output upper range (full scale) +/- tolerance
where:-
MV1 = measured output with lower range input value
MV2 = meaasure output with upper range (f.s) value
constant = 16mA
output upper range value = 20mA
adjustment tolerance = +/-1uA

Does anyone know how this formula was derived :-A = (MV2 * constant)/(MV2-MV1)

Thanks!

with regards,
how come?

Dear how come?,
Can you please send here the data sheet of this isolator here to be able to help you.
And also from the first looking at the formula you have sent, it looks like the linear equation between the input and output of the isolator.
Thus because MV2 - MV1 is the differences between measured outputs.
constant = 16mA is (20 mA - 4 mA).
And I think that:
The zero Pot will adjust the zero ( i.e. When 4 mA applied on the input of the isolator, adjust the zero pot until the output of it become 4mA).
The Span Pot will adjust the slope of the linear relation ship beteen input and output of the isolator: (so you can select the point of 20 mA to be adjusted and then force from the transducer for instance 16 mA and see what will you recieve in the output of the isolator. it should be 16mA too.

Regards,

=== Edited by Ayman Elotaify @ 7/12/2009 8:14 PM [GMT ] ===

Ayman Elotaify
MISC Egypt
www.miscegypt.com

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7/13/2009, 11:02 AM

how come

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Dear Ayman,

Thank you very much! Your clear explaination make me understand better!

Please find the attach of the formula which comes with the isolator.
Thanks!

with regards,
how come?

Attachment:

Isolator Table 4-20mA.JPG (27 Downloads)

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7/13/2009, 11:57 AM

Ayman Elotaify

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Dear how come?,
Let us define these terms first:
MV1: is the lower output value (here is your case the output of the isolator when 4mA is applied at its input).
MV2: is the upper output value (here is your case the output of the isolator when 20mA is applied at its input).
Constant = 20mA - 4 mA = 16 mA.

Then If you draw the relation ship between Input and output of the isolator.
Input is on the X-axis, and Output on the Y-Axis.

Then The slope = (MV2 - MV1) / constant.
And the value of the output at any point "A" will = A * Slope.
Then A = Output * constant / (MV2- MV1).

In the data sheet; it's written MV2 and not output in the formula! Strange!. I think it's a typing mistake and you have to return to your supplier.

Regards,

Ayman Elotaify
MISC Egypt
www.miscegypt.com

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7/17/2009, 11:32 AM

how come

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Dear Ayman,

Thank you very much! Its a very clear explaination!

You are right, why MV2. I will feedback to my supplier and see what they say.
Thanks!

with regards,
how come?