Zero/span application question

[shiftman]

Recently, I ran across an old removed "dumb" Rosemount DP xmitter (didn't get the model number) with only a zero and span adjustment on the outside of the xmitter housing. I was wondering if this type of old transmitter allowed the entire range to be shifted via the zero adjust? In other words, if the instrument was originally set up to read 0-100 in/h20 with 4-20 mAdc out, could the zero be shifted to provide for a 20 mAdc out at 0 delta p? Maybe for use in a wet leg type application or something similar.

Also, if the range could be adjusted (swapped)in this manner, would that mean that you would then use the zero as a span adjustment and vise-versa - since 20 mAdc would then equate to 0 delta P, or would the normal zero/span convention still apply?

Thanks.

 

[DRWeig]

You should get the model number and look up the manual.

Most likely, the zero and span are not interactive. Changing the zero will not change the span and vice versa. That's a probably-so in my mind.

1. Get the manual, or,
2. Put it on the bench and hook it up, or,
3. Wait on other comments here.

Best to you,

Goober Dave

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[shiftman]

It was in a scrap metal bin and couldn't be removed for "experimenting purposes" or I would have eagerly dissected it................... I'm sure that it was likely a very popular model in its day. I can't imagine the zero having enough turns on the pot to shift the zero to the opposite end of the range, but I can't say for sure.

 

[danw2]

From the 1151 analog manual:

Span Adjustment Range
The span on a Rosemount 1151 Analog with E and G output options is continuously adjustable to allow calibration anywhere between maximum span and one-sixth of maximum span.

For example, the span on a Range 4 transmitter can be adjusted between 25 and 150 inH2O (6.2 and 37.2 kPa).

Zero Adjustment Range
The zero on a Rosemount 1151 Analog with the E or G output options can be
adjusted for up to 500% suppression or 600% elevation. See Figure 4-8. pg 4-14.

[URL unfurl="true"]http://www2.emersonprocess.com/siteadmincenter/PM%20Rosemount%20Documents/00809-0100-4360.pdf[/url]

I noticed in the press release when Rosemount Nuclear recently introduced its 3150 replacement for its 40 year old 1150 (both of which are analog (not microprocessor-based) transmitters) that a 'coarse zero' is used for elevation and suppression:

The release stated that the 3150 Series has simplified electronics with:
a. Single board electronics vs. two boards for 1150 Series
b. Easily adjustable coarse zero selectable jumper for elevated or suppressed zero calibrations (vs. 1150 Series with turret post design requiring jumper wire and soldering)

 

[shiftman]

Thanks Dan,

I'm sure that this type of old xmitter was very popular in its day.

 

[waross]

Zero and span affect the input, not the output.
Example:
The process set point is 86 PSIG.
Shutdown alarms are set at 78 PSIG and 102 PSIG.
Solution 1; zero = 0 PSIG and span/range = 120 PSI.
Solution 2; zero = 70 PSIG and span/range = 40 PSI. (70 PSIG = 4 MA, 110 psig = 20 ma)
Pressures below 70 PSIG and above 110 PSIG are off scale.
This also has the effect of dividing the proportional band by 3.
Remember that PID controllers, ( and P and PI controllers) were originally mechanical/pneumatic devices. The accuracy of the modern electronic equipment was usually unobtainable. It was fairly common to use elevated zeros and reduced spans to realize tighter control.
Interaction between the span and zero was fairly common with the pneumatic transmitters. Old school calibration always included several iterations to check for and correct zero:span interaction.
When the first electronic transmitters hit the market, many still exhibited some interaction, often one way. eg: span may affect zero but zero didn't affect span.
Old school calibrators would always check for interaction until they were confident that a specific model had no or limited interaction.
If the span adjustment did not affect the zero, then the zero would be adjusted first so as to reduce the number of calibration iterations.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[shiftman]

Thanks for the reply.

"Zero and span affect the input, not the output" That is certainly one way to describe the zero/span relationship.
Zero adds and subtracts while span multiplies and divides; Zero shifts the input/output curve along the "x" axis, while span changes the input/output slope along the "y" axis are just a couple of additional definitions that one might find, but your point is well taken and appreciated.

I have seen xmitters that have a direct correlation between zero and span; xmitters that have no interaction whatsoever and xmitters that one affects the other to some degree. As I am finding out, the world of instrumentation is very broad indeed.