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What is Zero suppressor and Zero elevation in level transmitter?

Regards,
Shailesh C Patel

If you are mounting the transmitter below the datum line of the tank, the transmitter signal will go above 4 mA when the High side impulse line fills up and this will give a zero error compared to the tank actual zero level. So you have to suppress that head pressure. This is done by adjusting the signal to 4 ma. This is zero supression. Similarly if you mount the transmitter above the datum line you have to raise the signal from 4 ma to avoid the error when the hi-side impuse line fills up. This is zero elevation. But if you are using a transmitter with HART protocal you can do this just by changing the cal range using the HART communicator.

Pretty basic stuff... cut and paste this link... it will clarify your questions.

http://www.emersonprocess.com/rosemount/document/pds/Pressfund.pdf

Cheers!!

In signal conditioners in general, zero suppression and zero elevation refers to lowering (suppressing) or raising (elevating) the nominal "zero" output such that it is not at zero volts (or milli-amps).

A typical application for this is where it is desired to use an instrument which produces a +/- 10 volt signal, and take this signal into a 0 - 10 volt input. To use the full measurement range, you would need to "elevate" the zero point to +5 volts, and rescale the range to fit within 0 to 10.

Vinod has this backwards I believe. A suppressed zero would be say 20 to 100 "W.C. Elevated would be -20 to 100 " W.C.

Cut and paste this link and you will find some clarification:

http://www.emersonprocess.com/rosemount/document/pds/Pressfund.pdf

Hope this helps.

Cheers!!

It relates to the value corresponding to 4 mA in your transmitter range.

If your 4 mA point is above zero, e.g. range of 48.75 to 50 then you are doing suppression.

If your 4 mA point is below zero, e.g. range of -50 to -48.75 then you are doing elevation.

Elevation and suppression are chiefly used in differential pressure measurement since this make it possible to measure liquid levels with wet legs and remote seals etc. and was a "big deal" in analog days. These days it is all done in software and nobody ever asks for it anymore. You will find that the capability of a transmitter to do elevation and suppression was stated in % and relates to rangeability (turndown). A popular old pressure transmitter had a 6:1 rangeability and therefore used to sport 600% elevation and 500% suppression. These days most pressure transmitters have capabilities in order of several thousand percent so it is never even discussed.

Jonas Berge
SMAR
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As a little historical footnote to the previous responses that cover the question very well it should be noted that elevation & suppression option kits were mounted to pneumatic dp cells/transmitters in order to adjust for the location of the transmitter relative to the (lower) tapping point when the required adjustment was beyond the narrow adjustment capacity. Since the zero is now adjusted electronically as part of normal adjustments without the use of a separate device on a measuring cell the whole concept of elevation/suppression is no longer relevant.

As far as the terms suppression/elevation there was always confusion since there were two schools of definition (i.e. Honeywell & Foxboro) and they were opposite (I can not remember which was which). It depends on how you look at what happens to the zero as to whether you call it suppression or elevation. With a transmitter mounted below the lower impulse tap the transmitter output was higher than was needed. Hence one school of definition would say that the zero signal would need to be suppressed. However, the other school of definition would refer to the kit as an elevation kit since it would raise the "virtual" position of the transmitter. So both terms were used for the same installation. What happened from physical calibration sense was identical... just that the definitions came from opposite veiwpoints (cf with the two definitions of reset/integral action in the PID algorithm from Honeywell and Foxboro... in the end it provides the same algorithm).

I am in a training program for Instrumentation Technician, as a Steamfitter. I became frustrated seeing contradictory answers to which is Zero Suppression or Elevation on our practice tests and the instructional material from ISA.org. After some research here and following the leads given in the previous posts, I gathered varied theories and methods are followed.

What I would like to wrap my head around is how my school is working their method. From what I understood of the explanation: If the question is asking about a Level Transmitter located below the 0% level of the tank and it is asking for a solution, outside of the instrument, then it's "Zero Elevation" -- because it's a virtual displacement. Yet, if the question is asking for a solution involving a mechanical or electronic adjustment "inside" the instrument, then it's a "Zero Suppression" -- because the zero is being "pushed down".
The simplified explanation of this was, "If the difference is on the High side, it's "Zero Elevation"; if it's on the low side, it's "Zero Suppression".

I've done a lot of reading and research on this and found the conventional method to be; when the sensing element is below 0% of the tank it's "Zero Suppression". However, to get the right answer on the test, I need to understand how they are working their theory. My best guess as to why they took the route they did is to develop a homogeneous method which works for methods and when I show up at a plant and they will have their own method. To get there, I'll need to learn the training given and ace the test.

I need a light bulb here.

This may be of assistance:

http://www.superdcs.com/zl/1199.PDF

http://www.spartancontrols.com/~/media/Library%20Documents/Measurem ent-and-Analytical/Rosemount/Pressure/Manual/Rosemount_2024_Differential_ Pressure_Transmitter_Manual_March_2007.ashx

William (Bill) L. Mostia, Jr. PE
Sr. Consultant
SIS-Tech Solutions

Hi Michael,

Yes, this is a confusing area. The confusion is because there are two different "zero"s being referenced. One is where the value of the measured parameter is 0 in engineering units - 0 degC, 0 kPa, 0 mm. The other is 0 % of the measured range - better referred to as per ISA definitions as the LRV or lower range value.

Zero suppression is when the zero value of the measured parameter (or input to a system element) is below the LRV of the output range - for example, the standard 4-20 mA current range has a suppressed zero since 0 mA is effectively -20 %. Zero elevation is where the zero value of the parameter is above the LRV - for example, a temperature range of -50 to +150 degC.

With a level transmitter located below the 0 % level of the tank, there will be a differential pressure present at 0 % level. If we look at the relationship between the level range being measured and the differential pressure, zero differential pressure (the "output) occurs at a level lower than 0 % on the level range (the input). Looking at the conversion of level to differential pressure we have an elevated zero. The differential pressure transmitter will be set up so that the 0 % or LRV output is developed at a positive differential pressure, so this stage of the conversion will be suppressed zero. The desired indication is for 0 % level to give 0 % signal out, so one part of the system will have a suppressed zero and the other will have an elevated zero. If you sketch a graph showing the value of each of the effects considered (differential pressure, transmitter output) in terms of the original level range you should see this quite clearly.

Hope this helps,

Bruce.

For elevation and suppression, please refer to the technical white paper on this page:
http://www.eddl.org/DeviceManagement/Pages/Calibration.aspx

As per ISA51.1:
Suppression: The lower range value (4 mA point) is above zero, for instance a range of 20 to 100

Elevation: The lower range value (4 mA point) is below zero, for instance range of -25 to +100, or -100 to 0, or -100 to -20

Cheers,
Jonas