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Getting real about pressure transmitter accuracy - ISA Interchange

04 Jun.,2025

Getting real about pressure transmitter accuracy - ISA Interchange

This guest post is authored by Ted Dimm, field instrumentation marketing leader for Honeywell Process Solutions.

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Specifying the right pressure transmitter for replacement of non-working units to install in a plant-wide upgrade or for use in a start-up facility is critical. Seemingly small differences in accuracy and performance can have significant long-term effects. Product quality can vary, and downtime for troubleshooting and repairs can increase. Money, time and other equally precious resources can burn up quickly, affecting the efficiency, profitability and sustainability of the plant. Pressure and other critical measurements must be reliable, stable and accurate.

Those who are tasked with specifying pressure transmitters for industrial applications know how essential it is to get units in place that will work well and long under the harsh and variable conditions of their process plant. However, when considering what transmitter to use in an application, specifying engineers or consultants may perceive a risk in justifying a change to a new brand or type of transmitter (“We’ve used these for 20 years at this plant, and they work just fine!”). And after working with the same equipment for years, the plant crew has learned the ins and outs of the pressure equipment they work with every day, so there may be reluctance to become familiar with new “unknown” equipment. So unless there are obvious plant performance problems easily tied back to the status quo, the human tendency for inertia can influence a company to just keep ordering “more of the same.” The trouble with this strategy is that technology marches on – and due diligence must be done every time equipment is purchased to make certain that buying decisions are based on the current state of technology, and not on history or tradition.

There are budget concerns in any decision, so the cost vs. performance trade-off can also be a determining factor in specifying equipment. The accuracy vs. investment conundrum should never totally drive the purchasing decision, because in the long-run the costs associated with a less accurate or less hardy pressure transducer will be outweighed by its negative impact on the plant’s processes, financial position, and maybe even employee safety.

So considering the known impact that a pressure transducer has on plant performance and optimization, the manufacturer’s specifications are a logical place to start in the decision process. Why not just line up all the product brochures, compare them, and select the one with the tightest specifications?

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The answer is that product specs are a good starting point for analysis. But they don’t tell you the whole story – and what they (unavoidably) leave out is the answer to questions like these: How will this transmitter work at MY plant under MY operating conditions? Will it work in the dirty, noisy environment of the process line? What happens to the pressure output accuracy when ambient temperature or static pressure changes? Is the effect of these changes on output accuracy uniform across a range from zero to span? What happens if the operator changes the span after installation?

It doesn’t pay to make hypercritical decisions about pressure transmitter accuracy based simply on the manufacturer’s specification sheets, especially when the specs were written while the product was calibrated and tested under controlled laboratory conditions. An industrial plant is many things – but it is definitely not a laboratory.

The five elements with a meaningful effect on the overall performance of pressure transmitters are:

Turn-down-ration effects (resulting from customer-selected span setting)
Temperature effects on zero
Temperature effects on span
Static pressure effects on zero
Static pressure effects on span

Compensating for these influencing factors is the key to accurate measurements from zero to span. Sensors, which lie at the heart of the transmitter, are now available with built-in automatic compensation for these variables so that the output is accurate and dependable, potentially saving everything from lives to dollars and cents. Keep your facility up and running with the lowest maintenance costs, the least installation headaches, and best overall accuracy in the real world by using a pressure transmitter with compensation at the sensor level. Your plant is probably down and dirty – and you need a reliable pressure monitoring source that will perform under whatever conditions come its way.

Are you interested in learning more about pressure transmitters manufacturers? Contact us today to secure an expert consultation!

Question for Pressure Transmitter Installation Location in Liquid ...

Hi,

I would like to get opinions about pressure transmitter installation location in liquid service.
When measuring liquid pressure in pipeline, the pressure transmitter generally is installed below pipeline with a side pressure tap on pipeline.
This purpose is to vent freely entrapped gas/vapor(it can cause the measuring error) in liquid fluid through impulse line back to pipeline. Moreover, it can prevent the slurry or sediment in liquid fluid from entering impulse line(it can block the impulse line).
This is general practice as i know.

However, some owner or licensor recommend always installing a pressure transmitter above pipeline regardless the fluid status like vapor/gas, condensate, liquid, steam), and they tell it is for preventing measuring error caused by static head pressure.

I agree with that, but it is hard to understand easily.
So, i tried understanding with the fluid basis and Bernoulli's equation and the below is my conclusion.

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If the transmitter is installed at the below of pipeline with side pressure tap, the hydrostatic pressure generates due to an effect of elevation “h” which is impacted to gravity.
This makes the arbitrary datum plane “z” change from pipeline’s elevation(z) as the original reference level to transmitter’s elevation ground(h).

Vice versa, if the transmitter is installed above pipeline, the transmitter can measure only static pressure in pipeline since there is no impact of the hydrostatic pressure.
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Is it right? please give your help for my understanding. If your impulse line is at an angel upwards then yes, I suppose you could argue that the side orientation prevents gas being trapped there.

However trapped gas doesn't impact the pressure reading, but would affect transmission of pulses of pressure.

In practice though most above ground pipes run on low level sleepers or supports and having your instrument at ground level is difficult to access and protect.

A lot of pressure transmitters are just direct mounted or run off the top of the pipe ( your small tapping is more protected and doesn't cause an obstruction) and then the TX is placed on a pole close to the tapping point.

Unless the TX is located many metres different in height, the Z value is often ignored in reality unless you're dealing with very low pressure fluids.

Your diagram is providing the difference between a tapping point on the outside of a pipe versus a pitot tube pointing opposite to the direction of flow. That's something completely different. Again in reality unless you've got a very high velocity low pressure liquid, the dynamic pressure is often ignored as it's very small in comparison to static pressure.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it. Perhaps there are some authoritative sources which recommend locating the pressure tap on the side of the pipe, but I haven't seen them. And even if they exist, I'm not buying it. A vertically oriented tap minimizes accumulation of debris, and it doesn't cause any error at all in the pressure reading. The presence of a gas pocket in the instrument tubing simply dampens the pressure impulses but it doesn't change the pressure value. It seems to me that a gas pocket is actually beneficial because pressure readings in an incompressible system can be noisy.

Also, side mounted pressure taps will unnecessarily obstruct space in the piperack. This issue is usually only relevant to flow meters or level sensors which are measuring low differential pressures (<100 in H2O). In these cases if both lines are kept full of liquid or gas, then the error cancels out. So impulse lines are often purged to insure this, and to keep out dirt. In most other cases the error due to impulse lines can be ignored. However, every situation has to be evaluated individually. Thanks for all your replies.
I get to know that the gas entrapped doesn't give any impact to reading and I agree with the side tap orientation tends to make obstruction in the pipe rack.

However, unlike your replies, API RP 551 and PIP which are international standards, as well as most of the transmitter manufacturers such like Emerson/Yokogawa/ABB/E+H etc, have been recommending the preferred tap position as 45 degrees below horizontal plane or the side for liquid measurement.(of course, these documents recommend using a short impulse line as possible.)

Could you give any explanation to me why international standards and manufacturers still prefer and recommend this design against your reply as a top of the pipe?

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