why the output of a pneumatic instrument is 3 - 15 psi. 3 psi for 0% and 15psi

I suspect its analogous to the 1 - 5V range used by some electric sensors. The zero point of 3 psi can be used to indicate a fault condition (< 3psi means "no primary pressure" (power supply), or "disconnected or leaking feed from sensor" (broken wire).

Why the maximum of 15 psi? Don't know, but it's got to stop somewhere!

I think you can tune sensors, actuators and controllers to act in certain ranges. If you have 4 actuators in an air handling unit, and want to set up a progressive open situation on the dampers, you can stack them, and have them ranged to open at different pressures. I know that also, they use this strategy for cooling valves, free cooling (outdoor air dampers) as a pair, or alternatively, exhaust dampers and heating valves.. They use a graph of the rise in air pressure, versus actuator percent open, to communicate the sequence of operation.

its been a decade or so, but i would chase up an ASHRAE set of handbooks to tell you all about it.

Chris

There is a certain amount of hysteresis in pneumatic actuators.The 3 psi acts as a preload to help overcome this.The standard was adopted long before electronic controls, so when the electronic controls (4 20 millamps) was adopted, it was backward compatable. Not all pneumatic controls follow the 3 15 psi standard.I have seen 8 to 13 psi on some controls and many variations, however, they all include a certain degree of preload to offset hysteresis.

Differential voltage signals in electronics are a different matter,but similar goal.

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Why did you post twice?

Why not put a post in the first one (as someone else did for you!!) to get them to go to your second attempt only?

Wioll there be a third attempt? Should I wait for it?

Physic's, process pressures against valve bench set via diaphram size thus transmitter/controller outputs and that is just the tip of the iceberg.

3 to 15 PSI is standard but not the only.

I have used sensors of various ranges for various applications and a general answer for you is to tht with many transducers the output range of the sensor is 0 20mA.

at 0 signal output, you do not know if you are getting 0 or your bottom pressure range or if you have lost excitation to your sensor. Therefore, having your 0 outpt at 3 or 4 mA is giving you an actual signal and you know you are getting a proper reading. secondly the 15 high reading is to limit the range of the sensor to an reasonable operating / output range. if you had a sensor that could read up to 100 then when operating only up to 15 you would be using a very small portion of the output and your ability to read detail would be lost. Look at it this way, if your speedometer on your car read 0 to 1000 and you typically drove at 25, your needle would hardly move.

Typical pressure sensors of insturmentation is most accurate between 20% and 80% of the sensor scale if you operated close to the low or high end of scale you loose accuracy. it is best to choose an appropriate pneumatic insturment for the range of operating conditions for accuracy.

Protocol.

Without knowing the specific model instrument or the instrument designer's objective, one can only make wild guesses. We're good at that, here. Don't forget, at these pressures you should also ask is this relative (above atmosphere) or absolute pressure levels. Possibly, the designer wisely recognized that internal friction forces meant readings below 3 PSI were inaccurate and meaningless.

very interesting! GA.. please tell more!

Chris

It is called "LIVE ZERO pressure" in the same way 4 to 20mA signal & 6 to 20V the min. 4mA and 6V are called live zero signal. That Is the is in an open circuit the control instrumentation will detect the breakdown and it will not treat open or short circuit as a signal.

An I/p converter works for the input signal between 4 to 20 ma.So for 4 ma input signal the I/p output should be 3psi & for 20 ma the i/p output should be 15psi

I know I am late to the party, but I feel compelled to note the posting by Keith Bowers on 3/23/09 cannot be true. He states that ISA developed the 3-15 psi standard in 1935-ish, prior to WWII. This is untrue since ISA was not even founded until 1945, after WWII (http://www.isa.org/Content/NavigationMenu/General_Information/About_ISA1/ISAs_History/ISAs_History.htm)

Now perhaps that is a minor detail, but the point is that if the 3-15 psi standard was established in 1935-ish, then it wasn't ISA that had anything to do with it. Frankly I am not sure where it originated, but perhaps it evolved as a de-facto standard that ISA, once it came into being, adopted. If anyone can provide more detail, I would appreciate it.

FWIW:

ISA's monthly magazine, Intech, published an article in August 2003 entitled "Top Technologies and Events" in which they cite the ISA S7 standard as, "S7, the original 3–15 pounds-per-square-inch air pressure standard;".

However, S7 is titled "Quality Standard for Instrument Air" and that is exactly what it is, a standard that defines how not-dirty (allowed particle size) and how dry instrument air needs to be. But S7 does not define 3-15 psi as a standard range, like the ANSI-ISA 50 standard does for 4-20mA.

With regard to ranges, SP7's table B.2 actually provides minimum supply pressures for several pneumatic transmission ranges:

The comment below reveals that 15 psi on the high side was being settled on in the 1920's:

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"The process for cracking crude oil at very high temperatures and pressures was developed in the 1920s. During this period, Thomas Neilan founded the Neilan Company on the West Coast to serve the needs of the petroleum industry. The company developed a line of regulators as well as models of remarkably sophisticated pressure and temperature controllers. By this time, automatic control was booming, and the concept of using 15 psi air pressure for actuation came into being. The live zero lower end of the scale remained unsettled for some years."

http://www.globalspec.com/reference/13607/179909/Chapter-2-The-History-of-Control-Valves
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A Bailey history provides this insight into 3-15 psi, but without any corresponding dates.

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BAILEY METERS IN CROYDON, SURREY, UK
Development of pneumatic systems

The first control devices to be sold by the company were based on US Bailey's "Customline" design. These cumbersome mechanical components communicated with each other by means of air-pressure signals transmitted along copper pipes. The original design worked on a pressure range of 5-25 psi (roughly 0.3 to 1.7 Bar in today's terminology). This meant that when a parameter such as temperature was at the lower end of the measurement scale the instrument transmitted a pressure of 5 psi; at the opposite extreme of the scale it transmitted 25 psi.

The industry – which was largely dominated by American manufacturers in those days – used a variety of pressure ranges for these signals, which meant that it was not always possible to assemble a system that used, say, transmitters made by one manufacturer with controllers made by another. Eventually, common sense prevailed, and the industry gravitated to a single range of 3-15 psi (0.2-1.0 Bar). This range is still in use after almost 100 years of modern process-control development!

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Maybe some old guy will crop up and enlighten us.

Iris, Thank you very much. This is looking more and more like a de-facto standard that was already in place before ISA was established. I am thinking 4-20 milliamps came about the same way, in the 1960s. My understanding is that some IBM machines used 4-20 current loop before it became widely used in process control.

Pat, check out Dick Caro's article where he covers the process generating a standard in industrial networking - it's fascinating (link below)

In that article, he mentions the 11 year ordeal of establishing 4-20mA as a standard, given Foxboro's market position with 10-50mA.

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Before the 4-20 mA standard, there was the 3-15 psi
standard. It was 40-200 kilopascals internationally. It's the
same stuff, and actually that's pretty close, but a little different.
The 4-20 mA became the world standard for the electronic
analog transmission that had been reduced from two.
There was a 10-50 mA range that Foxboro was proposing,
and a 4-20 that everybody else wanted. Foxboro held out
because they didn't have the technology early enough to do
4-20. Eventually, they got the technology in place and agreed.
That alone took eight years. The 4-20 mA standard took
11 years.
http://www.industrialethernetu.com/pdf/109.pdf
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My personal anecdote: I met a former Honeywell engineer some years ago who worked on the very first 4-20mA two wire transmitters. He told me that "live zero" was in part marketing to cover up the fact that the first generation transistors wouldn't completely turn off, so using a live zero at 4mA helped disguise that. But selling 'live zero' required no magic in an industry already accustomed to the live zero in pneumatic transmission ranges.

Iris,

You are a wealth of information, thank you. The article by Caro is very helpful

Welcome to CR4. fyi, we just click the button and give a "GA" (good answer) vote to posts that are helpful and true, to show others what most believe is valuable. It helps separate the wheat from the chaff.

Cheers,

Chris