Answer / carl_ellis

Because it's a standard, widely used conventional pneumatic
control transmission range.

The 3-15 psi signal is a standard (ISA SP7) that came from
widespread use even before that standard came into being.
As early as the 1920's pneumatic regulators, pressure and
temperature controllers were using 15 psi at the high end.
Various low end 'live' or elevated zero values were used
before standardization.

Any pneumatic control or transmission signal starts with an
'elevated' or 'live' zero' at 3 psi because there is
internal friction and a fair amount of hysteresis in the
movement of pneumatic components, the flapper, nozzle,
diaphragms, spring preloads, pivots and bellows that make up
the transmitters and receivers. This friction and
hysteresis manifests as non-linearity below 3 psi. A 3 psi
minimum pressure puts the transmission signal into a linear
response range. Obviously, control system measurements,
signal transmission and the receivers need a linear response
to effect accurate and precise control.

Not all pneumatic control signals are in the 3-15 psi range.
The ISA SP7 standard (1996) that addresses instrument air
quality standards also lists 5-25 psi, 6-30 psi, 3-27 psi as
standard pneumatic transmission ranges, along with the
minimum supply pressure associated with each range. These
ranges all include an elevated zero.

Carl Ellis
Measure First

Answer / guest

posinar

HOW DPT WORKS?

6 Answers   Essar,

---

how to design orifice plate?what is the equation?

0 Answers   REL,

---

how is a displacer type level transmitter calibrated.explain the method?

2 Answers   Emirates,

---

what is the significance of hook up

3 Answers  

---

The DCS has the following signal type:

3 Answers  

---

What is the principle of capacitance type level transmitter?

0 Answers  

---

which type signal in & out in dcs

4 Answers  

---

How to adjust the zero & span of a control valve from the posiitioner(MIL)?

1 Answers   Bhel, SMC,

---

what is meant by cold junction compensation. where it is used.

1 Answers   SRACO,

---

what is rtd

4 Answers   Petrofac,

---

Good Evening all.My question is - For a feed water line to an equipment if i need to measure the flow we will use orifice /magnetic/vortex depend on cost/spec.If i need to measure a ''Alarm''signal and ''Interlock'' from one point i think we can use orifice type in one location with 2 different tapping points - one for alarm and one for an interlock to flow valve..If in case i need to have both these signals(Alarm & interlock) from magnetic flow meter,is it possible??will 2 seperate sensors will be located inbuilt r we can program it ??..i thk we can have one transmitter with these 2 signals...need to know any technical issues are there?pls clarify

3 Answers  

---

how 3 out of 2 Logic/interlocks works? give brief explanation with help of diagram.

0 Answers   Adani Power,

---