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SEMI E28-92 © SEMI 1992, 2004 2 4.1.3 ambient press ure — the absolute pressure of the medium surrounding the MFC (see Figure 2). Figure 2 Pressure Definitions for MFCs 4.1.4 burst pressure — th e gas pressure at which t…

SEMI E28-92 © SEMI 1992, 2004 1

SEMI E28-92 (Reapproved 1104)

GUIDELINE FOR PRESSURE SPECIFICATIONS OF THE MASS FLOW

CONTROLLER

This guideline was technically reapproved by the Global Gases Committee and is the direct responsibility of

the North American Gases Committee. Current edition approved by the North American Regional Standards

Committee in July 11, 2004. Initially available at www.semi.org September 2004; to be published November

2004. Originally published in 1992; last published February 1999.

1 Purpose

1.1 The purpose of this guideline is to establish a

uniform, worldwide means to describe pressure

parameters as they relate to mass flow controllers. It is

intended to prevent confusion and misunderstanding

between manufacturers and users.

2 Scope

2.1 This guideline contains definitions of terms which

describe gas pressure in mass flow controllers as used

in the semiconductor industry. SI units are the

reference units for this document.

NOTICE: This standard does not purport to address

safety issues, if any, associated with its use. It is the

responsibility of the users of this standard to establish

appropriate safety and health practices and determine

the applicability of regulatory or other limitations prior

to use.

3 Referenced Standards

3.1 SEMI Standard

SEMI E12 — Standard for Standard Pressure and

Standard Temperature for Flow Units Used in Mass

Flow Meters and Mass Flow Controllers

NOTICE: Unless otherwise indicated, all documents

cited shall be the latest published versions.

4 Terminology

4.1 Definitions

4.1.1 units of pressure — several units of pressure are

commonly used in conjunction with MFCs. The Pascal

is the preferred unit of pressure for use within the

semiconductor industry. Units of pressure include the

following:

 Pascal (Pa)

 Pounds per square inch (psi)

 Torr (T)

 Kilograms per square centimeter (kg/cm

)

 Bar (B)

NOTE 1: Units of pressure are sometimes expressed as an

equivalent height of a column of some liquid, such as

millimeters of mercury or inches of water. These units

require correction to some standard for liquid density and

gravity. As these corrections are neither broadly standardized

nor often even addressed, their use should be avoided.

4.1.2 absolute pressure — the pressure measured

relative to zero pressure (perfect vacuum) (see Figure

1).

NOTE 2: Absolute pressure is the pressure illustrated by the

ideal gas law, PV = nRT. For example, when the number of

moles, n, equals zero (no molecules), absolute pressure, P,

equals zero. To indicate unambiguously that a pressure

measurement is absolute, the following abbreviations should

be used:

 Pa — Pascal (absolute assumed)

 psi (a) — Pounds per square inch, absolute

 Torr — Torr (absolute assumed)

 kg/cm

(a) — Kilograms per square centimeter, absolute

 B (a) — Bar, absolute

4.1.2.1 Units such as Pascal and Torr are customarily

absolute units.

Figure 1

Relationship Between Absolute, Gauge,

and Ambient Pressure

SEMI E28-92 © SEMI 1992, 2004 2

4.1.3 ambient pressure — the absolute pressure of the

medium surrounding the MFC (see Figure 2).

Figure 2

Pressure Definitions for MFCs

4.1.4 burst pressure — the gas pressure at which the

MFC may rupture.

4.1.5 differential pressure — the difference in absolute

pressure between two points of measurement in a

system (see Figure 3).

NOTE 3: To indicate unambiguously that a pressure

measurement is differential, the following abbreviations

should be used:

 Pa (d) — Pascal, differential

 psi (d) — Pounds per square inch, differential

 Torr (d) — Torr, differential

 kg/cm

(d) — Kilograms per square centimeter,

differential

 B (d) — Bar, differential

4.1.5.1 Gauge pressures may also be used in the

differential pressure calculation if consistency is

maintained. A common error would be to take the

difference between an inlet gauge pressure and an outlet

absolute pressure without first converting to common

units.

4.1.5.2 As it applies to an MFC, differential pressure is

usually the measured difference in pressures between

the gas inlet and outlet fittings of the MFC.

Figure 3

Definition of Differential Pressure for MFCs

4.1.6 gauge pressure — the differential pressure

measured relative to ambient pressure. For example,

when the pressure within a system equals the prevailing

ambient pressure, the gauge pressure equals zero (see

Figure 1).

NOTE 4: To indicate unambiguously that a pressure

measurement is gauge, the following abbreviations should be

used:

 Pa (g) — Pascal, gauge

 psi (g) — Pounds per square inch, gauge

 Torr (g) — Torr, gauge

 kg/cm

— Kilograms per square centimeter, gauge

 B (g) — Bar, gauge

NOTE 5: The performance of MFCs can vary significantly

with gas density. Atmospheric pressure varies with the

weather and altitude at various geographical locations. Gauge

pressure units commonly reference atmospheric pressure.

Therefore, the same gauge pressures measured at different

geographical locations may correspond to different gas

densities. For this reason, the use of gauge pressure units

with MFCs can be imprecise and should be avoided.

4.1.7 inlet pressure — the pressure at the inlet fitting

of the MFC (see Figure 3).

4.1.8 maximum operating pressure — operation is

permitted up to this inlet pressure, but performance is

not specified above normal operating pressure (see

Figure 2).

4.1.9 maximum overrange pressure — the maximum

gas pressure to which the MFC may be subjected

without degrading specified performance. When

returned to normal operating pressure, the MFC must

require no adjustment to return to specified

performance (see Figure 2).

SEMI E28-92 © SEMI 1992, 2004 3

4.1.10 MFC calibration pressure, inlet and outlet —

the inlet and outlet pressure at which the MFC was

calibrated (see Figure 2).

4.1.11 normal operating pressure, inlet and outlet —

the pressure range within which the MFC meets its

stated performance specifications (see Figure 2).

4.1.12 normal operating differential pressure — the

range of differential pressure (see Section 4.1.5)

required by the MFC to meet its stated performance

specifications.

NOTE 6: The upper and lower limits are dependent upon the

absolute inlet or outlet pressure. These limits are

manufacturer-specific.

4.1.13 outlet pressure — the pressure at the outlet

fitting of the MFC (see Figure 3).

NOTE 7: To completely specify the pressure operating

environment for MFCs, at least two of the following three

pressures must be listed: inlet, outlet, and differential.

4.1.14 proof pressure — the maximum gas pressure

the MFC may be subjected to without permanent

damage. Some adjustment may be necessary to make it

meet its specified performance when returning to

normal operating pressure. (see Figure 2.)

4.1.15 reference ambient — the composition and

pressure range of the ambient medium surrounding the

MFC within which performance specifications apply

without requiring correction for changes in the ambient

medium.

4.1.16 reference operating pressure, inlet and outlet —

the range of gas pressures on the inlet of the MFC and

across the MFC within which performance

specifications apply without requiring correction for gas

pressure effects (see Figure 2).

4.1.17 standard pressure — SEMI E12 defines

standard pressure as 760 Torr (101.32 kPa) (see Figure

2).

5 Gas Pressure Effects

5.1 Specified Gas — Gas pressure effects may be gas

species sensitive. The gas must be specified when

stating gas pressure effects. Nitrogen is recommended

as the standard gas.

5.2 Pressure Measurement Point — In this section,

pressure is assumed to be measured at the fitting of the

MFC, inlet or outlet, that is adjacent to the flow

transducer.

5.3 Total Calibration Effect — The change in output,

including zero and span, due to a change in gas pressure

from one normal operating pressure to a second normal

operating pressure. All other conditions must be held

within the limits of reference operating conditions.

5.4 Zero Calibration Effect — the change in zero due

to a change in gas pressure from one normal operating

pressure to a second normal operating pressure. All

other conditions must be held within the limits of

reference operating conditions.

5.4.1 The effect of gas pressure change on zero may be

expressed as a coefficient calculated as the ratio of full-

scale percent change in output to the corresponding

change in gas pressure. The change in gas pressure

should be specified. This coefficient is defined as the

“pressure coefficient of zero.”

5.4.1.1 Example — Pressure coefficient of zero may be

expressed as:

0.2% of full scale

240 kPa - 220 kPa



0.01% of full scale/kPa with N

NOTE 8: If the relation between gas pressure and change in

output is linear, one coefficient will suffice.

5.4.2 If the gas pressure influence is non-linear, a

different method of expression may be used. Two

examples:

5.4.2.1 The percent of full-scale change in output will

not exceed a specified value for any value of gas

pressure within a specified gas pressure range.

5.4.2.1.1 Example — “± 0.15% of full-scale maximum

error over 200 kPa to 250 kPa with N

”

5.4.2.2 It may be desirable to state a series of

coefficients for successive increments of gas pressure

within a specified gas pressure range.

5.5 Span Calibration Effect — The change in span due

to a change in gas pressure from one normal operating

pressure to a second normal operating pressure. All

other conditions must be held within the limits of

reference operating conditions.

5.5.1 The effect of gas pressure change on span may be

expressed as a coefficient calculated as the ratio of

percent of reading change in output to the

corresponding change in gas pressure. The change in

gas pressure should be specified. This coefficient is

defined as the “pressure coefficient of span.”