Sierra 860, 840, 830 Calculating Dual Gas Mixtures, Important Note about K-FACTORS

Appendix A Series 830/840/860 Instruction Manual

Calculating Dual Gas Mixtures

Equation (6) is used for gas mixtures, but we must calculate N/ρCp for the mixture. The equivalent values of ρ, Cp, and N for a dual gas mixture are given as follows:

The equivalent gas density is:

!

•

•

!

•

•

!

= ( m

/ m )

1

+ ( m

/ m )

2

1

T

2

T

Where:

•

•

•

= Total mass flow rate (gm/min),

mT

= m1 +

m2

(

)1

= Refers to gas #1, and

(

)2

= Refers to gas #2

The equivalent specific heat is:

Cp = F1Cp1 + F2Cp2

Where:

•

!

•

!

) and

F1 = (m1

1 )/(mT

•

!

•

!

)

F2 = (m2

2)/(mT

The equivalent value of N is:

•

•

•

•

! = ( m1 / mT ) !1 + ( m2

/ mT ) !2

The equivalency relationships for ρ, Cp, and N for mixtures of more than two gases have a form similar to the dual-gas relation- ship given above.

IMPORTANT NOTE ABOUT K-FACTORS:

Please note that if you have a transducer calibrated for a gas such as methane and wish to use the K-factors to measure a gas such as air, that the inaccuracy of the measurement can range from ±5 to 10%. The use of K-factors is, at best, only a rough approximation and should not be used in applications that require a better than ±5 to 10% accuracy.

It should also be noted that certain gases, in similar “families,” will work exceptionally well with K-factors; however, those instances are only true when similar thermal properties of the gas are pre- sent.

A-6 IM-83/84/86-H