Sierra 830, 860, 840 instruction manual Example

For our purposes, we want the ratio of the flow rate, Q1, for an ac- tual gas to the flow rate of a reference gas, Q2, which will produce the same output voltage in a particular mass flow meter or control- ler. We get this by combining equations (1) and (5):

Please note that the constant b cancels out. Equation (6) is the funda- mental relationship used in the accompanying tables. For conven- ience, the tables give “relative” K-factors, which are the ratios K1/K2, instead of the K-factors themselves. In the tables, the relative K-factor is

Kactual/KN2 where the reference gas is the commonly used gas, nitrogen (N2). The remaining columns give Cp and ρ, enabling you to calculate K1/K2 directly using Equation (6). In some instances, K1/K2 from the tables may be different from that which you calculate directly. The value from the tables is preferred because in many cases it was ob- tained by experiment. Sierra calibrates every transducer with primary standards using the actual gas or a molecular equivalent reference gas. The calibration certificate accompanying the transducer cites the refer- ence gas used.

Example 1:

A transducer is calibrated for nitrogen (N2), and the flow rate is 1000 sccm for a 5.000 VDC output signal. The flow rate for car- bon dioxide at a 5.000 VDC output is:

QCO2/QN2 = KCO2/K N2, or

QCO2 = (0.74/1.000)1000 = 740 sccm

Example 2:

A transducer is calibrated for hydrogen (H2), and the flow rate is 100 sccm for a 5.000 VDC output signal. The flow rate for nitrous oxide (N2O) is found as follows:

QN2O/QH2 = K N2O/K H2, or

Q N2O = (0.71/1.01) 100 = 70.3 sccm

Note that the K-factors relative to nitrogen must be used in each case.

Example 3:

We want a transducer to be calibrated for use with dichlorosilane (SiH2Cl2) at a 100 sccm full scale flow. We wish to use the pre- ferred reference gas Freon-14 (CF4). What flow of CF4 must we generate to do the calibration?