 We are discussing how to keep our flow measurements (and our heat-treat processes) accurate. Part 1 talked about flowmeter sizing and gas measurement. The discussion continues in Part 2.

Pressure Compensation
Equation (2) allows us to calculate the actual flow when a change in pressure occurs.

(2) Fa = Fi x √P2/P1

where:
Fa = actual flow
Fi = flow indicated scale reading
P2 = new inlet pressure + 14.7 (English system) or new inlet pressure + 101.3 kPa (Metric system)
P1 = nameplate (calibrated) inlet pressure + 14.7 (English system) or nameplate (calibrated) inlet pressure + 101.3 kPa (Metric system)

A summary for some common values is shown in Table 2. If it is necessary to compensate for both temperature and pressure, Equation (3) should be used.

(3) Fa = Fi √ P2/P1 x T1/T2

Specific Gravity Conversion
One of the most common problems seen in heat-treat shops is that operating personnel and supervisors are unaware of the consequences of flowing a process gas through a flowmeter that is calibrated for a different gas.

When switching gases, the difference in specific gravity of the two gases is the principle factor that must be taken into account. Specific gravity is the ratio of the density of a gas to the density of dry air at standard temperature and pressure, 77ºF (25ºC) and 1 atmosphere (14.7 psi or 101 kPa).

Equation (4) can be used to calculate the actual flow when another gas is being metered:

(4) Fa = Fi x √SG1 x SG2

where:
Fa = actual flow
Fi = flow indicated by the scale reading on the flowmeter
SG1 = specific gravity of the “nameplate” gas (the gas the meter is calibrated for)
SG2 = specific gravity of the gas to be used in the flowmeter

Specific gravities of many of the gases commonly used in heat treating are listed in Table 3.