Optimising Response Time; Non-Interfering; Interfering; Contaminating - Ametek 303B User Manual

Optimizing Response Time
The built-in bypass system is always used to optimize response time. Un-
less the sample gas is hazardous or costly, bypass flow should be as high
as possible. The bypass system also is valuable with the instrument in
portable service analyzing discrete samples. In these applications, bypass
helps to purge gas cylinder valves, pressure regulators and tubing prior to
sampling.
Non-interfering, Interfering, and Contaminating Gases

Non-interfering

The cell is not affected by gases such as nitrogen, methane, and many
aliphatic and aromatic hydrocarbons. The instrument is ideally suited for
analysis of these gases.

Interfering

When analyzing moisture vapor in hydrogen, oxygen, or gas mixtures
containing a high percentage of either gas, the display will read artificially
high. On air for example, it may read up to 10 percent high. This is caused
by the electrolysis of moisture formed by recombination of free hydrogen
or oxygen. A recommended flow doubling technique to correct for hydro-
gen and oxygen is described in Chapter 4.

Contaminating

These substances damage the instrument or cell. Instrument readings may
or may not be immediately affected.
Certain corrosive acid gases will corrode the tubing and flow control
sys-tem, but will not harm the cell. The reading will not be affected until
the damage is sufficient to cause line leaks, poor flow control, etc. Typical
ex-amples are chlorine and sulfur dioxide.
Certain unsaturated hydrocarbons gradually coat the cell with a polymer-
ic coating. The reading is affected gradually as the cell becomes inactive. A
typical example is butadiene.
Entrained liquids and solids also can affect cell performance. Filtering
usually prevents this problem.
Do not allow liquids to pass through the cell.
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