Ambient THC Monitor

The APHA-370 uses a combination of the flame ionization detection method and selective-combustion.
This gives it the advantage of the single-detector method plus the ability to perform continuous measurements, free of zero-drift, for THC, NMHC, and CH₄. The design gives great stability and high sensitivity (0-5 ppm F.S.)

The APHA-370 has a relative-sensitivity correction function for CH₄ and NMHC.

All the necessary features are built right into a single rack-sized instrument, including a catalytic unit for selective combustion (i.e., an NMHC cutter); a cataytic unit for generating reference gas and auxiliary combustion air (standard); and a sampling pump. The only supplemental gas required is H₂.

Flame ionization detection method (FID) with selective-combustion
The flame ionization detection method (FID) — used in combination with the selective-combustion system — utilizes the ionization that occurs as the result of the high-temperature energy from combus-tion at the tip of the burner jet when organic carbon compounds are introduced into the hydrogen flame. The hydrogen flame is located between two elect-rodes.
When an electrical voltage is applied across these electrodes a minute ion current proportional to the hydrocarbon concentration is produced. This current is monitored by a low leakage amplifier, giving a voltage readout for THC. To measure CH₄ the sample gas is passed through the selective catalytic combustion unit (the NMHC cutter), which oxidizes NMHC without oxidizing CH₄. This is shown as A below. B represents the THC concentration measured without passing the gas through the NMHC cutter. Thus B - A will give the concentration of NMHC. The final concentration value is calculated using a relative-sensitivity correction coefficient, k, as shown below.
CH₄ Concentration A
NMHC Concentration k (B - A)
THC Concentration A + k (B - A)