Interferometer Method Sensor: FI
Stationary sensor
Example: FI-23

1. Brief description

This gas detector, one of the oldest gas sensors of ours, recognizes changes in therefractive index of gas. With a high accuracy, it maintains stability over the long term.
In early times, it was used inside coal mines to measure the methane concentrationand in recent years, it is widely used to measure solvent concentrations or heat
quantities of fuel gases such as natural gas.

Category

Detectable gas

OpticalCombustible

2. Structure and principles[Structure]

The light source emits light, which is split by a parallel planemirror into two light rays (A and B) and reflected by a prism.
Ray A makes one round trip within the gas chamber, D,
where the detectable gas flows, and ray B makes one roundtrip within the gas chamber, E, where the reference gas flows.
The two light rays, A and B, meet each other at point C of theparallel plane mirror, and form an interference pattern on theimage sensor through the mirror and lens.

[Conceptual rendering of the sensor elements]

An interference pattern moves in proportion to the differencein the refractive index between the detectable gas andreference gas. The light wave interferometer-based sensor
measures the distance the interference pattern has travelled
to determine the refractive index of the detectable gas andconvert it to a gas concentration or heat quantity.

Lightsource(LED)[Principles]

3. Features
The travel distance of the interference pattern, ∆θ,measured by this sensor is represented as theequation below:

273.15101.325

L: Gas chamber length
nGAS : Refractive index of the detectable gas
nREF : Refractive index of the reference gas
λ: Light source wavelength
T : Temperature
P : Pressure

Gas chamber
DParallelplane mirror

CPrismEBDALightsource(LED)

MirrorInterference
pattern
ImagesensorLens○ Output characteristics

Since the change in the refractive index is proportional to the change in gas
concentration, the sensor provides a very high linearity.

○ ResponsivenessThe sensor finishes measurement by completing the substitution within thegas chamber with a volume of 0.5 to 5 mL. Some models finishmeasurement in 5 to 10 seconds with a 90% response.

○ Aging characteristics

The most striking feature of this sensor is that it does not degrade insensitivity. The sensitivity of the sensor depends only on the gas chamber
length, L, and the light source wavelength, λ. Since both of theseparameters are invariant, the sensor provides stable sensitivity over the longterm. The optical element, even if soiled, does not affect the travel distanceof the interference pattern; therefore, the sensor does not degrade insensitivity so long as it can recognize the pattern.

○ Pressure and temperature characteristics

Although the refractive index of gas varies depending on the temperature, T,and pressure, P, the sensor measures the temperature and pressure tocorrect them and therefore is not affected by them.

4. Measurement type, detectable gas, molecular formula, anddetection range (examples)
Measurement type
Puritymeasurement
Solvent
concentration
measurement
Calorimetricmeasurement

Detectable gas

Molecular formulaHydrogenH2

Sulfur hexafluoride

SF6

Carbon dioxide

CO2Toluene

C 7H 8l

Detection range
0-100 vol%99.50-100.00 vol%Vinyl chloride

C2H3ClMethyl ethyl ketone

C4H8O

Natural gas-25-55 MJ/m3Propane air-0-75 MJ/m3

Butane air-0-70 MJ/m3

5. Products of this type (examples)○Stationary products... FI-900, FI-915○Portable products... FI-8000

0-100% LELFI-900