DE9315508U1 - Infrared absorption gas analyzer with separate optical cuvette - Google Patents

Description

··: "IHamburg, 14.07.1993

INFRARED ABSORPTION GAS ANALYZER WITH SEPARATE OPTICAL CUVETTE

1. Brief description of the utility model

Infrared gas absorption is one of the most important methods for measuring the gas concentration of heteroatomic gases. The product of the optical path length and the concentration of the gases determines the size of the gas absorption. For small gas concentrations, the length of the cuvette is of crucial importance for the absorption of the gas to be measured, since the number of gas atoms, which in turn depends on the optical path length in the cuvette, is directly proportional to the measurement signal. In traditional gas analysis devices, the gas measuring cuvette is housed in the housing itself. If the housing length is limited, e.g. 19", the length of the cuvette is also limited. A length of 200 mm can rarely be exceeded.

When using pyroelectric multi-channel detectors, the cuvette tube consisting of the emitter and measuring receiver can be housed separately, i.e. outside the housing, e.g. in the gas sampling probe itself. This means that optical path lengths of up to approx. 1000 mm can be achieved without any problems. Such handy gas sampling probes with an internal gas cuvette can play a major role, especially in portable infrared measuring devices.

2. State of the art

Infrared gas absorption uses two important methods to measure the concentration of heteroatomic gases. The older method uses a membrane capacitor as a measuring sensor. The newer methods use semiconductor or pyroelectric measuring cells. The measuring method with the membrane capacitor uses a gold-plated cuvette separated by a partition, the halves of which are alternately exposed to the infrared rays. This pulsating darkening and exposure is generated with a chopper wheel, usually driven by a synchronous motor.

The radiator usually has a diameter of at least 40 to 50 mm. The membrane capacitor is even larger and consists of a thin metal foil, usually made of titanium or aluminum, stretched over a frame. This foil bends as a result of the infrared radiation or the gas pressure generated by the heat, thereby changing the dielectric constant of the capacitor. The stretched foil must have a minimum size to allow effective bending. Membrane capacitors usually have a cross-section of at least 600 ... 700 mm ² . The outer diameter of the measuring receiver is then 50 ... 60 mm.

Neither the radiator nor the membrane connection: syfld for ttSrfBihbau in gas sampling probes

suitable. They are too large and too sensitive for that. For example, the membrane capacitor bends not only when exposed to heat or pressure, but also when subjected to simple mechanical shocks.

The analyzers with a semiconductor or pyroelectric measuring head have a more robust design. However, these receivers also have to be chopping because it is not the static, but the dynamic loading of these measuring cells that generates an analyzable measuring signal. In these devices, either the emitter or the detector must therefore be clocked. The chopping again leads to the isolation or release of the infrared beam path from the emitter to the cuvette.

Usually, a device is used that is similar in principle to the chopper of gas analyzers with membrane capacitors. This means that a motor is used again, coupled with an aperture wheel. This design again results in a bulky device that is also sensitive to mechanical shocks. The axis of the motor can be damaged if it hits something hard.

Both methods are not very suitable for mobile use and therefore for installation in a portable gas sampling probe that is subject to high mechanical stress.

3. Description of the utility model

There are pyroelectric measuring cells for the multiple detection of heteroatomic gases. There are receivers that can receive four infrared bands combined using a narrow band filter. These combined measuring cells can record all four channels, ie one reference band and three measuring bands, for one heteroatomic gas each, due to the simultaneous application. The measuring signals from the measuring filters, eg for CO ₂ , CO and CH _{4 ,} are related to the measuring signal of a neighboring but completely independent band. This means either the difference or the quotient formation from the signal peaks of the respective measuring and comparison bands.

The chopping of the combined pyroelectric measuring cell can be carried out by means of an aperture wheel or by clocking the power supply of the lamp's filament. For the accommodation of the measuring gas cuvette outside the actual measuring device, e.g. in the gas sampling probe, the simple electrical pulsation of the lamp is more advantageous than chopping using an aperture wheel. Fig. 1 shows the schematic structure of a measuring gas cuvette that is built into a gas sampling probe. The cuvette length can be between approx. 1000 mm (for very small concentrations, e.g.

for values below 10 ppm) and 10 mm long (9) · It is important to have good thermostat control of the entire cuvette (2). The emitter (7) should be clocked according to the inertia of the entire system, e.g. with a frequency between 1 and 12 Hz. The measuring signals should be amplified directly at the detector (8). SMD-equipped microcircuit boards, which are directly coupled to the four-channel detector, e.g. encapsulated (14), are particularly suitable for this.

This simple structure means that, for example, a measuring range adjustment can be carried out without any problem. All that is required is to move the emitter along the protective tube (Fig. 2). In this case, the protective tube itself is the measuring gas cuvette. For small concentrations, the emitter (3) moves very close to the measuring cell (5), and for large concentrations it moves far away from it. The distance between the emitter and the IR receiver can be adjusted both manually and automatically, e.g. with the motor (7) and the axis (4).

1 protective tube

2 heating mat

3 Sintered bronze filter· A Fine filter cartridge

7 infrared heaters

Infrared 4-channel detector Measuring gas cuvette Coupling for electrical current

5 DC microbump 11 Pneumatic coupling

6 Hose 12 Handle

13 Hand protection

14 Amplifier board.

15 Insulation

Fig.

!Filter 5 Four-channel IR detector

2 Gas pump 6 SMD-equipped micro amplifier (possibly

3 emitters 7 stepper motor cast)

Linear drive (e.g. toothed rack)

Fig. 2 Measuring gas cuvette with adjustable optical

Away

Claims (5)

Protection claims • · 1. IR gas measuring cuvette, built into a gas sampling probe, characterized in that '. that the cuvette is portable. 2. IR gas measuring cuvette, built into a gas sampling probe, characterized in that a mobile gas filtering and conveying unit is housed in front of the cuvette. 3. IR gas measuring cuvette, built into a gas sampling probe, characterized in that the gas sampling probe can be heated. 4. IR gas measuring cuvette, characterized in that the emitter in the protective tube can be continuously adjusted. The optimal cuvette length corresponding to the respective measuring ranges can thus be set automatically or manually. 5. IR gas measuring cuvette, characterized in that the detector can be used both as a single measuring cell and as a multiple measuring cell with a reference band.