|NDT.net January 2003, Vol. 8 No.01|
The requirements on the quality standards of refrigerating machines are increasing steadily. New refrigerants are being developed continuously and used in numerous products world-wide. During leak tests with halogen leak detectors commonly used today, frequent false alarms significantly hamper the leak detection process. In addition the sensors are subject to rapid ageing resulting in high maintenance costs and restricted uptime. On the basis of a new sensor principle a handy unit has been developed which removes the disadvantages and which moreover satisfies the requirements of today’s production lines.
The requirements as to the quality standards of refrigerating machines are increasing steadily. New refrigerants are being developed continuously and are used in numerous products world-wide. Through the Europe-wide ban on CFC usage to protect the ozone layer, partially halogenated H-CFCs and H-FCs have established since the beginning of the Nineties. These gases offer a low ozone depletion potential, but their global warming potential is still 100 to 5,000 times greater compared to CO2.
Moreover the manufacturers must ensure a hermetic seal for the entire refrigerant circuit and its components over many years.
|Fig 1: Overview on different refrigerants|
Typical leak tightness requirements for complete refrigerating systems today are in the range of only a few grams of lost refrigerant per year. Consequently the leak tightness on requirements individual components of the system and its joints are more demanding. In the supporting industry the test gas method has become widespread and commonly helium as the test gas is being detected.
During final testing of refrigerating machines, helium is not suitable as a test gas since at that time the refrigerant circuit is already filled with the refrigerant. Therefore one has to determine the leak tightness of the joints between the individual components and subassemblies using a test instrument cable of detecting the type of refrigerant used at a sufficient low detection limit.
The new refrigerant leak detector HLD 5000 is mainly used to detect leaks in connection with the following applications:
|Fig 2: Refrigerant leak detector HLD 5000.|
Instruments on the market today are based on principles where, for example, the refrigerant to be detected gives rise to a chemical reaction in a heated sensor generating a measurable electric current or on where principles the refrigerant temporarily removes oxygen molecules from a ceramic surface, resulting in a measurable change in electric resistance. However, these methods have significant disadvantages which become evident in an everyday production environment:
These disadvantages have been avoided by the sensor principle used in the HLD 5000. Here the refrigerant which is to be detected is exposed to infrared light. Refrigerants have – depending on their composition – the ability to absorb certain frequency components in the infrared light.
To detect gases selectively in each case, a filter which matches the typical absorption line of the refrigerant is placed in front of the sensor. That model of the refrigerant detector which is to detect R134a is correspondingly fitted with a
7.7 µm interference filter whereas the model for R22 for example is equipped with a 9 µm filter.
|Fig 3: Absorption spectrum of R134a and R22.|
The gas to be measured flows through a cell, the ends of which are equipped with an infrared source and a pyroelectric sensor. Ahead of the pyroelectric sensor a narrow-band infrared filter is placed. To protec the sensor as well as the infrared source against dirt CF2-windows have been put to the ends of the cell.
These sensors rely on the operating principle that infrared light which is modulated heats the (sensor) crystal via an absorption coating. By this change in temperature electrical charges are released in certain materials like LiTaO3. By modulating the infrared light this heating will generate an electric AC signal. Pyroelectric sensors of this kind are used in infrared motion detectors.
Only a small part of the diffuse light emitted by the infrared source can reach the sensor directly; over 90% of the light is reflected several times at the cell’s surface, before being detected by the sensor. Using this arrangement no optical system is necessary. Coating the surface of the aluminum cell which is manufactured in an extrusion process with gold or alike is not necessary.
|Fig 4: Cell with sensor and infrared source.|
In the HLD 5000, however, it is not the infrared light which is modulated as common in infrared measurements, but the composition of the gas is modulated instead. A 3/2 way valve changes the gas entering the cell several times per second between ambient air (“reference gas”) and the gas which is escaping from a leak.
|Fig 5: Basic circuit diagram for the HLD 5000.|
By the quasi-permanent reference measurement, false indications are avoided which normally would be caused by the natural content of water vapour in the ambient air.
In addition, relatively high concentrations of freon 12 escaping from insulating foams and/or from the filling stations close by, are usually present at the production sites of refrigerating and air conditioning units.
Owing to the quasi parallel admission via the reference and the test gas inlet ports these gases will also not produce incorrect leak rate readings.
At the same time modulating the gas will avoid drift which is typical for pyroelectric sensors, this drift being caused by temperature changes. Since in the case of this new type of arrangement the gas flow and not the source of infrared light is modulated, a relatively “slow” infrared source with higher power could be used.
|Fig 6: Absorption spectrum of water vapour and R 134a.|
The gas admission ports at the tip of the sniffer are designed in a way that the opening for the reference gas is located as close as possible to the test gas inlet, because the concentrations of interfering gases are not constant, but have gradients even over short distances. On the other hand the two openings must not be so close to each other that gas from the leak will enter through the reference gas inlet to the sensor.
|Fig 7: Sniffer tip with test gas and reference gas inlets.|
In the following figure the design of the handle is shown which contains the entire sensor system and thus the “brains” of the HLD 5000.
|Fig 8: Handle.|
Via the line between the unit and the handle not only the already digitised measurement data is transmitted but it also links the gas supply pump in the operating unit to the gas inlet through which the leak gas enters the cell where it is detected.
Besides the microprocessor board, display, power supply and gas supply pump the operating unit contains a calibrated leak which is filled with R 134a. This calibrated leak is located at the bottom of the unit and may be as simply exchanged as a battery. The calibration process is initiated automatically through a light barrier as soon as the tip of the handle is inserted into the opening of the calibrated leak on the front.
|Fig 9: Bottom view of the instrument with calibrated leak.|
Since the calibrated leak is filled with liquid R134a, is has been possible to keep it small. The gas constantly diffuses through a diaphragm to the outside. Because of the diffusion characteristic of the diaphragm the quantity of gas escaping increases exponentially with temperature. This effect is compensated by software. For this purpose a temperature sensor is located at the bottom of the calibrated leak’s reservoir. The leak rate of the calibrated leak is determined before delivery and documented in the factory test certificate. Thus measurements made with the HLD 5000 are traceable to national standards so that the instrument complies with the requirements of DIN/ISO 9001.
Compared to conventional methods, the operating costs of this new refrigerant leak detector are significantly lower while at the same time usage in on everyday production environment is significantly simplified. The different versions of the HLD 5000 are capable of detecting the following gases:
R134a, R404A, R407C, R410A and R22
Versions for other gases are being prepared.
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