Classifications

• • G—PHYSICS
  • G05—CONTROLLING; REGULATING
  • G05D—SYSTEMS FOR CONTROLLING OR REGULATING NON-ELECTRIC VARIABLES
  • G05D7/00—Control of flow
  • G05D7/06—Control of flow characterised by the use of electric means
  • G05D7/0617—Control of flow characterised by the use of electric means specially adapted for fluid materials
  • G05D7/0629—Control of flow characterised by the use of electric means specially adapted for fluid materials characterised by the type of regulator means
  • G05D7/0635—Control of flow characterised by the use of electric means specially adapted for fluid materials characterised by the type of regulator means by action on throttling means
  • G05D7/0641—Control of flow characterised by the use of electric means specially adapted for fluid materials characterised by the type of regulator means by action on throttling means using a plurality of throttling means

Definitions

• the invention relates to a calibration system for the calibration of mass flow controllers of an apparatus used for a chemical vapour phase growth.
• vapour phase growth is used generally in semi-conductor industries and in connection with the production of optic fibers (MCVD modified chemical vapour deposition -technique, OVPO outside vapour phase oxidation -technique, VAD vapour phase axial deposition -method and the like).
• Apparatuses applying said methods comprise vapour channels intended for different materials (such as SiCl., GeCl., POCl- BBr 3 ) and including mass flow controllers.
• FIG. 1 A conventional apparatus for controlling a vapour flow and for the transportation thereof into a system to be used is illustrated in Figure 1.
• a carrier gas e.g., oxygen
• M e.g., oxygen
• the empty upper portion of the bubbler B is filled with a saturated (or at least by near saturated) vapour of the used lignin F.
• the carrier gas carries the vapour into the using system.
• the vapour flow rate Q (moles/min) is
• a conventional characteristic of a mass flow controller (gas flow rate as a function of the control voltage) is shown in Figure 2, in which a curve a shows an ideal characteristic and a curve b an actual characteristic.
• a vapour control system of the type described is more advisable than a flow controller based on direct measuring of the amount of vapour (a so called source controller, e.g., Source V, manuf. Tylan).
• source controller e.g., Source V, manuf. Tylan
• the thermal conductivity of the carrier gas and the vapour is therein measured by means of a special measuring element which comprises parts which are usually made of stainless steel.
• the chemicals used in the production of optic fibers e.g., cause corrosion in the metal parts and dissolved metals may deteriorate the quality of the fiber.
• the curve b shows the characteristic of a thermal mass flow controller.
• the offset value o typically approx. 2 %.
• the fullscale flow fs e.g. 1000 ml/min
• the optic fiber production a wide linear range of control as well as an accurate control over the whole range are of importance.
• the present calibration system which is characterized in that it comprises at least one mass flow meter for measuring the characteristics of the mass flow controllers and an apparatus for calculating correction coefficients for the different controllers.
• Said system preferably comprises at least one flow meter for measuring the flow range of one mass flow controller or at least the adjoining ranges of mass flow controllers covering adjacent flow ranges.
• FIG. 1 illustrates a system according to the prior art
• Figure 2 shows a characteristic of an ideal and an actual mass flow controller
• Figure 3 shows the characteristics of two control ⁇ lers included in the same system
• Figure 4 illustrates the principle of a CVD vapour directing apparatus used in the invention
• Figure 5 illustrates the principal structure of the calibration system according to the invention.
• FIG. 4 The principal structures of the apparatus used in the invention is illustrated in Figure 4. Said figure shows two parallel mass flow controllers 1 and 2 and stop valves 3 and 4 attached thereto, whereby said controller 1 is intended for minor flow rates and said controller 2 for major flow rates, whereby the characteristics there ⁇ of can be similar to those according to Figure 3, for instance.
• Oxygen or some other corresponding gas is fed through said controllers 1 and 2 into a bubbling bottle (bubbler) 5 containing,e.g. , SiCl.
• the bubbler 5 is shown to further comprise a so called condenser 14 which is provided with an electrical or a water bath heating 16 and by means of which a more accurate control is achieved, said control being more independent of, for instance, the variation in the surface level of the bubbler 5 and the temperature of the bubbler.
• the temperature of the bubb ⁇ ler 5 slightly exceeds that of the condenser 14 in such a manner that even if the vapour contained in the empty space of the bubbler is not saturated by 100 %, the lower temperature of the condenser ensures that the vapour is saturated by 100 % at that temperature.
• vapour flow rate is directly proportional to the carrier gas flow rate and error due to a variation in the surface level and/or the temperature of the bubbler, for instance, is eliminated, assuming that the temperature of the condenser 14 is stable and sufficiently lower than that of the bubbler.
• the system can comprise a carrier gas heating system 15 which is operated electrically or by means of water, for instance, and by means of which a tem ⁇ perature change (usually cooling) in the bubbler 5, which change is caused by the vaporization or a wrong temperature of the gas, can be compensated by introducing heat or ex ⁇ tracting heat by means of the gas into or from the liquid contained in the bubbler.
• said apparatus is a heat regulating system (e.g., an elec ⁇ trical heating or water bath) built around a steel tube which transfers gas and by means of which system the gas contained in the tube is brought into a required determined temperature.
• the system illustrated in Figure 5 comprises three parallel branches for the gas passed therein, said branches being provided with stop valves 7,8 and 9.
• the valves 8 and 9 comprise mass flow meters MFM.. and MFM 2 used in the cali- bration process, the measuring ranges of said meters comple ⁇ ting each other.
• the gas flow is passed through the valve 7. Thereafter the flow is led through the stop valves 10,11 and 12 into the mass flow controllers MFC j , MFC 2 and MFC 3 , whereby MFC. and MFC 2 are connected in parallel.
• the system includes means, e.g., a computer, by means of which the characteristics of each mass flow con ⁇ troller is determined point by point, for instance. These measured properties enable the determination of correction factors for correcting the operation of the mass flow controllers into the ideal. In this way, when using a certain control voltage (flow rate value), the controllers are corrected by the calculated correction in order to obtain the ideal flow rate value.
• Standardmass flow meters can be used as mass flow meters, because the relative proportions and, on the other hand, a seamless joining of the control ranges of controllers having different flow ranges are essential for the process. This is achieved, first, by measuring the controllers of different materials by means of the same meter (possibly using the same gas (nitrogen) and by calculating the final calibration co ⁇ efficient of the used gas by means of the physical co ⁇ efficients of the gas) and, second, by measuring at least the adjoining ranges of controllers covering adjacent flow ranges by means of the same meter, too. Thus the errors caused by the possible inaccuracy of the mass flow meters can be essentially eliminated.
• the system By means of the system, all the controllers or, in connection with the replacing or cleaning of a controller, this controller only can be calibrated, whereby also the effect of the replacement of a controller on the quality of the product can be eliminated.
• the quality of the fiber can be improved by means of the system even to such an extent that band widths which are four to six .times higher than previously are easily obtained. Also the accuracy and the reproducibility of the process are improved, whereby, for instance, the flow sequences used in the fiber production can be transferred from one device to another. Because the calibration can be completely carried out by means of a computer (wherein the calibration values are stored), it can be effected quickly and accurately so that one or a few accurate mass flow meters make the whole apparatus accurate by means of the calibration process.
• the apparatus shown in Figure 4 and 5 and the calibration -system attached thereto provide an accurate absolute flow rate control and a wide linear control range required from a CVD -vapour system, for instance, in the production of optic fibers, by means of commercially available standard mass flow controllers which are advantageous in price.
• a fully continuous control is provided at a point in which the mass flow controller of the lowest flow is replaced with the controller of an upper flow.

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Citations (7)

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• 1984
  • 1984-06-15 FI FI842441A patent/FI73315C/fi not_active IP Right Cessation
• 1985
  • 1985-06-14 WO PCT/FI1985/000053 patent/WO1986000153A1/en active IP Right Grant
  • 1985-06-14 DE DE8585902993T patent/DE3574631D1/de not_active Expired - Lifetime
  • 1985-06-14 EP EP85902993A patent/EP0218588B1/en not_active Expired
  • 1985-06-14 US US06/842,257 patent/US4671097A/en not_active Expired - Fee Related
• 1986
  • 1986-02-14 DK DK73386A patent/DK73386A/da not_active Application Discontinuation

Patent Citations (7)

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