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        Spectrometer Monitoring the sparking process: SATEUS, SETEME

          Burn spot assessment is only sensitive enough to detect incorrect analyses with extreme interference affecting the discharge circle. It is, however, a useful aid when no other is available.

          Monitoring the sparking-in and homogenization processes on the one hand, and the sparking process during intensity measurements on the other, is important for the RSD and SR of the analyses. Intensity measurements should be taken in SS. The pre-sparking time can be determined on numerous samples but some require a longer pre-sparking time eg. if, due to carelessness, slag is poured with the sample, or if C has precipitated in pig-iron samples. The pre-sparking time should not be unnecessarily prolonged.

          Monitoring of the sparking-in processes is described. The zero order radiation is measured during the pre-sparking time. If the integrated signal does not reach the value preset for the type of sample at the end of the pre-sparking time, in the simplest case sparking is discontinued or the analysis is discarded.

          Note: Zero order radiation reaches its maximum by the end of the sparking-in process, so that no information on the state of homogenization can be obtained by monitoring this radiation.

          The SEREPS method can thus be used for sample quality control. This can be done by specifying that, during the first 3s at 400Hz at least half of these discharges must be concentrated. Otherwise, sparking will be discontinued, with the note: “poor burn spot”. This function is known as SATEUS which is an abbreviation of: Sample Test of Usefulness.

          Monitoring of homogenization is assured by SEREPS but one special example will be given.

          Cast-iron which has undergone partial grey solidification is of special example will be given.

          Cast-iron which has undergone partial grey solidification is of special interest. Such samples cannot be identified in production analyses. Cast-iron undergoing grey solidification with slow cooling can solidify partly grey with rapid cooling in bright Cu moulds, with minimum sample dimensions. At present, such samples are only recognized during sparking if they are “very grey”, so that during sparking times of≈20s, mainly diffuse discharges with white burn spots are obtained. Measurement values are so impossible that the resulting concentration exceed all quality limits.

          Cast-iron reference samples and production samples were measured with a pre-sparking time of 25s.

          This points to precipitated C. In fact, the C intensities for all production samples except No.12 are stable after a few s.

          The burn-off curve for C can be used to assess white solidification. If this curve has no peak, no C is precipitated. On the other hand, with even distribution and constant size of the C precipitates and approximately the same proportions and sizes of precipitates from sample to sample, the surface integral of the burn-off curve peak is, to a first approximation, proportional to the concentration of precipitated C.

          Monitoring the intensity of the reference or of zero order during measurements is useful. This test is known as SETEME. If, during this time, there is a change from the concentrated to the diffuse type of discharge, the intensities will be wrong. The process can be such short duration that it cannot be recognized from the burn spot.

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