Production • Processing • Handling
Table 2. Signal Parameters
Velocity of Sound (VOS)
Velocity of sound is measured together concurrently with the velocity of the flow. Since the two values are calculated from the same transit time measurement (the travel time of the ultrasonic signal), the correctness of the velocity of sound will also indicate the quality of the flow measurements. A check of the transducer health on each path can be made by comparing the velocity of sound measured on the different paths. Since each path has an individual VOS measurement and they are measuring the same liquid, the VOS should be the approximately the same for each path.
Gain
The gain is a measure of how much amplification is being applied by the electronics to effectively detect the transmitted ultrasonic signal. This is controlled by the automatic gain control (AGC) function built into the software. The AGC tries to keep the received signal level constant. The amplification needed to achieve this is represented by the gain value. This value is displayed for each individual transducer, both upstream and downstream for each path. A high gain is an indication that the signal strength may be too low for proper measurement.
Signals %
Signal to Noise Ratio (S/N)
This value describes how many of the ultrasonic signals are acceptable to be used for custody transfer flow measurement. The value is displayed as a percentage indicating how many of the transmitted signals are being used.
The signal to noise ratio is monitored both on the sampled signal and the processed signal. The S/N ratio is calculated by measuring the RMS (Root Mean Square) value of both the received signal and the ambient ultrasonic noise. A low S/N ratio is an indication that the signal is being attenuated by the product.
Table 2 shows the signal parameters for the Smith Meter Ultra6
6-path flow meter. Like the flow
profile parameters, limits can be set on each of these parameters to indicate a deviation from the reference condition. If an alarm condition is present on one of the signal parameters, it indicates that something is causing the ultrasonic signal to degrade as it is transmitted through the product. Some causes of this signal degradation could be entrained air or particulates in the product, a damaged transducer or cabling or debris fouling in the meter. Naturally, any alarm condition needs to be investigated to correct the root cause of the problem. Isolating the cause of the alarm to specific flow profile or signal parameters will assist in quickly identifying the problem. Winscreen also includes higher level diagnostic functions that can be used to identify the root cause of the problem. One such diagnostic function is the Signal Analyser window which shows the transmitted and received signals for each of the 12 transducers that make up the six paths in the meter. Each of these windows can be view individually and expanded to analyse the signal in detail. As discussed, proper operation of liquid
ultrasonic flow meters is dependent on aspects of the fluid flow stream that have never been visible using traditional metering technology. Tese flow profile and signal parameters can be used not only to verify proper operation of the meter but also
to identify changes in the flow stream that could lead to inaccurate measurement. By properly using the diagnostic capabilities of LUFMs, the user can be assured that highly accurate custody transfer measurement is being achieved. l
Enter 28 or ✔ at
www.engineerlive.com/iog
* Tis article was originally presented at the International School of Hydrocarbon Measurement (ISHM).
Joshua W Rose is Product Marketing Manager, , FMC Technologies, Erie, Pennsylvania USA.
www.fmctechnologies.com.
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Fig. 5. Winscreen Measured values display.
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