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Ctf878 Startup Guide
- 1. eimagination at work GE Sensing CTF878 Flowmeter Startup Guide
- 2. eimagination at work GE Sensing Principle of Operation Flow Channel 1 Transmit Channel 2 Transmit Channel 1 Receive Channel 2 Receive • Continuous wave (CW) acoustic signals are transmitted from the transmit transducers • Flow eddies modulate the amplitude of each acoustic beam as they pass through • Electronics console demodulates signals from both receive transducers similar to AM radio • DSP cross-correlates similar patterns in each demodulated signal to calculate the delay – Tao between these patterns • The distance between the transducer pairs or Tag Path is divided by Tao to obtain flow velocity
- 3. eimagination at work GE Sensing Signal Processing Carrier Up Channel 1 Carrier Down Channel 2 Modulation Up Channel 1 Modulation Down Channel 2 Tao
- 4. eimagination at work GE Sensing Cross-Correlation Noise Signal Cross-Correlation SNR = Signal Amplitude Noise Amplitude
- 5. eimagination at work GE Sensing Obstacles to Flow Measurement
- 6. eimagination at work GE Sensing Signal Through Gas • Most of the energy remains in the pipe wall due to the acoustic impedance (density • SOS) mismatch between the pipe and the gas. A gas with higher density (pressure) provides better Signal Strength • Signal through gas is attenuated more with longer path length, which is compensated by lowering transducer frequency with a sacrifice in the magnitude of signal modulation (Caution: Lower transducer frequency is for larger pipes only!)
- 7. eimagination at work GE Sensing Short Circuit Noise • Pipe wall conducts acoustic short circuit or SC through the circumference and heavy wall pipes have more SC due to more acoustic modes generated in the thick wall • Acoustic signal is also conducted through the pipe wall between the transducers of each path causing cross talk • Lower frequency transducers generate more SC which excludes their application for smaller pipes • For these reasons, the dampening material is required for all applications Gas “Signal” Short Circuit “Noise”
- 8. eimagination at work GE Sensing Liquid on the Pipe Wall • Liquid on the inside or outside surfaces of the pipe is a major obstacle to CTF878 – Amplitude of the acoustic SC signal traveling in the pipe wall modulates in the presence of a liquid on the wall – SC modulation cannot be distinguished from modulation caused by flow turbulence • Dampening material installed outside of the clamping fixture reduces SC modulation due to rain on uninsulated pipes and SC reflected from any nearby circumferential welds
- 9. eimagination at work GE Sensing Application Verification Gas type and minimumpressure: – Dry air, oxygen, nitrogen or argon at or above ambient pressure (1 bara, 14.5 psia) – Sweet natural gas at or above 2 bara, 30 psia – Other acoustically conductive gases with density of 0.074 lbs/ft3 , 1.185 kg/m3 Transducerfrequency: – 0.5 MHz for ANSI (DIN) 3 (75) to 16 (400) pipes – 0.2 MHz for ANSI (DIN) 16 (400) to 24 (600) Pipes: – ANSI (DIN) 4 (100) to 24 (600) – Non-lined steel (double minimum pressure/density for Duplex), copper and most other metals – HDPE (High Density Polyethylene), PVC, CPVC and most other non-lined plastic pipes (fiberglass requires demo) Velocity Limits: – Bi-directional flow – Between 3.5 ft/s (1.1 m/s) and 150 ft/s (46 m/s)
- 10. eimagination at work GE Sensing Installation
- 11. eimagination at work GE Sensing Pipe Geometry 20D 10 D Flow MinimumStraight Run: – 10 to 20 (preferred) diameters upstream – 5 to 10 (preferred) diameters downstream – Minimum of 20 diameters between circumferential welds P T
- 12. eimagination at work GE Sensing Pipe Survey Thickness Variation – Measure the thickness at 5 points along the axis of the pipe – Maximum variation should be less than 0.01 in. (0.25 mm) Pipe Cross-section – Measure the average OD – Measure the thickness at 8 points – Program the meter with the average thickness value TransducerLocation – Nearest the horizontal plane – Location with the greatest thickness difference of the opposing walls (W3 and W7 ) – Away from circumferential weld(s) or seam 1 2 3 4 5 W1 W2W8 W4 W3W7 W6 W5
- 13. eimagination at work GE Sensing Pipe Preparation Fixture Location – Remove 30 inches (750 mm) of insulation if any – Remove any loose paint and/or rust – Do not alter the curvature of the pipe if using a grinder TransducerLocation – Mark two correctly spaced areas on either side of the pipe, 4 inches (100 mm) long by 2 inches (50 mm) wide – Grind off paint if it is thick or uneven without altering the pipe curvature – Polish with fine sand paper
- 14. eimagination at work GE Sensing Dampening Installation Wrapping DMP-1 (MaximumTemperature 150 ºF, 65 ºC) – Warm up the DMP-1 if the pipe surface is below 45 ºF (7 ºC) – Apply the DMP-1 starting at the bottom – Align it parallel to the axis of the pipe – Pull DMP-1 while wrapping, taking care to minimize air pockets by pressing it and sliding back and forth with the free hand Removing DMP-1 fromTransducerLocations – Temporarily install the clamping fixture with the yokes correctly spaced – Install the transducers without any couplant and mark their locations – Remove the transducers and cut off the dampening material from marked locations
- 15. eimagination at work GE Sensing High Temp. Dampening Installation Safety Equipment and Precautions – Wear high temperature gloves and goggles – Use hardhat to avoid head injuries and burns from overhead pipes – The area must have good ventilation because of fumes and smoke during installation Installing the Pipe Dampening Jacket (PDJ) – Orient the PDJ to allow clearance for transducers and junction boxes – Remove the paper liner from the inside of the PDJ – Wrap the PDJ around the pipe with the fasteners close to the bottom – Place a metal bucket and a drop cloth under the PDJ to collect the hot fluid runoff
- 16. eimagination at work GE Sensing Final Installation Clamping Fixture – Replace any plastic parts with metal versions for high temperature – Adjust the transducer yoke positions to correct spacing – Align fixture correctly during installation by maintaining equal distance between pipe brackets on top and bottom and placing yokes over transducer openings Transducers – Screw the transducers into the junction boxes and verify the orientation – Apply a thin layer of couplant to the whole face of each transducer – CPL-01/CPL-04 for temporary normal/high temperature installations – CPL-16 for permanent normal and high temperature installations – Allow the CPL-16 to “skin” on the pipe for 15 minutes before fully tightening the transducer hold down bolts and locking nuts
- 17. eimagination at work GE Sensing SystemWiring Wiring the Preamplifier(s) – Connect the preamplifier cable in the junction box to the Channel 1 receive or upstream transducer of the upstream path – The Channel 1 Transmit cable from the flowmeter should be connected to the downstream transducer of the upstream path – Repeat connections for the Channel 2 (downstream path) transmit and receive cables Wiring the Flowmeter – If used, wire the pressure and temperature transducers to the Analog/RTD inputs – Hook up the Analog or Frequency output and Alarms, if required – Verify that the power source is not energized and connect the power to the CTF878 flowmeter
- 18. eimagination at work GE Sensing Wiring Diagram Flow P T Ch. 1 TX Ch. 2 TX Ch. 2 RX Ch. 1 RX Preamplifiers (1080) installed on receiving transducers Transducer Spacing = X1 – X2 X 1 X2 Tag Path DigiFlow CTF878 g
- 19. eimagination at work GE Sensing FlowmeterProgramming DigiFlow CTF878 g
- 20. eimagination at work GE Sensing Pipe and TransducerParameters • TransducerSelection – Number 312/310 for 0.2/0.5 MHz up to 266°F (130 °C) – Number 318 for 0.2 MHz up to 350°F (176°C) – Number 307 for 0.5 MHz up to 450°F (232°C) – Wedge temperature is an average of ambient and gas temperatures • Pipe Properties – Select the pipe material from the list – Program OD and wall thickness measured in “Pipe Survey” • Gas Properties – Select the type of Fluid being measured – Use Sonicware™ or the NIST website for Kinematic Viscosity • TransducerSpacing – Physical transducer spacing for each path must be the same as calculated by the CTF878 – Up to 0.25 inch (6.4 mm) difference from calculated spacing is allowed • Tag Path – Tag Path or the distance between paths must be the same as calculated by the CTF878 – Based on pipe ID with minimum 5-inches (125 mm) and maximum 10-inches (250 mm)
- 21. eimagination at work GE Sensing Standard/Normal Flow Parameters • Enabling Analog/RTDInputs – Enable Analog inputs in the “Analog I/O” menu • Temperature Input – Program correct “Base” temperature (check with customer) – Measure the “Fixed” temperature as close as possible to the measurement location and under the insulation, if used – Program “Active” temperature input with appropriate limits for the probe • Pressure Input – Program correct “Base” pressure (check with customer) – “Fixed” pressure must be obtained from a probe as close as possible to the measurement location without pressure drops (elbows, tees, valves or reducers) in between – Program “Active” pressure input with appropriate limits for the probe
- 22. eimagination at work GE Sensing ErrorLimits • Diagnostic Parameters – SNR Min. sets Low SNR Error (E8) trigger point, default is 10 – Carrier Limits set Carrier Under (E4) and Over (E5) trigger points, defaults are 75 mV and 1200 mV – DMod Limits set Modulation Under (E6) and Over (E7) trigger points, defaults are 75 mV and 1500 mV • Flow Parameters – Velocity Limits set Velocity Range Error (E9) trigger point which should be programmed to application maximum and minimum – Acceleration Limit sets Acceleration Error (E10) trigger point, decrease if flow will not change a lot and increase otherwise to improve response
- 23. eimagination at work GE Sensing Signal Setup Parameters • Signal Menu – Reverse Flow should be checked to reverse the flow reading – Zero Cutoff is the lowest measured velocity, lower values will be displayed as zero – Velocity Averaging is the number of readings that are averaged together, the greater the number, the slower the response – Errors Allowed sets the number of errors that is allowed before an error message is displayed, the default is 2 • AGC/MGC Menu – Gain control should always be set to AGC or automatic, manual control or MGC is used for diagnosis of problems – Carrier set point limits HFHI and HFLO for AGC should be set to 1000 mV and 200 mV – Modulation or low frequency set point LFSP should be set to 590 mV • Cross-Correlation (CC) Averaging – CCorr Average should be set to 75%, increase to improve SNR for low flow • Low Pass (LP) Filter – Electrical (Elec.) System should be set to the frequency of the AC power system
- 24. eimagination at work GE Sensing Calibrating Analog I/O • Calibrate Analog Output(s) forData orInput Calibration – Connect DMM or Calibrator to the Output being calibrated – Select “Main Board (0)” or Slot # corresponding to the Analog Output option card in the “Calibrate/Test” submenu from the “Service menu” – Calibrate upper and lower limits then repeat for the other output(s) • Calibrate Analog Input(s) – Connect a Calibrator or Analog Output A to Input (A) – Select the appropriate Slot number – Select the appropriate Analog Input in the “Calibrate/Test” submenu from the “Service menu” – Calibrate upper and lower limits then repeat for the other input(s) • Calibrate RTDInput(s) – Connect RTD Calibrator to the appropriate RTD Input on the Option Card – Select the appropriate Slot # and Input in the “Calibrate/Test” submenu – Calibrate upper and lower limits for each RTD Input used
- 25. eimagination at work GE Sensing Operation Verification and Troubleshooting
- 26. eimagination at work GE Sensing Signal Verification • Cross-Correlation SNR(CC SNR) – CC SNR should be greater than 10 – If not, the flow is too low • CarrierAmplitude and Gain – Carrier Amplitude should be between 75 mV and 1200 mV – If Carrier Gain is close to –6 dB and Amplitude is high, the carrier may be saturated – remove the preamplifiers and disable preamplifier power – If Carrier Amplitude is below 75 mV and Gain is 31 dB, the carrier is too weak, add preamplifiers or use 0.2 MHz transducers and receiver card • Modulation (Mod) Amplitude and Gain – Mod Amplitude should be between 75 mV and 1500 mV – If Mod Amplitude is below 75 mV and Gain is 31.5 dB, the Carrier is weak – If Mod Amplitude is above 1500 mV and Gain is –16.5 dB, there is inadequate straight run upstream or high liquid content in the gas
- 27. eimagination at work GE Sensing Flow Measurement Verification • Delay (Tau) – Tau should be stable and reasonable – Unstable Tao could be caused by low CC SNR • Velocity/Volumetric Accuracy – Check the physical Tag Spacing and programmed setting consistency – Check the Temperature and Pressure settings or probe parameters – Check whether there is enough straight run of pipe and no disturbances upstream • Sanity Check – Reverse the transducer cables between channels – Verify that Tau and velocity/volumetric are negative with the same magnitude – The sum of Carrier and Demodulation gains for Channel 1 should be equal to Channel 2, which indicates that the cross-talk is cancelled out
- 28. eimagination at work GE Sensing Troubleshooting Flow Chart CARRIER UNDER (E4) CARRIER OVER (E5) MOD UNDER (E6) MOD OVER (E7) XDCR SPACING? COUPLANT? GAS DENSITY LOW? ADD PREAMPS CHANGE TO 0.2 MHz DAMPENING? CHANGE TO 0.5 MHz TAG PATH? COUPLANT? LOW VELOCITY? CHANGE TO 0.5 MHz REMOVE PREAMPS CHANGE TO 0.2 MHz LIQUID CONTENT? LOW SNR (E8) CARRIER GAIN < 0? MOD GAIN < 0? STRAIGHT RUN? DAMPENING? REMOVE PREAMPS YesNo YesNo LOW VELOCITY?
Editor's Notes
- The CTF878 flowmeter operates using a Tag principle. A flow eddy passes through the first continuous wave (CW) acoustic beam and modulates its amplitude in a certain pattern, then it passes through the second acoustic beam and modulates its amplitude in a similar pattern. The electronics console demodulates the received signal from each channel or path similar to AM radio. Then the digital signal processor or DSP in the electronics console cross-correlates these signals to calculate the delay Tao, between similar patterns in them. The flow velocity is the ratio of the Tag path to delay Tao.
- The following slides provide explanations to the major obstacles to measuring natural gas flow with clamp-on technology.
- The magnitude of the signal that is transmitted through the gas is much smaller than what remains in the wall of the pipe. This phenomena is caused by the difference in acoustic impedance of the pipe wall and the gas or mismatch. The acoustic impedance is the product of the density and the soundspeed of the material. In gas, the density depends on the pressure, so the greater the pressure, the better the impedance matching, the stronger the signal traveling through the gas. In larger pipes the strength of the received signal is lower. The larger the pipe, the greater the acoustic path, the higher the attenuation of the signal through the gas. To compensate, lower frequency transducers are used. The only downside is the reduction of the magnitude of acoustic signal modulation due to a wider beam.
- The short circuit or SC signal is conducted through the circumference of the pipe to the receiving transducers. Also, more acoustic modes are generated in a thick wall, increasing the amplitude of the SC. Acoustic signal is also conducted through the wall between the pairs causing cross talk Lower frequency transducers increase the amount of SC noise, which makes them impractical for use on smaller pipes. Even though CTF878 does not require a good signal-to-noise ratio or SNR to work, the dampening material is still required for all applications except plastic pipes.
- A major obstacle to CTF878 is liquid on the inner or outer surfaces of the pipe wall. Any liquid on the wall surface causes modulation in the SC noise, which is indistinguishable from that caused by flow turbulence. Dampening material installed on the pipe outside of the clamping fixture protects the pipe from rain, snow or condensation in the immediate area of the installation and also reduces any SC reflections from nearby circumferential welds.
- Verify that an application conforms to the following: Gas type and minimum pressure adequate for a strong signal and a reliable measurement Pipe size and material within specification. Transducer frequency is chosen to optimize modulation and signal strength Velocity is within minimum and maximum limits
- Straight run of pipe must be adequate to avoid flow turbulence which degrades measurement accuracy, any P and T ports must be downstream to avoid flow disturbance Pipes must be long enough to avoid acoustic short circuit reverberation between ends (pipe acts like a tuning fork)
- The wall thickness measurement along the axis helps identify pipes in poor condition (internal rust or roughness, acoustic properties of the pipe wall) OD measurement improves transducer alignment, which is important for optimal signal quality especially for small pipes and improves accuracy of volumetric calculations Thickness measurement around the circumference helps determine pipe concentricity and avoid beam deflection and improves accuracy of transducer spacing and volumetric calculations Program the average wall thickness of the pipe into the electronics Install transducers as near as possible to the horizontal plane to avoid debris buildup at the bottom of the pipe Transducers should be installed away from seams to avoid signal blockage or distortion
- It is very important to prepare the pipe surface before the installation. If there is any insulation on the pipe, it must be removed from the location of the measurement. The surface of the pipe must be free of any loose paint or rust particles and smooth enough for acoustic coupling of the dampening material and the transducers. Leave the surface slightly rough for adhesion of the dampening material. During any grinding, care must be taken not to alter the curvature of the pipe, so as to avoid beam steering. Mark areas where transducers will be installed. If necessary, use the clamping fixture with transducer yokes properly spaced and install the transducers without the couplant. If the paint is very thick, it must be removed from transducer locations, otherwise polish with fine sand paper.
- The dampening material must be at least 45 F (7 C) to adhere to the pipe. Use blow drier or heat gun, if necessary. Start wrapping from the bottom of and parallel with the axis of the pipe. Hold and pull while wrapping with one hand and press and slide back and forth with the other to minimize air pockets. If the pipe is located outside or can be exposed to moisture or is sweating, install additional dampening that is the same width, on either side of the fixture. Install the dampening material while the pipe is dry. Mark areas where transducers will be installed. If necessary, use the clamping fixture with transducer yokes properly spaced and install the transducers without the couplant. Remove the transducers and cut off the dampening material from marked locations.
- Safety precautions must be taken during installation on hot pipes. Wear high temperature gloves, goggles and hardhat. Fumes and hot liquid runoff occur during installation, so locate a bucket and a drop cloth under the PDJ and ventilate the area. Remove the paper lining from the inside of the PDJ and orient it so that the transducer openings are as close as possible to the horizontal plane and there is enough clearance for transducers and junction boxes. Wrap it around the pipe with the fasteners close to the bottom. Tighten the fasteners evenly and firmly. The dampening effect will increase as the DMP-3 dries out.
- If the application is at a temperature above 150 F (65 C), remove the thumb caps from the yoke set screws and replace plastic spacers with metal versions. Also, replace the plastic transducer inserts with metal versions if the temperature is above 194 F (90 C). When installing the clamping fixture verify the yoke spacing and align them with the openings in the dampening. Check that the two halves of the fixture are 180 degrees apart during installation by maintaining equal distance on the top and the bottom of each pipe bracket. Install the junction boxes, if used, onto the transducers and verify their orientation so that the clearance is adequate. Apply a thin layer of appropriate couplant to the whole face of each transducer. Use CPL-01 couplant for normal temperature or CPL-04 for high temperature temporary applications. For permanent applications, apply CPL-16, but allow it to “skin” for 15 minutes before fully tightening transducer hold down bolts. Tighten the locking nuts to prevent loosening due to temperature variation or vibration.
- Connect the preamplifier(s) in the junction boxes to the upstream transducers of each path. The flowmeter and preamplifiers must be connected with RG62 type cables, which can be up to 500 ft (150 m) long.
- See previous slide for description
- The following slides provide instructions on how to program CTF878 flowmeter to perform a flow measurement
- The first parameters to program are the transducer type and pipe size CTF878 flowmeter has a built in table of parameters for different types of transducers. There are transducers of two different frequencies and three different temperature ranges. The temperature rating applies to the gas and not the wedge property. The wedge temperature programmed must be the average of the gas and the ambient temperatures Program the type of pipe material from the list provided. If the pipe material is not listed, select “Other” and program the shear wave soundspeed (longitudinal wave for plastics) of that material Program the OD and wall thickness from the values acquired in the “Pipe Survey” procedures Select the gas being measured from the list, as fluid The kinematic viscosity is automatically selected for listed gases at ambient temperature and pressure, for different conditions or other gases, it can be calculated using SonicwareTM software package or the NIST website at www.webbook/chemistry/fluid (use methane for natural gas) or check with the customer. Often, the viscosity provided is dynamic, in these cases the kinematic viscosity can be calculated by dividing it by the density. The units should be ft – lbs/ft3 – sec (kg – m/m3 – sec) for dynamic viscosity, lbs/ft3 (kg/m3) for density and ft2/sec (m2/sec) for kinematic viscosity. This parameter is used to determine the minimum velocity limit by the CTF878 Program the physical spacing between transducers in each path. If necessary, the physical spacing can be as much as 0.25 inch (6.4 mm) different from the calculated value and must be reflected in the flowmeter programming The Tag Spacing is the distance between the paths and must be set properly for accurate flow measurement. Use a caliper to precisely adjust this parameter by referencing the measurement from the same point such as the inner surface of the pipe bracket. Measure to the same surface on each transducer yoke, for example the left side of each yoke. The difference between the distances to each yoke from the same reference point is the Tag Spacing
- If standard or normal volumetric flow is required, enable the Analog Inputs in the “Analog I/O” menu Program the correct “Base” temperature which is usually 68 F (20 C) or check with the customer. If a temperature probe is not used, measure and program the “Fixed” temperature as close as possible to the measurement location and under the insulation, if used. Select “Active” temperature measurement and enter the limits for the probe being used Program the correct “Base” pressure which is usually 14.7 psi (1 bar) or check with the customer. If a pressure probe is not used, program the “Fixed” pressure that is measured as close as possible to the measurement location without any pressure drops such as elbows, valves, tees, or pipe reducers in between. Select “Active” pressure measurement and enter the limits of the probe being used
- The Diagnostic Parameters set the limits of acceptable received signals. The Minimum SNR sets the Low SNR Error (E8) trigger point, with a default value of 10 The Carrier Limits set the Carrier Over (E5) and Under (E4) Error trigger points, with default values of 75 and 1200 mV for minimum and maximum, respectively The DMod Limits set the Modulation Over (E7) and Under (E6) Error trigger points, with default values of 75 and 1500 mV for minimum and maximum, respectively The Flow Parameters monitor the validity of the measured flow The Velocity Limits set the Velocity Range Error (E9) trigger point and should be programmed to the minimum and maximum velocity of the application The Acceleration Limit sets the Acceleration Error (E10) trigger point. If flow is stable, the limit should be decreased. An improved response time is achieved for flow that is going to change a lot by increasing this limit
- Parameters in the Signal menu control the velocity measurement settings Reverse Flow should be selected if the measured flow is negative The Zero Cutoff is calculated by the CTF878 based on gas and pipe parameters. This value is the lowest velocity that can be reliably measured. Below this value the CTF878 displays zero The Velocity Averaging controls the number of readings that are averaged for each flow measurement. Higher values decrease the response to flow change The parameters in the AGC/MGC menu control the operation of the carrier and demodulation gain. It should always be set to AGC or automatic. The manual or MGC mode and the gain set points are for diagnostics purposes only The Cross-Correlation average should not be changed from 75% The Low (LP) Pass Filter should be set to the frequency of the of the local AC power grid, either 50 Hz (Europe, Middle East, etc.) or 60 Hz (USA, Canada, Japan, etc.)
- The Analog Inputs and Outputs must be calibrated before wiring The Analog Output(s) can be used for temperature and/or pressure input/probe calibration needed for standard or mass flow measurements or to output data. To calibrate, connect the DMM (digital multimeter) or a Calibrator to the output being calibrated. Select “Slot #” in the CAL menu for GC868 or Analog Output tab in Calibrate submenu in Service menu for PT878GC. Calibrate upper and lower limits and then test the that output. Repeat for the other output (GC868) To calibrate Analog Input(s), connect the Calibrator, DMM or Analog Output A (GC868) to Input A and select the appropriate Slot # in the CAL menu for GC868 or Analog Input tab in the Calibrate submenu from Service menu for PT878GC. Calibrate upper and lower limits and then test that input. Repeat for the other input(s) (GC868) To calibrate the RTD input(s) for GC868, connect the RTD Calibrator to the RTD input on the Option card and select the appropriate Slot # in the CAL menu. Calibrate the upper (Slope) and lower (Zero) limits for each RTD input and test them
- Use the following slides to verify the diagnostics and flow measurement
- Cross-Correlation SNR (CC SNR) should be above 10:1 to have a reliable flow measurement. A low CC SNR indicates a lack of turbulence due to low flow or the turbulence changes while traveling between the beams. If the Carrier Amplitude is above 1200 mV and the Gain is close to –6 dB, the Carrier may be saturated. To correct the problem, remove the preamplifiers and disable their power by moving the jumper next to the transducer connectors to the OFF position. If the Carrier Amplitude is below 75 mV and the Gain is 31 dB, the Carrier may be weak. Either connect the preamplifier and change the power jumper appropriately, or change to 0.2 MHz transducers with the appropriate receiver card change. If the Demodulation Amplitude is lower than 75 mV and the Gain is 31.5 dB, there may not be enough modulation due to a weak Carrier. To correct the problem, add preamplifiers or change to 0.5 MHz transducers and appropriate receiver card. An excessive Demodulation Amplitude with –16.5 dB Gain may be caused by a saturated Carrier due to high liquid content in the gas.
- An unstable delay – Tao, may be caused by low Cross-Correlation SNR or flow below specification. If the application is within specifications, verify all other diagnostics and take appropriate action, as indicated. The most common reason for incorrect flow measurement is improper Tag Spacing. Verify the distance between the transducer pairs and the programmed value. If Standard Volumetric or Mass Flow readings are incorrect, verify temperature and pressure probe parameters and calibration. Also, check that these probes are downstream of the transducers and there are no other upstream disturbances near the setup. If there is a large difference between the CTF878 flow measurement and a reference, the reliability of the measurement can be verified by swapping the cables between the channels. The resulting Tao and velocity/volumetric readings should be negative with the same magnitude. The sum of Carrier upstream and Demodulation downstream gains should be equal to the sum of the reverse (Carr. Up + Demod. Down = Carr. Down + Demod. Up). This procedure indicates that the cross-talk between paths is cancelled out.