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How to Troubleshoot a Pressure Transducer

Hands-on troubleshooting for voltage and current sensors

Pressure transducers today are more rugged and reliable than ever before. Made of rugged stainless steel construction, they provide abundent overpressure protection, improved total error band, and offer negligible orientation and vibration effects. They are ideal for long-term use even in harsh environments of extreme temperature, humidity, and vibration.

Still, pressure transducer installation occasionally fail. Typical problems include improper wiring, incorrect polarity, short circuits, inadequate power supply, multiply grounds, system operation issues or there may be a problem with the pressure transducer itself.

Fortunately, it's easy for technicians to perform simple troubleshooting techniques to determine if the transducer is operational, regardless of whether it's a 3-wire voltage output, 4-wire voltage output, or a 4-20 mA loop transmitter. The troubleshooting procedures will guide a technician through three or four simple steps to quickly and easily determine transducer status.

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How to Troubleshoot a Pressure Transducer

  1. 1. 1 HOW TO TROUBLESHOOT A PRESSURE TRANSDUCER Hands-On Troubleshooting For Voltage Sensors Pressure transducers today are more rugged and reliable than ever before. Made of rugged stainless steel construction, they provide abundant overpressure protection, improved to- tal error band (TEB) and offer negligible orientation and vibration effects. They are ideal for long-term use in harsh environments of extreme temperature, humidity and vibration. Still, pressure transducer installations occasionally fail. Typical problems include: improper wiring, incorrect polarity, short circuits, inadequate power supply, multiple grounds, system operations issues or a problem with the pressure transducer itself. Fortunately, it’s easy for technicians to perform simple troubleshooting techniques to de- termine if the transducer is operational, regardless of whether it’s a 3-wire voltage output, 4-wire voltage output, or a 4-20 mA loop transmitter. The troubleshooting procedures be- low will help a technician quickly and easily determine transducer status. 1. Troubleshooting Assumptions These troubleshooting guidelines assume that the technician is trained, has access to a 24 VDC power source and knows how to properly use a digital multimeter to measure voltage (voltmeter), current (milliamp meter) and resistance. This guide also assumes that the technician knows that the best and safest method to test any transducer is to remove it entirely from the pipeline and control circuit. That’s because an installed transducer can be injected with unwanted voltage from another source. For example, consider the actual application of an original equipment manufacturer (OEM) that mounted a transducer in a chiller’s water pipe. Unknowingly, another company’s arc weld- ing operation in an adjacent building used a ground clamp on that same water pipe. Inter- mittent arc restrikes induced huge sporadic voltage spikes that were carried through the pipe and water to the transducer, ruining its circuitry and functionality. This problem can be eliminated by removing the sensor from the pipeline to a workbench, which allows the troubleshooting technician to focus solely on troubleshooting the transmitter. AlthoughthemachineisbeingbuiltontheOEM’smanufacturingfloor,considerationshould also be given to the environment into which the machine ultimately will be installed. For WHITEPAPER What’s Inside Section 1: Troubleshooting Assumptions Section 2: 3-Wire Transducer Removed From a Pipeline Section 3: 3-Wire Transducer Connected To a Pipeline Section 4: 4-Wire Transducer Removed From a Pipeline Section 5: 4-Wire Transducer Connected To a Pipeline Section 6: 4-20 mA Transducer Removed From a Pipeline Section 7: 4-20 mA Transducer Connected To a Pipeline Section 8: Customer Application Stories
  2. 2. 2 example, a pressure transducer installed on a steam line very close to where the steam is being generated can cause a negative change in the dynamics of the sensor. This issue can be eliminated by moving the transducer further away from the steam line. 2. 3-Wire Transducer Removed From a Pipeline The 3-wire voltage output transducer is the most commonly used and easiest to trouble- shoot voltage transducer. Typically, the problem with this type of unit is either no signal or the signal is different from what was expected. After the transducer has been removed from the pipeline and control circuit, the technician must first identify all terminals (+excitation, –excitation, +signal, –signal and ground) for the unit being examined. This is necessary because transducer terminal configurations vary by both model and manufacturer, which can be found in the model’s operation instructions. Additionally, the technician must understand that the –excitation and –signal are com- moned within a 3-wire transducer. (“Common”refers to both negatives.) Once terminal configuration has been determined, the technician can power the unit by placing the +24VDC power supply to the transducer’s +excitation and –24 VDC to the –ex- citation. To establish if the transmitter is operating properly, place the voltmeter +lead onto the +signal and then the voltmeter –lead onto common. With no pressure applied the voltmeter, the reading should equal the analog signal for zero applied pressure such as 0.0 VDC, 0.1 VDC or 0.03 VDC. The transducer is operating if the reading is what was expected. 3. 3-Wire Transducer Connected to a Pipeline Although it may not be practical to remove the transmitter from the pipeline and control circuit, it still must be tested. First, make sure +24 VDC is connected to the transducer’s +ex- citation and –24 VDC to common. Next, disconnect the wire that runs from the transmit- ter’s +signal to the control circuit. This disconnects the sensor from the control circuit, while leaving the transducer in the pipeline. Now place the voltmeter +lead onto the transducer’s +signal and the voltmeter –lead onto common. With no pressure applied, the transmitter should provide a voltage output as specified on the unit’s data sheet, confirming that its operational. However, one more check is necessary. Reconnect the control circuit wire that runs from the transmitter’s +signal to the control cir- cuit. Once again, place the voltmeter +lead onto the transducer +signal and the voltmeter –lead onto common. The voltage reading should display just as before. If not, the cause is a circuit-wiring problem, not a faulty transducer. It is up to the technician to carefully trouble- shoot all wiring connections.The manufacturer’s operating instructions should be reviewed to verify terminal identification and check that all wiring was connected properly. 4. 4-Wire Transducer Removed From a Pipeline A 4-wire voltage output transducer has several different troubleshooting steps compared to the 3-wire transducer. Again, it is assumed a trained technician has access to a 24 VDC power source, knows how to properly use a digital multimeter and can identify all of the unit’s terminals.
  3. 3. 3 What differentiates the 4-wire transducer configuration from the 3-wire system is that the -excitation and –signal are not common within the transducer. Instead they are electrically at a separate voltage potential and all four wires must remain isolated from each other. There is a VDC difference between –excitation and –signal in a 4-wire circuit, known as a Common Mode Voltage (CMV), whereas the two minus signals are at the same potential in a 3-wire circuit. This is a very important difference, and the technician must understand it because if the worker commons the –signal and –excitation, a short circuit will occur. Once the unit has been removed from the pipeline and control circuit and terminal configu- ration has been determined, the technician can power the unit by connecting the +24 VDC power supply to the transducer’s +excitation and –24VDC to the –excitation.To determine if the unit is operational, connect the voltmeter +lead to the +signal, then place the voltmeter –lead onto the –signal.When no pressure is applied, the voltmeter reading should equal the analog signal for zero applied pressure. 5. 4-Wire Transducer Connected to a Pipeline With the transducer connected to the pipeline and control circuit, confirm that +24 VDC is connected to the transducer’s +excitation and –24 VDC to the –excitation. Next, discon- nect the wire that runs from the transmitter’s +signal to the control circuit. This disconnects the sensor from the control circuit, while leaving the transducer in the pipeline. Place the digital voltmeter +lead onto the transducer’s +signal and the digital voltmeter –lead onto the transducer’s –signal. With no pressure applied, the transducer should provide a voltage output specified on the unit’s data sheet. If it is, then the transducer is operational with one caveat. If the digital meter reading is 0.0 VDC, the functionality of the transducer can’t be estab- lished. To determine functionality, the technician must disconnect the transducer +signal and –signal from the circuit wiring. By placing the voltmeter +lead to the transducer -signal and the voltmeter –lead to the transducer –excitation, you should be able to read the CMV. Relocating the voltmeter +lead to the transducer +signal should provide the same CMV reading. This establishes that the transducer is functioning. Keeping the voltmeter attached as above, touch the transducer –lead to the –input terminal. You should be able to read the CMV, if not the two negatives are shorted together someplace in the system. It is now up to the technician to determine the cause and location of the short circuit. Generally, this problem originates from technicians familiar with 3-wire systems, but un- familiar with 4-wire systems. They don’t understand that the input channels to their data acquisition systems are all common to themselves. Although power is taken from a nearby source, and the wire is tied to the + and –terminals of the input card, that input card neg- ative is also tied to the power supply. This causes the two negatives get shorted out. Not isolating the two negatives is the most common error on 4-wire transducers. 6. 4-20 mA Transducer Removed From a Pipeline Troubleshooting a 4-20 mA transducer is more difficult, because the only instrument that directly measures current is an analog meter, which uses a pointer to indicate current. In an analog meter, electric current flows through a coil to produce a magnetic field creating a magnetic torque that causes a pointer needle to deflect proportionally to the current. To- day, digital instrumentation is more commonly used than an analog meter.
  4. 4. 4 Digital metering is based on voltage values. Here’s how it works for current. The technician powers the transducer by connecting its +terminal (red wire) to the +terminal of the 24 VDC power supply. 4-20 mA flows from the transducer’s –terminal (black wire) that is then connected to the +lead of a digital milliamp meter. The meter’s –lead is then connected to the –terminal of the 24 VDC power supply. This meter has an internal dropping resistor that develops a voltage, which is converted to a milliamp value displayed on the meter’s screen. This is the same method used in automation control. A 250 ohm dropping resistor is placed in series with the output of the transducer and the –terminal of the power source. The volt- age developed (1 to 5 VDC) across this resistor is fed into a channel of the control system. (A 500 ohm resistor converts 4-20 mA current to 2-10 VDC). If the milliamp readout is 4 mA with no pressure applied, the transducer is operational. Technicians can use these steps to troubleshoot any 4-20 mA circuit, whether it’s a tempera- ture sensor, pressure sensor, vibration sensor or transducer. 7. 4-20 mA Transducer Connected To a Pipeline If the technician is to troubleshoots transducer while still connected to the pipeline and control circuit, follow the next steps. As in earlier examples, these steps are different from troubleshooting with the unit removed from the pipeline. Connect the 24 VDC to the red wire (+terminal) of the transducer. (Note: Check the trans- ducer model’s operating instructions because some manufacturers may use different no- menclature). Disconnect the wire from the transducer that is connected to the control cir- cuit and place the +lead from the digital milliamp meter to the black wire (–terminal) of the transducer. Next, connect the –lead from the digital milliamp meter to the lead going to the control circuit. In this wiring configuration, power goes through the digital milliamp meter and small re- sistor, into the control circuit’s larger 250 ohm resistor, back to the –terminal of the power source. If the transducer provides a 4 mA output signal with no pressure applied, then the sensor is operational. If the transducer provides 0.0 mA output, then the problem is either with the transducer or the input to the control circuit. Whichever it is, it has been established that there is no conductive path from the transmitter –terminal back to the –post of the power source. If the circuit is the problem, the typical cause is the absence of the dropping resistor in the circuit. When 0.0 mA readout occurs, it is recommended that the technician remove the unit from the pipeline and control circuit to determine if the transmitter is working. If the technician has completed these troubleshooting steps for 3-wire, 4-wire or 4-20 mA transducers and still can’t solve the problem, contact the transducer manufacturer’s tech- nical support. They may be able to provide other troubleshooting steps, engineer a unique solution or go on-site to diagnose the problem. If the supplier’s technical support can’t re- solve the issue, return the unit to the manufacturer for repair. The manufacturer can only determine what the problem is, not the cause. If the transducer is found to be operational, then the problem exists within the installation and will be up the technician to correct it. 8. Customer Application Stories Below are customer testimonials on how Setra engineers have diagnosed various transduc- er problems.
  5. 5. 5 A customer had complained that Setra’s gauge pressure sensors were getting damaged during their fluid control loop. A phone call with the customer wasn’t successful in deter- mining the problem, so Setra engineers went out to visit the installation. The first thing the customer did was walk the engineers through a normal system operation of all of their processes. By observing each step within the process, the engineers were able to pinpoint the moment that the sensor output went to full scale and didn’t return to normal condition. Next, the engineers reviewed the system programing to see what was being triggered in the control system. It was determined that poor valve timing was causing a fluid hammer in the water line that was damaging the transducers. The valves were closing so quickly that a pressure wave was created in the fluid. Since the fluid was incompressible, the pressure wave created a very brief but high pressure spike that damaged the transducer. The engi- neers were able to solve the problem by preventing the fluid hammer effect from occurring. By modifying the timing and the speed of the valves, they were able to stop the generation of the damaging pressure wave. The customer found that the best option was to prevent the fluid hammer effect, but if it wasn’t preventable more options were available. For example, Setra could have provided a snubber or restrictor on the transducer. A snubber is often an adaptor screwed onto the pressure connection side of the pressure transducer. It evens out the rate of the fluid enter- ing the pressure transducer. Inside the snubber is a porous disk or filter-like screen that dis- rupts a pressure wave, allowing normal fluid pressure measurements. A restrictor is a small orifice installed inside of the pressure side of the pressure transducer. This small hole does the same job as the snubber, but eliminates the need for an adaptor. Another customer had called Setra about their high accuracy transducers that appeared to be drifting in the engine test cell equipment. During a phone conversation, the application setup appeared to be correct. Returned units were still in calibration and the engineers couldn’t replicate the failure in the factory. A Setra engineer went to the customer’s factory to look at the process, and concluded that the test cell appeared to be functioning properly. While looking at historical data, however, they noticed that a pressure drift happened daily from 12-1 pm, which corresponded to the technician’s lunch. To discover if this time frame had anything to do with the recurring drift, the engineer asked to sit in and observe just before the technician’s lunch. At noon all of the associates left their station and opened a large garage door to have lunch together outside. The open door raised the ambient room temperature, causing larger-than-normal thermal errors in all of the transducers. The high accuracy transducer drift was actually caused by external factors. When the door was kept shut, the engine test cell and the transducers at- tached remained at a constant temperature and worked properly. A manufacturer of factory mobile equipment and forklifts had a problem with pressure transducers failing while their equipment was being assembled. The sensors were returned to Setra and the failure analysis showed that the sensors’electronics were exposed to high voltages, causing their failure. Setra reengineered and supplied the customer with a more electrically robust sensor. This prevented the transducer from getting damaged during as- sembly, but the customer started experiencing failures in more expensive equipment. The customer asked Setra for the original units so they could replace a sensor instead of a more expensive part.When a Setra engineer visited the customer months later and watched the assembly process, the manufacturing team admitted that when they had a short cir- cuit in the system and turned it on, there was a power surge that damaged the pressure
  6. 6. 6 transducer. The engineer recommended that the manufacturing team use an ohmmeter to check for short circuits before turning the power on, conducting that simple check, the customer eliminated failures during assembly. Author Bio: Dick Pansire is a Senior Application Engineer at Setra Systems, Inc. A 35 year expert in pressure sensing and capacitance technology, Dick provides customer troubleshooting and support for HVAC and Industrial products. Dick has a background in manufacturing, engineering and technical support. About Setra: Founded by former professors of Engineering at Massachusetts Institute of Technology (M.I.T.), Setra has been designing and manufacturing sensor products since 1967. Our specialty is in the pressure and sensing in a wide range of markets including HVAC/R building automation, pharmaceutical, energy, medical sterilization, industrial OEM, test & measurement, meteorology and semiconductor. Setra Creates Solutions: Over 40 years of expertise in sensing and sensing applications R&D and Design Engineerings focused providing application solutions Sensors cover a wide range of pressure rages with unique expertise in low pressures Sales and manufacturing in the U.S., Europe, and Asia for fast solutions and products

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