GUIDE TO PRESSURE TRANSDUCER SELECTION
The purpose of this article is to provide some guidelines for helping a prospective pressure transducer user detect atypical conditions and compensate for them through the proper selection and application of the transducer. This article addresses most questions users most often ask about the selection and application of pressure transducers. If you have any of the problems mentioned here, then it might be wise to consult directly with your transducer supplier. Tell them what your application is and what you hope to achieve through the pressure measurement. A little time spent up front talking with the an experienced sensor engineer can identify and compensate for potentially expensive problems before they have a chance to materialize.
Why Use a Pressure Transducer or Sensor?
Pressure transducers are used to generate an electrical output for a variety of uses that may include data collection, process monitoring and control, or electronic transmission of a pressure reading to a remote display. Pressure transducers provide accuracies ranging from 0.1 percent of full scale output to a more typical accuracy of 0.5 percent. The greater the accuracy required, the more expensive the pressure measuring device. Many times product quality is directly related to how accurately the pressure is maintained and where the expense of a more accurate device will be justified.
Dealing with Abnormal Conditions
Once the need for a pressure transducer has been determined, it is prudent to determine if there might be some out of the ordinary conditions that could upset transducer performance. The balance of this article will identify some of these conditions and what can be done to compensate for them.
Two sources of temperature variation can affect transducer performance-ambient conditions and the fluid or gaseous medium itself. Ambient conditions include such things as abrupt changes in air temperature due to heating and cooling systems cycling on and off and the effects of radiant energy impinging on the transducer. Variations in the fluid or gas temperature occur most often during start-up of a process as the fluid goes from room temperature to a higher operating temperature. Dynamic temperature problems may arise because pressure transducers are calibrated under static conditions of both pressure and temperature. Dynamic temperature effects in the field, therefore, may be superimposed on a steady pressure condition and cause an unstable reading. The easiest solution to this problem is to wait for the temperature to achieve a steady state before making a pressure reading.
If accurate pressure readings must be made during large ambient temperature transitions, then corrective measures must be taken. Shielding the transducer may reduce ambient temperature shifts;
you can build a baffle around it or wrap it with some insulating material. One-way to compensate for temperature effects of the media is to buffer the transducer by placing it at the end of a short length of stainless steel or copper tubing. The tubing dissipates excess heat at a rate determined by the material, diameter, and length of the tubing.
Most Pressure transducer media contact surfaces are made from stainless steel, either 17-4PH (precipitation hardenable). This is an excellent material providing corrosion resistance similar to 303 and 304 grades of stainless steel. These materials handle most fluid media very well. If you know from previous experience that these materials are unacceptable, then ask your transducer supplier. It is very likely that other options are available.
Mechanical Shock and Vibration
Excessive mechanical shock and vibration could develop due to mounting on reciprocating engines, piston pumps, hydraulic cylinders, cycling valves, as well as vehicles. Cables that are subjected to continuous vibration may also eventually break, and pressure fittings can eventually work loose. Proper selection and support is critical. Several technologies (such as strain gage based pressure sensors) typically hold up extremely well under shock and vibration, and field-testing has brought about continual improvement in this area. If a pressure transducer is likely to be exposed to a great deal of shock and vibration, strain relief the cabling and use a high-quality pressure fitting that has been properly tightened.
A dynamic overload is a transient pressure spike of a magnitude greater than normal operating pressure. Such overloads may be present due to complex system dynamics and are very hard to trace. They can be as much as 10 times greater than the system pressure the transducer was designed to measure.
Dynamic overloads can place unacceptable stress on the transducer diaphragm and could cause transducer damage. If a zero shift due to an overload is suspected, drop the pressure in the system to zero and check the transducer’s zero output. Usually, a high positive zero shift is an indication of a pressure overload. If pressure spikes are a persistent problem, you may need to buy a higher pressure ranged transducer or correct the system dynamics with surge tanks and/or snubbing devices. Also, relocation of the transducer may diminish or eliminate the overload condition. If a problem does exist, contact your local pressure sensor support engineer who will help you evaluate the options.
Most transducer users want to measure pressure in a steady or a quasi- steady state. They don’t want to know what is happening to the pressure as fast changes take place. However some users want to
measure dynamic pressure changes in the system. For these users, it is very important that they tell their transducer support engineer that this is what they want to do.
The dynamic response of a system is a very complicated subject. This complexity is further compounded by the sometimes-misunderstood usage of such terms as frequency response, dynamic response, response to a step input, and rise time. These terms mean one thing when applied to the transducer and another when referring to the system. If dynamic measurements are required, it is wise to discuss the application with the pressure sensor manufacturer and seek his advice. In addition to entering into a dialogue with your supplier, here are a few additional guidelines for using transducers to measure dynamic pressure:
1. Couple the transducer as close as possible to the measuring point
2. Use a flush diaphragm transducer instead of a cavity type for better frequency response.
3. Avoid isolating the transducer with tubing or snubbers because system dynamics will change.
Handling and Installation
As mentioned earlier, today’s pressure transducer is an exceptionally rugged device. Even so, it pays to exercise a little common sense during handling and installation. Here are a few easy-to follow tips:
1. Be careful with the electrical termination. Damage to the electrical connector or cable could put the transducer out of service.
2. Never poke the diaphragm of a transducer with a pencil point or other stiff object. This could damage the diaphragm.
3. Do tighten the transducer well, making certain there is no leakage at the pressure fitting connection. 4. Make sure the pressure fitting is made of a compatible material. Combining materials with different thermal expansion characteristics (say a brass fitting and a stainless steel transducer) could result in a leak.
5. If the transducer has a zero and span adjustment, position it so the adjustment can be reached with a screwdriver.
6. Upon installation of the transducer, check the entire system for proper wiring, integrity of connections, and proper grounding. Poor electrical connections and improper grounding could cause elusive intermittent problems. If the problem persists even after a thorough system check, it may be reasonable to suspect that the transducer is faulty.