5 Things to Consider when Working with a Pressure Transducer
A pressure transducer data sheet can be a formidable document, with lots of numbers and specifications. Here is a list of the five most important areas to consider when selecting a pressure transducer for your application.
This may seem obvious, but it is important to know what pressure you have to measure – or at least have some idea. Often this is not a precisely known value – if it were, perhaps you wouldn’t need to buy the pressure – but at least an estimate of the range of pressures expected is helpful. You want to select a pressure transducer whose range is about 120% of the expected maximum pressure. This will leave ‘headroom’ to be able to sense when a pressure is higher than expected. It is not considered good practice to make measurements in the bottom 20% of a transducers range as this sacrifices too much accuracy. The accuracy of the transducer is always stated as a percent of full scale, and not as a percent of reading. If you wanted to select a pressure transducer to measure pressures between 80 and 100 psi, for example, a 125 psi full scale would be ideal and a 500 psi full scale would likely be too large to take satisfactory readings.
The type of pressure to be measured is also important. There are gauge pressures referenced to the atmosphere, absolute pressures referenced to a full vacuum and differential pressures – the difference in pressure between the two pressure transducer ports, For differential pressure transducers, the static pressure in the system is important. The pressure drop across an air filter is a duct will probably not involve a significant static pressure, but the pressure drop across a hydraulic filter will. The possibility of over-pressure – pressures that may occur that are higher than the transducer full scale – must also be considered. The limits for static pressure and over-pressure can be found in the Validyne data sheet for each pressure measurement type.
Power and Signal:
Pressure transducers require a power source and produce an electronic signal proportional to the pressure sensed. Power and signal must be matched to the environment and the receiving device. In an industrial environment, where long distances between the pressure sensor and a programmable controller are common, the two-wire 4-20 mA current signal is standard. This signal can be up to one mile and has the added advantage that only two wires are required for both power and signal. In laboratories with data acquisition systems, a voltage signal is often preferred such as 0 to 5 Vdc or 0 to 10 Vdc. This provides more resolution for the A/D converter but has a limit of about 10 feet between transducer and the data acquisition system before noise pickup becomes an issue.
Transducer response is often an issue – how fast is the pressure changing and can the transducer keep up? Or perhaps the opposite – how much can the output signal be filtered to smooth out readings in a noisy pressure environment. Validyne transducers are mechanically simple and respond to a pressure change very quickly. The transducer data sheets will have specifications for low pass output filters or damping.
Some transducers can provide a digital signal directly to a computer via USB or IEEE488 serial protocols. The readings from the transducer are given directly in engineering units and this can be a great convenience. The rate at which the readings can be collected and the software needed are important considerations, however.
Validyne transducers have a wide variety of power and signal combinations for most models. For digital transducers, Validyne will supply data logging software or driver code.
The fluid type is an important parameter and will determine what sensor materials are best suited to the application. Air, gas or liquid pressure media can be accommodated, but the sensor body material must be chosen for protection against corrosion. Validyne offers 410 stainless steel for air, benign gases and hydrocarbons in addition to 316 SST and Inconel for more corrosive fluids. The O-Ring seals must also be compatible with the fluid type and Validyne offers a wide selection of elastomer compounds. Validyne can help you choose the best materials for your application. (Parker O-Ring Compatibility Chart)
Environment and Enclosure:
The ambient installed environment is an important consideration. If the transducer is to be outside, then a NEMA 4 enclosure type is a standard requirement – Validyne transmitters such as model DR800 and P532 have spray-resistant enclosures and conduit connections to protect the power and signal wiring connections. If the pressure transducer is inside, the enclosure requirements are less rigorous and Validyne pressure transducers are happy in the typical industrial or lab environment.
Temperature is also a consideration – steam line pressures can be measured safely with a pressure transducer when a foot of bare steel tubing is used connect the pressure sensor to the steam pipe. Validyne has pressure transducer models that can operate in an ambient temperature of 250 F or as low as -65 F. Vibration and mounting can be important in mobile applications or when moving machinery is involved – be sure to minimize stress on the transducer pressure ports in these applications. If a differential pressure transducer is to operate at high static pressure, be sure to allow space for a three valve manifold.
You can buy a pressure transducer from a catalog on-line or on e-bay, but if the pressure transducer is for a critical application, it is always a good idea to check with the manufacturer and discuss the technical details. You should ask if their pressure transducers have been used successfully in similar applications, or if another model would be better. Often the manufacturer will recommend a model that is suitable and also saves money. Validyne has always believed in providing strong application engineering assistance to all our customers – large or small.
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