When selecting a pressure transducer, it is important to consider the following factors
to insure safety and accuracy.
Introduction to Pressure Transducers & Transmitters
Pressure Transducer Types
Output Signal
Pressure Ranges
Maximum Pressure
Burst Pressure
Supply Voltage or Excitation
Calibration
Pressure/Process Connection
Electrical Connection
Accuracy & Performance
Temperature
Pressure Fluid Composition
Electrical Protection
Shock & Vibration Resistance
Introduction to Pressure Transducers & Transmitters
Pressure transducers and transmitters convert an applied pressure into an electrical signal. This signal is both linear and proportional to the applied pressure. Pressure sensors, transducers, and transmitters are commonly referred to as just pressure transducers. The output electrical signal is sent to computers, PLC's, chart recorders, digital panel meters or other devices that interpret this signal and use it to display, record, and/or change the pressure in the system being monitored. The most popular output signal is a 4-20 mA 2 wire current loop. Other voltage signals such as 0-10 VDC are also used in some applications. All transducers require an input (also referred to as excitation or supply voltage) in order to power the internal circuitry. Wika transducers use two types of strain gauge sensors, piezoresistive and thin film. Piezoresistive sensors are used in low pressure applications up to 300 PSI and thin film sensors above 300 PSI.
Pressure Transducer Types
General Purpose Transducers provide excellent performance, reliability and value for a wide variety of electronic pressure measurement applications. The accuracy is less than or equal to 0.5% BFSL full span. These units are available in pressure ranges 0-15 PSIG up to 0-5000 PSIG.
General purpose transducers are available with a 4-20mA or 0-10VDC output.
Industrial Grade Transducers are precision engineered to fit the majority of industrial pressure measurement applications. Each unit undergoes quality control testing and calibration to achieve an accuracy of 0.25% BFSL full scale. The printed circuit board uses state of the art surface mount
technology and is potted in silicone gel for protection against vibration, shock, and humidity. Industrial models have a 4-20mA output signal and offer pressure ranges including vacuum, compound, and pressures up to 5000 PSIG. Advantages of the industrial grade model include higher accuracy, zero & span adjustment for recalibration, and better resistance to vibration, shock and humidity than most transducers on the market.
Industrial NEMA 4X transducers are designed for extremely harsh, washdown, and corrosion resistant industrial pressure measurement applications. State of the art surface mount technology ensures that the unit is protected against vibration, shock and humidity. The compact design features an integral NEMA 4X junction box and accuracy of 0.25% BFSL full scale.
Submersible Liquid Level Transmitters and transducers are engineered for a wide variety of industrial and municipal liquid level measurement applications. Typical applications include: water and wastewater treatment, well depth measurement, and offshore water depth measurement. These transmitters feature a high strength watertight, vented polyurethane cable and meet NEMA 6P and IP68 requirements for total submersion.
Output Signal
We offer transducers with either an industry standard 4-20mA current or 0-10VDC voltage output. This signal is linear and proportional to the input pressure. The 4-20mA output signal is the most popular and offers the following advantages: immunity to interference from electrical noise (RFI), low installation cost-only 2 wires required, distances up to 10,000 feet are possible, and the minimum 4mA signal can be used for diagnostic capabilities. The output signal is sent to computers, PLC’s, chart recorders, digital panel meters, or other devices that interpret this signal and use it to display, record and/or change the pressure in the system being monitored.
Pressure Ranges
Wika transducers are fixed range. The range selected is determined by the application requirements. The standard range should be equal to or greater than the highest expected working pressure generated by the application. There are four basic pressure references as follows:
Vacuum transducers are vented to atmosphere and produce a 20 mA output when no vacuum is applied. The output decreases as the vacuum applied increases.
Gauge Pressure transducers are referenced to atmospheric pressure. With no pressure applied, the output is 4 mA or 0 VDC. Gauge pressure transducers usually refer to the pressure range as just PSI (pounds per square inch) or PSIG (PSI gauge) and are the most common transducers.
Compound ranges are vented to atmosphere, and measure both vacuum or pressure relative to atmospheric pressure.
Absolute Pressure transducers are referenced to a sealed vacuum chamber located behind the sensor element. These transducers sense changes in barometric pressure as the applied pressure is compared to a known vacuum. Absolute units are used where barometric changes affect the operation of the system being measured.
Maximum Pressure
The maximum pressure defines the safety margin of pressure the transducer can tolerate (for short periods of time) without damage.
Burst Pressure
The maximum pressure causing permanent, nonadjustable damage or destruction of the transducer.
Supply Voltage or Excitation
All transducers require an input voltage in order to power the internal circuitry. The standard optimal input voltage is 24 VDC, although any voltage within the published range will work. Our transducers feature a built in power regulation circuit, an unregulated power supply is acceptable.
Calibration
Industrial grade transducers feature on board adjustable zero and span potentiometers for recalibration (up to +/- 5% of span) as required for maintenance procedures. The general purpose models can be recalibrated by changing the scaling of the digital panel meter or PLC to compensate for changes in transducer output caused by over pressurization or long term drift.
Pressure/Process Connection
Transducers are available in several types of process connections including NPT (national pipe thread), flush mount diaphragm, 3A sanitary 1.5 or 2 inch Tri-Clamp or submersible with vented cable. NPT connections are for use with non-clogging pressure media only. When measuring media that is viscous, crystallizing, or contains particulates a flush diaphragm type transducer or a transducer with a chemical diaphragm seal should be installed to protect the transducer. The 3A sanitary transducers are commonly used in food and pharmaceutical applications.
Electrical Connection
Most of our standard general purpose and industrial grade transducers are shipped with an easily removable black L-shaped DIN 43650 style plug connector. The connector has a built-in compression fitting and a solderless screw terminal block for easy installation of your own wiring. These connectors have an IP65/NEMA 5 rating which is dust tight and weather resistant. The same connector with a 1/2" NPT female conduit connection is available under transducer options. The heavy duty NEMA 4X transducers feature a junction box for washdown, corrosion resistance, and protection from electromagnetic interference with a female 1/2 inch conduit connection.
Accuracy & Performance
Accuracy of the pressure transducer is one measure of performance and includes the following:
Linearity is the error defined by the maximum deviation of a transducer output from a best fit straight line (BFSL) during any one calibration cycle. This is measured as a percent of span. An accuracy of +/- 0.25% BFSL is equivalent to +/- 0.50% terminal point accuracy for calibration purposes. Ex. A 0-100 PSI unit with a +/- 0.25% BFSL accuracy could deviate up to +/- 0.50 PSI terminal point throughout its working pressure range.
Hysteresis is the error defined by the maximum measured separation between the upscale and the downscale indications of the measured pressure during a full range traverse from 0 to the maximum pressure.
Repeatability is the error defined by the ability of a transducer to reproduce an identical signal when the same pressure is applied to it consecutively, under the same conditions and in the same direction. In industrial applications repeatability is usually the most important specification.
Response Time is the length of time required for the output to change in response to a pressure change.
Temperature
Media Temperature is the maximum allowable temperature of the pressure media being measured without causing damage to the transducer. For steam applications, consider using a steam siphon to reduce the media temperature.
Ambient Temperature is the maximum allowable temperature around the case of the transducer without causing damage.
Temperature Compensation is the range in which temperature changes will cause an output change no greater than the temperature error.
Pressure Fluid Composition
Since the sensing element of a pressure transducer may be exposed directly to the measured medium, consider the characteristics of this medium. It may be corrosive, it may solidify at various temperatures, or it may contain solids that will clog or leave deposits inside the sensing element.
All general purpose and industrial grade transducers in our store have 316 stainless steel wetted parts and are for use with pressure fluids that will not solidify (non clogging) under normal conditions or leave deposits. Chemical diaphragm seals or flush diaphragm transducer are typically used to isolate transducers from viscous, corrosive, high temperature, or media containing particulates.
Electrical Protection
Wika transducers are protected against reverse polarity, short circuit output and incorporate a suppressor diode for high-voltage protection.
Shock & Vibration Resistance
Excessive vibration can damage transducers. Whenever possible, the user should try to install the transducer in an area that minimizes vibration. Vibration specifications on Wika transducers are among the best in the industry.
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