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Introduction
to Pressure Transducers & Transmitters
What is a pressure
transducer? A pressure transducer is a transducer that converts
pressure into an analog electrical signal.
A transducer produces
millivolts, amplified voltage, or current output; however, a transmitter
produces current output only.
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 current output.
Other voltage signals such as 0-5/10 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.
FTB transducers use FTB-patented and developed sensing/sensory system to
accurately measure the pressure from the application. Other than those
pressure sensors (strain gauge, piezoresistive and thin film) available
on market for most of the use on pressure transducers, FTB pressure
transducers are unique and capable of being used in both low and high
pressure applications up to 15,000 PSI.
Pressure Transducer Types
General Purpose
Transducers provide excellent performance, reliability and value for
a wide variety of electronic pressure measurement applications. 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 a higher
accuracy. 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, 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. However, FTB pressure transducers are designed to meet the
needs of the applications in general purpose and industrial grade with
pressures up to 15,000 PSIG.
The Electrical Output of Pressure Transducers
FTB Pressure transducers are
available with two types of electrical output; volt with 0-5 VDC or 0-10
VDC, and 4-20mA. We
offer transducers with either an industry standard 4-20mA current or
0-5/10VDC 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, 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.
Below is a summary of the
outputs and when they are best used.
1) 0-5 or 0-10
Voltage Output Pressure Transducers
Voltage output
transducers include integral signal conditioning which provide a much
higher output than a millivolt transducer. The output is normally
0-5Vdc or 0-10Vdc. Although model specific, the output of the
transducer is not normally a direct function of excitation. This means
unregulated power supplies are often sufficient as long as they fall
within a specified power range. Because they have a higher level
output, these transducers are not as susceptible to electrical noise as
millivolt transducers and can therefore be used in much more industrial
environments.
2) 4-20 mA Current
Output Pressure Transducers
These types of
transducers are also known as pressure transmitters. As
a transducer produces
millivolts, amplified voltage, or current output, a transmitter produces
current output only.
Since a 4-20mA signal is
least affected by electrical noise and resistance in the signal wires,
these transducers are best used when the signal must be transmitted long
distances. It is not uncommon to use these transducers in applications
where the lead wire must be 1000 feet or more.
Pressure Ranges
FTB transducers are
fixed range. The range selected is determined by the application
requirements. The standard range should ALWAYS be greater than
the highest expected working pressure generated by the application. The
most recommended range for the application is 2 times of the working
pressure. Basic pressure references are as follows:
Vacuum transducers are vented to atmosphere and produce a 4 mA
output when no vacuum is applied. The output increases as the vacuum
applied increases (20 mA maximum).
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.
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 5-24 VDC (amplified voltage output
transducers) 8-30 VDC (4-20 mA current output transducers), although any
voltage within the published range will work. Our transducers feature a
built in power regulation circuit, an unregulated power supply in
between 5-24 VDC or 8-30 VDC is acceptable. It is strongly recommended
using a regulated power supply of 24 VDC on amplified voltage output
transducers to maintain the best accuracy of the transducer. If an
unregulated power supply in between 5-23 VDC is applied, the accuracy of
the transducer will be slightly different from what an 24 VDC regulated
power is supplied. For minimum voltage required for current 4-20 mA
output transducers, please refer to
FTB Pressure Transducers - Installation & Use for details.
Calibration
FTB transducers feature on board zero adjustment for recalibration as
required for maintenance procedures on both Industrial Grade and General
Purpose transducers.
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
transducer with a chemical diaphragm seal is commonly used in food and
pharmaceutical applications.
Electrical
Connection
Our transducers are shipped
with an easy-to-install wiring (3 wires). The wiring is therefore
connected directly to the wiring of the application.
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.
FTB transducers are all with 1% F.S. (full scale) accuracy.
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. However, all FTB-patented transducers are FREE from Temperature
Compensation. There will have NO output change caused when the
temperature changes. This makes FTB transducers more reliable over the
ones available on market.
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 FTB transducers 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. Optional brass fitting transducers are also
available upon request.
Electrical
Protection
FTB 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 FTB transducers are among the
best in the industry.
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Stainless Steel
316 Ti Stem for long-lasting life span to be exposed under extreme
conditions.
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Important
components make the accuracy accurate.
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