Pressure Transducers - Installation & Use
INTRODUCTION
Common
problems or questions concerning the use of pressure transducers are:
1.
Transducer outputs and their wiring configurations.
2. Wiring one transducer to multiple readouts, recorders, computers, and
etc.
3. Wiring multiple transducers to one readout, recorder, computer, and
etc.
4. Using a milliamp (current) signal with voltage input instrumentation.
5. Determining how many transducers can be excited from one power
supply.
Each of
these problems, or questions are discussed in detail in the following
article.
1)
TRANSDUCER OUTPUTS AND THEIR WIRING CONFIGURATIONS
FTB
transducers have two main types of electrical outputs; volts (V), and
current (mA). It is important for the user to know which output
suits his application to ensure proper selection of a transducer.
The following will describe the advantages, disadvantages, and wiring for
volt and current output transducers.
Type I: Amplified Voltage Output (0-5 VDC or 0-10 VDC)
Transducers with an amplified voltage output are generally used in a light
industrial environment and computer interface systems, where a higher
level DC signal is required. Due to the built-in signal
conditioning, they are higher cost and larger in size than the millivolt
output transducers. Amplified voltage signals can travel up to
medium distances and are much better in their immunity to stray electrical
interference than the millivolt signal. Typical wiring
configurations are shown in Figure 1 below.

Figure 1
Transducers with an
amplified voltage output
Type II:
Milliamp (Current) 4-20 mA Output
A transducer
produces millivolts, amplified voltage, or current output; however, a
transmitter produces current output only. Again, due to the built-in
signal conditioning, the transmitters are higher cost and larger in size
than the millivolt output transducers. Unlike the millivolt and voltage
output transducers, a current signal is immune to any stray electrical
interference, a valuable asset in the factory. A current signal also can
be transmitted long distances. Please be noticed that FTB current
output transducers are 3-wire wiring transducers. The wiring
configurations are the same as amplified voltage output transducers.
Typical wiring configurations are shown in Figure 2 below.

Figure
2
Transducers
with a current (4-20 mA) output
WHERE:
Iout
= current output 4-20 mA
Iin Maximum
= 80 mA
Vin Minimum
= 8 Volts
Vin Maximum
= 30 Volts
2) WIRING
ONE TRANSDUCER TO MULTIPLE READOUTS, RECORDERS, COMPUTERS, AND ETC.
One of the
great advantages of a current signal is the simplicity in setting up a
multi-instrument system. Long distance transmission from instrument to
instrument without electrical interference make multi-instrument systems
easy. For example, a material test center may have one control room for
all the different test labs, enabling operation from one central location. Instrument calibration and troubleshooting are simple in a
multi-instrument current loop. The only limitation for the number of
instruments is the amount of voltage from the power supply driving the
current loop. The minimum voltage required is determined by Ohms law, V-IR
(voltage equals current times resistance). This is shown and explained in
Figure 3 below.

Figure 3
Wiring
one 4-20 current transducer to multiple instrumentations
WHERE:
RLINE
= resistance due to wire
RLOAD
= combined instrumentation resistances
VsTRANSDUCER
= minimum supply voltage for transducer
For
example, let's assume you have the following:
1.
Pressure transmitter PT-75 (4-20 mA) with 8-30 VDC supply voltage;
2. Panel
meter with a 10 ohm input impedance;
3.
Recorder with a 25 ohm input impedance;
4.
Computer with a 200 ohm input impedance;
5. Lead
wire resistance of 5 ohms.
Minimum
voltage required = (.020). (5 + 10 + 25 + 200) + 8 = 12.8 volts. 24
volts is the most common power supply in a 4-20 mA current loop.
Wiring a voltage signal to multiple instruments also can be done as shown
in Figure 4. The installation is the same and as easy as our current
output 4-20 mA transducers. The voltage signal can be wired in parallel to multiple
instruments as shown in Figure 4 below. This method assumes a very high
input impedance in the instruments being wired. If this is not the
case, an analog output can be used instead to retransmit the signal.

Figure 4
Wiring
one amplified voltage transducer to multiple instrumentations
3)
WIRING MULTIPLE TRANSDUCERS TO ONE READOUT, RECORDER, COMPUTER, AND ETC.
In
measuring multiple pressures, it is a common mistake trying to use
multiple transducers, a switching device, and just one panel meter, thus
saving money on multiple panel meters (or any other instrumentation).
The problem is that each transducer has a unique zero point and the
readout only has one zero screw. The net result is that the total
accuracy increases to about 3%, even though each sensor is 0.5-1%
accurate. In most cases, this larger error is intolerable.
The correct
method of using multiple transducers with one readout device is to use
transducers that have built-in zero and span adjustments screws, the same
output (voltage or current), and the same pressure range. Each
transducer is adjusted by applying a known pressure, so that they all have
identical outputs. When they all have identical outputs, the meter
is scaled and a switch can be used.
Another
solution to using multiple transducers with one readout is to use a
scanner instead of meter and a switch. There are many types of
scanners. The type of scanner that works with multiple pressure
transducers must have independent scaling on each channel.
Some
scanners, besides having independent scaling on each channel, also offer
independent current, voltage, or millivolt inputs to each channel.
These types of scanners enable you to use transducers with different
outputs as well as different pressure ranges with the same instrument.
Figure 1.
Typical wiring configuration for voltage output transducer
(-excitation and -signal are common)
Figure 2.
Typical wiring configuration for current output transducer
Figure 3.
Multi-instrument 4-20mA current loop (panel meters, chart recorders,
computers, etc.)
Minimum
voltage required = (0.20 Amps)(R LINE + R LOAD) + Vs TRANSDUCER
Figure 4.
Multiple instruments wired in parallel to a voltage output (0-5 VDC or
0-10 VDC) transducer
Figure 5.
Converting current into voltage for instrumentation set up for voltage
4) USING
A MILLIAMP SIGNAL WITH VOLTAGE INPUT INSTRUMENTATION
Most
instrumentation is set up to receive voltage. A commonly asked
question is how to use a current signal with instrumentation set up for
voltage. This is simply done by installing a resistor across
the input terminals of the instrumentation. The value of the resistor is
determined by Ohms law (V = IR). For example, installing a 500 ohm
resistor will convert 20 mA to 10 volts (V = IR = .020 x 500). This
is shown in Figure 5 below. The only other consideration is the zero
offset. Since most current loops have a low end of 4 mA, there will
be a zero offset. Using the same value resistor as above 4 mA will
convert to 2 volts.
R=V/I
Where: R = Size of Resistor
V = Desired Voltage
I = Current
Minimum Power Required for
1. 10 Volts (output) + 8 Volts (for transducer operation) = 18
Volts, but not exceeding 30 Volts
2. 5 Volts (output) + 8 Volts (for transducer operation) = 13
Volts, but not exceeding 30 Volts
Example:
To Convert 4-20 mA into 2-10 V
R = V/I = 10/.02 = 500 Ohms
To Convert 4-20 mA into 1-5 V
R = V/I = 5/.02 = 250 Ohms
A 500/250
Ohm Resistor Would be Installed Across the (+) and (-) Terminals on the
Instrumentation

Figure 5
Convert
current into voltage for instrumentation set up for voltage
5)
DETERMINING HOW MANY TRANSDUCERS CAN BE EXCITED FROM ONE POWER SUPPLY
Multiple
transducers can be excited from one power supply. The number of
transducers that can be used is simply determined by the current draw of
each transducer and the current capacity of the supply source. The
sum of the current draw of the transducers can not exceed the total
current capacity of the supply. For example, if you have 50
transducers drawing 13 milliamps, you will need a power supply having at
least 650 milliamps (50 x 13). There is also nothing wrong with
powering just one transducer with a power supply having high current
capacity.
HANDLING, LOCATING AND INSTALLING TRANSDUCERS
A.
Diaphragm - Do not press or touch the diaphragm as you may
damage or alter its calibration, particularly on low pressure range
models.
B.
Fittings and Hardware - Use appropriate pressure rated fittings
and hardware. Make sure you have the correct thread type and size
fitting. Use pressure limiters, capacity chambers, snubbers,
etc., if needed.
C.
Operate at Ambient Temperatures - Locate the transducer where
it can be readily inspected and serviced. Ambient temperature should
be within the transducer specifications. The temperature
coefficient effects on the overall accuracy of the transducer can be
minimized the closer the ambient temperature is to 25蚓. Avoid
locations with excessive vibration.
D.
Installation - Installation should be made only by qualified
personnel familiar with safety practices and knowledgeable with all
industry accepted standard relating to pressure systems.
Transducer calibration and/or zero may shift if it is over-torqued
when installing. Check for a zero shift after installing.
When installing transducers, refer to standard industry torque data
for thread size and material type. |
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