Pressure Transducers - Installation & Use
Common problems or questions concerning the use of pressure transducers are:
Each of these problems, or questions are discussed in detail in the following article.
1) TRANSDUCER OUTPUTS AND THEIR WIRING CONFIGURATIONS
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.
Milliamp (Current) 4-20 mA Output
Iout = current output 4-20 mA
Iin Maximum = 80 mA
Vin Minimum = 8 Volts
= 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.
RLINE = resistance due to wire
RLOAD = combined instrumentation resistances
VsTRANSDUCER = minimum supply voltage for transducer
For example, let's assume you have the following:
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.
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
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.
A 500/250 Ohm Resistor Would be Installed Across the (+) and (-) Terminals on the Instrumentation.
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°C. Avoid locations with excessive vibration.
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.