How to measure pressure with transducers?

Installation and Use of pressure transducers

Common problems or questions concerning the use of pressure transducers are:

Transducer outputs and their wiring configurations;
Wiring one transducer to multiple readouts, recorders, computers, etc.;
Wiring multiple transducers to one readout, recorder, computer, etc.;
Using a milliamp signal with voltage input instrumentation;
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.

TRANSDUCER OUTPUTS AND THEIR WIRING CONFIGURATIONS

OMEGA transducers have three main types of electrical outputs; millivolts (mV), 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 millivolt, volt and current output transducers.

Transducers with a millivolt output are generally used in laboratory applications. They are low cost, small in size, and require a regulated power supply. Remembering that the millivolt signal is very low level, it is limited to short distances (up to 200 feet is usually considered the limit) and is very prone to stray electrical interference from other nearby electrical signals (other instrumentation, high ac voltage lines, etc.). Typical wiring configurations are shown in Figure 1.

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 2.

A transducer produces millivolts, amplified voltage, or current output. 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. Typical wiring configurations are shown in Figure 3.

WIRING ONE TRANSDUCER TO MULTIPLE READOUTS, RECORDERS, COMPUTERS, 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 4.

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:

Pressure transmitter (4-20 mA) with 12-30 Vdc supply voltage;
Panel meter with a 10 ohm input impedance;
Recorder with a 25 ohm input impedance;
Computer with a 200 ohm input impedance;
Lead wire resistance of 5 ohms.

Minimum voltage required = (.020). (5 + 10 + 25 + 200) + 12 = 16.8 volts 24 volts is the most common power supply in a 4-20 mA current loop. Wiring a voltage or millivolt signal to multiple instruments also can be done, but is not as easy and does not have the calibration and troubleshooting advantages inherent in a current loop system. The voltage or millivolt signal can be wired in parallel to multiple instruments as shown in Figure 5. 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.

WIRING MULTIPLE TRANSDUCERS TO ONE READOUT, RECORDER, COMPUTER, 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 pressure sensor is 0.5% 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 2. Typical wiring configuration for voltage output transducer
(-excitation and -signal are common)

Figure 1. Typical wiring configuration for millivolt output transducer

Figure 3. Typical wiring configuration for current output transducer

Figure 4. Multi-instrument 4-20mA current loop (panel meters, chart recorder, computers, etc.)

Minimum voltage req'd = (0.20 Amps)(R LINE + R LOAD) + Vs TRANSDUCER
Figure 5. Multiple instruments wired in parallel to a voltage output transducer

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 7. 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
Example:
To Convert 4-20 mA into 2-10 V
R = V/I = 10/.02 = 500 Ohms A 500 Ohm Resistor Would be Installed Across the (+) and (-) Terminals on the Instrumentation

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.

Figure 6. Multiple transducers wired to one meter and one switch (transducers with built-in zero & span adjustments, same outputs & same pressure ranges)

Figure 7. Converting current into voltage for instrumentation set up for voltage

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.

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.