Measuring the speed of a car is done via the speedometer, measuring the flight altitude of an aircraft is done via the altimeter, but how can we monitor the convey performance of a pneumatic convey system? There is a limited amount of possibilities for such, one of which is measuring the “heartbeat” and provides the operator with all information on the vital condition of the pneumatic unloader.
In one of our previous blog, we explained:
Monitoring via the “Heartbeat”
As each step has its own start and end phase based on a specific pressure, which is monitored by the PLC via pressure transmitters. Let’s assume that the PLC will collect a pressure sample each second, we can visualize this into the following illustration per Tank within a timeframe:
A pressure transducer, often called a pressure transmitter, is a transducer that converts pressure into an analog electrical signal. Although there are various types of pressure transducers, one of the most common is the strain-gage base transducer.
The conversion of pressure into an electrical signal is achieved by the physical deformation of strain gages which are bonded into the diaphragm of the pressure transducer and wired into a wheatstone bridge configuration. Pressure applied to the pressure transducer produces a deflection of the diaphragm which introduces strain to the gages. The strain will produce an electrical resistance change proportional to the pressure.
The Electrical Output of Pressure Transducers
Pressure transducers are generally available with three types of electrical output; millivolt, amplified voltage and 4-20mA. Below is a summary of the outputs and when they are best used.
Millivolt Output Pressure Transducers
Transducers with millivolt output are normally the most economical pressure transducers. The output of the millivolt transducer is nominally around 30mV. The actual output is directly proportional to the pressure transducer input power or excitation. If the excitation fluctuates, the output will change also. Because of this dependence on the excitation level, regulated power supplies are suggested for use with millivolt transducers. Because the output signal is so low, the transducer should not be located in an electrically noisy environment. The distances between the transducer and the readout instrument should also be kept relatively short.
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.
4-20 mA Output Pressure Transducers
These types of transducers are also known as pressure transmitters. 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.
Diaphragm seals, also referred to as chemical seals, are used to isolate pressure gauges, switches and transmitters from clogging and/or corrosive media. Standard diaphragm seal bodies and diaphragms are made of stainless steel; however, a variety of materials from carbon steel to Hastelloy® C-276 are available to meet the demands of most applications.
Operating Principle of a Diaphragm Seal
The drawing below illustrates the operating principle of a diaphragm seal assembly. A pressure measurement instrument such as a conventional pressure gauge or electronic pressure transmitter is either mounted directly to the diaphragm seal or attached to the seal by means of a capillary or cooling element.
A diaphragm within the diaphragm seal separates the gauge/transmitter from the process medium. Any part of the diaphragm seal (i.e., diaphragm, lower housing, gaskets) which will be exposed to the process medium is selected from materials resistant to pressure, temperature and possible chemical attack by the process medium.
The diaphragm seal is also filled with a transmitting fluid or system fill fluid. Any pressure applied by the process medium to the seal diaphragm is hydraulically transmitted to the pressure element of the gauge/switch/transmitter thus generating a pressure reading.
Choosing Pressure Transducer
Still wondering how to decide what type of pressure transducer or pressure transmitter you need? To learn more about the criteria you should think about when making your decision, based on more than 50 years of experience in this industry, Lion Bulk Handling is able to assist you in advising the recommended pressure transmitters within your project. Find your parts here: http://www.lionbulkhandling.com/product/instrumentation/.