BASIC ELECTRONICS FOR FIELD MEASUREMENT John Culp Emerson Process Management Remote Automation Solutions Houston, Texas

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1 BASIC ELECTRONICS FOR FIELD MEASUREMENT John Culp Emerson Process Management Remote Automation Solutions Houston, Texas Introduction If you are able to install your own TV dish satellite system and a wireless home network for pes, you have a head starl 011 installing and maintaining electronic field measurement equipment. Today's measurement technicians and engineers are required to operate and maintain a variety of Hi-Tech field measurement equipment. Most of the field instrumentation is tightly integrated in a complete system functional environment. The larger the metering station, the more complex the system. The goal of this paper is to provide an introduction to electronics based on current field measurement technology. Basic field duties related to electronic instrumentation would include: installation calibration data collection, configuration, orifice plate changes, communication checks and maintenance. Some of the basic types of connections and measurements include power, analog signals and digital signals going into and out of the device. Each of these play unique circuitry roles and require different knowledge of their intended operation. The Basic Circuit Electricity has three components, Voltage (equivalent to pressure in a fluid piping system) measured in volts, Resistance (equivalent to pipe roughness) to the flow. measured in Ohms and current (equivalent to the fluid (gallons of water), measured in amps. If the pressure is one volt and the resistance one ohm the current will be one amp. V = I x R, In many tests voltage is referred to as the Electromotive Force and referred to as E but till measured in volts. So the text book fonnula is usually E = I x R The units of measure include V or E for Voltage (Volts), I for ClIrrent (Amps), R for Resistance (Ohms Q), P is for power measured in watts where: P=PxR p~ E x I The best way to understand this relationship is to look at a basic circuit. A simple circuit requires a power source to generate an electromotive (electrical power) source. In the circuit there is a load that provides the majority of the resistance to a flow in current and a switch to control the flow. As shown below the symbol for a battery or power source is on the left, the load is at the

2 top and the switch that controls the flow is one the bottom. Closing the switch completes the circuit and allows the current to flow. In this case the voltage applied is 12 volts and the resistance or load is two amps so the current is (12 16 = 6) 6 amps. Current flows from one tenninal to another only when there is a completed circuit. The device being powered must have enough current and voltage to insure proper operation (Inrush). Most Field electronics operate off of 6, 12 or 24 volts DC. A way to remember the relationship between voltage, resistance and current is shown in the diagram to the right. 20(ohm,), Battery (Powel Souce) Vois (E)' 12 Vois DC load (R) Ohms V =I R I= y R=y R 1 I=V IR Basic Electric Circuit P = J. 1,1 P :: 12. R P = y2 I :: Current (A) P :: Power ('0'01) R:: Resistance (Q) V :: Voltage (V) Relationship Diagram R In most text books the current is shown flowing from + to - on the voltage source but the actual electrons are negatively charged and flow from negative to positive. This is one of those "do we really care" facts. Current to vouage basics Most RTUs, PlCs and flow computers create outputs in the form of a transm itted signal that generates a current that is proportional to some level of measurement. The signal usually ranges from 4 mi ll iamps to 20 mi lliamps. 4 milliamps represents the minimum (sometimes zero) and 20 milliamps the maximum. So if I am measuring temperature I can calibrate my transmitter to send 4 milliamps when the temperature is O' Centigrade and 20 milliamps when the temperature is loo Centigrade. Thus a 12 milliamp signal would represent the halfway point and would indicate 50' Centigrade. Some people like to read their signals in voltage. By installing a 250 ohm resistor (shown below) 1 can convert a signal that will range from I to 5 volts. - ==1"P=;- Typical Resistor So a 4 milliamp signal would be measured as I volt and the 20 milliamp signal would be measured as 5 Volts. 4rnA x 2500 ~ I V 20rnA x 2500 ~ 5 V Current output devices are still common. When the transm itter is designed to deliver a current for a given signal it has several advantages. First it can be sent to more than one measuring device and can be sent to displays with thousands of feet between devices. In some cases the circuits become more complicated and more devices are connected. A more complicated circuit is shown below.

3 i 1 = 6 6= 2 +4 "- + 30(OhmS)~ i2=2a L i3 = 4 a I 6Vofts \: ~ T-1= VIR 30 (OhmS)~ 6 Vofts Complex Electric Circuit I I When the circuits get connected to equipment they can look like the diagram below. POl/-fER i 5.4 TO 36VDC) COMMON SIGNAL INPUT (O PEN CO LLE CTORi r "o------r.lax 1000 FEET 0 05 METERS) -----;.011 PULLUP RESISTOR END DEVICE " ~ Preamplifier OC Output Diagram Tl"Oubleshooling & Checkups 3-co N D U CTORS HIELDED CABLE. 2.AWG OR LARGER OC OUT MAX SOmA CONNECT SHIELD ONL Y AT END DEVICE PV-:R GND 0-5 OU T OC OUT COIL COIL- 24AWG OR ~ PICKUP LARGER PREAMPLIfiER TWISTED PAIR COIL I MAX 100 FEET I ~ i 30 METERS) ~ As-built drawings, Make & Model of equipment lists and the manuals usually are available from the manufacturer to describe the equipment. Most manuals can be found on the vendor's website. Other useful information as to support contacts, software upgrades/patches and FAQs ( Frequently Asked Questions) is also online. f'ield Checks I. Are there Lights, LCD, or other health indications'? 2. How's the Power'? 3. Check the Fuses. 4. How's the system grounding? (Another lengthy discussion)

4 The ~I way to verify comet operation is to interrogate the device by us ing its diagnostic modes. A laptop PC or PDA is needed. Some new technology even has predictive diagnostics, integrated documentation, calibration management and device configuration. Analog Signals 4 to 20 milliamp or I to 5 volts are generated so their level is in proportion to or analogous to the level of signal being sensed. As a result they are called analog signals. When more than one device is driven by an analog signal it is often useful to look at a Voltage Burden Chart. Loop loading can be seen on the Input Burden Chan below. This chart shows the maximum number of 2500 loads a device can power, based on loop voltage levels. This applies to both analog inputs and analog outputs. 30V., -., 20V '" ~ -:::l Q. c:..,., -:::l 10 V 1 Input Voltage Burden Chart V./ / ~20mA././ V"././'.J 10'" ~12mA././"././ I' 'f"./' 4mA / / o Output Load (0) Digital Signals Digital Signals is one that is ether on or off and are normally used as status type inputs that sense a voltage level transition. This could be from a valve limit switch. The signal is a low to high voltage level transition.

5 Freguency/Pulse Signals Some signals are in the form of a senes of pulses where the number of pulses in one second, represent the value of the signal. For instance ten pulses in one second could indicate ten volts or ten barrels of oil. Turbine meters, conolis meters and ultrasonic meters have a pulse or frequency type outputs. They are best measured by using an oscilloscope. This allows you to view the amplitude and frequency of the signal. Un-amplified signals are usually small amplitude sine waves. Amplified device outputs are more than likely a square wave. The chart below gives an example of the millivolt input lo frequency ratio on the input of a preamp. This shows that a high frequency (Noise) small amplitude signal would be filtered out... E :> - ~ 1 'g, E " E., E j l-=dd::ttI Frequency (Hz) Relays Preamp Frequency Input Graph Relay type circuits are magnetically operated switches. A wetting voltage usually DC is applied to the coil windings which produce a magnetic field used to open or close the relay switch. Relay terminals are referred to as "Normally Open", or "Normally Closed". Relays can be used to tum on or off different actuations such as: Solenoid valves, gas samplers, and radios. Open Collector Some signal inputs and outputs may require a "pull-up" resistor to source thc voltagc. These nrc mostly referred to as "Open Collector". Check the specifications to insure proper resistor sizing. The resistor has to be sized to allow only enough current to flow through it. Digital Signals Digital signals are normally low level, low voltage input types. They are typically looking for a voltage transition between 0 and 5 volts DC. Some digital signal types are as follows:

6 Digital Outputs are normally used to trigger an event like a smart gas sampler. This output could also be "OC" type. The signal is a low to high voltage level transition. Communications are digital in nature because they carry the binary code (Computer Talk) information needed for data transfer. While there are many different hardware formats for carrying data, RS232 is still the most common device interface today. The table shows the connector pin out for two types of common cables. Ethernet Ethernet connectivity is being used for most plant type operations involving line power flo w computers. This is a very common network medium that is spreading to more complex field measurement scenarios. Most RTUs, Gas Chromatographs and Gas Ultrasonic meters have Ethernet ports. Simply follow the instructions needed for setting up your laptop's Ethernet port. Fieldbus Connecting a set of measurement transducers to a flow computer has been traditionally an analog world. There is the low power 1 to 5vdc method and the 4 to 20 rna method for line powered applications. Fieldbus changes this in two ways. A standard set of wires and connectors are used between devices and a common communication protocol is used. Less fie ld wiring is needed and device interchangeability is added. Device interrogation is done digitally now instead of analog. Process variables along with performance diagnostics are now available at the device level. Protocols A protocol is a set of pulse codes that are designed to transmit information. Morse code used in telegraph messages is an example of a protocol. The de facto standard for flow computer protocol is ModSus, (developed by Modicon) or ASCII (developed by EIA). All major vendors otter some variation of this. This was the first step toward vendor interchangeability. The next step is the use of higher level protocols based from the internet standards. As communications become more complex more than one wire may be required to send information from one location to another. Multi-pin connections became the standard for the industry until the Ethernet moved in. Examples of multi-pin connectors are shown below. RS= D~~) OB9 D~~) 0826

7 Some Field Technician Tools Portable Laptop Computer with device software Cables Digital Voltmeter Transducer Calibration Test equipment Some Common Field Instruments Gas flow meter with solar panel for power. Area Classifications If the equipment is installed in a hazardous area for measuring natural gas or liquid hydrocarbons. it must meet certain safety requirements. In the Americas. U.L or CSA type ratings are required. Sanity Checks and Procedures Remember to shut down and start up electronic measurement & control equipment properly. Some valves and regulators will need to be placed in manual operation until the electronics are brought back online. Failure to operate and maintain the equipment in the manner it was designed for, might have undesired consequences. Results of this could be inaccurate measurement accounting and deliveries or worse. Before you attempt to work on any equipment, make sure that the area is safe. Sare from a hazardous area perspective and by general electrical safety precautions. Cautions such as, but not limited to: Working on (Hot) powered equipment while standing in water. Turning process control equipment off without setting proper manual, override or bypass conditions.

8 Conclusion As measurement devices and techniques change, so must the skill level of the Technician or Engineer. Training is a necessity and not a luxury. OJT (On the Job Training) in the measurement of natural gas does not lend itself to good results. Make sure there is a sound knowledge of the device's operation, especially when it relates to custody and gas control operations. Many times if an RTU or flow computer has to be configured, repaired or replaced, a software cold start or program memory re-flash will be required. Simply removing power will not work anymore! Learn the essentials.