Home | Forum | DAQ Fundamentals | DAQ Hardware | DAQ Software Input Devices | Data Loggers + Recorders | Books | Links + Resources |
AMAZON multi-meters discounts AMAZON oscilloscope discounts Instrumentation and transducer considerations There are many considerations that have to be taken into account when selecting instruments and transducers. The following is an explanation and index of the more important aspects of choice. Signal transmission pneumatic vs electronic Electronic means for signal transmission and control is becoming more favored, however pneumatic controls are still used and do have advantages in different applications. Advantages - electronic: • Lower installation cost • Lower maintenance • Higher accuracy (especially smart instruments) • Faster dynamic response • Suitable for long distances • Digital control system compatible. The primary reason for selecting electronic devices is their compatibility with the control system. With data exchange highways becoming more common it’s also easier to obtain more information from the sensor with smart electronics. AMAZON multi-meters discounts AMAZON oscilloscope discounts Advantages - pneumatic: • Lower initial hardware cost • Simple design • Less affected by corrosive environments • Easily connected with control valves • Pneumatics has a prime advantage because of their safety in hazardous locations. Signal conditioners Signal conditioners change or alter signals so that different process devices can effectively and accurately communicate with each other. They are typically used to link process instruments with indicators, recorders, and microprocessor-based control and monitoring systems. They consist of either: Signal conversion: A signal converter is used to change an analog signal from one form to another. This enables equipment with differing signals to communicate. Signal boosting: For analog signals (voltage) that are required to be transmitted over long distances, it’s possible that the signal may attenuate, or fade. For analog signals (current) in loops that have a number of loop-powered devices, the signal may not be strong enough. Noise Electrical noise, or interference, is unwanted electrical signals that cause disruptive errors, or even completely disable electronic control and measuring equipment. There are two main categories of electrical measurement noise: 1. Radio frequency interference (RFI) 2. Electromagnetic interference (EMI). Some examples of the more commonly encountered sources of interference are: • Hand-held (walkie-talkie) • Cellular phones • AC and DC motors • Transformers • Arc welders • Large solenoids, contactors and relays • High power cabling, both voltage and current • High speed power switching, such as SCRs and thyristor • Variable frequency drives • Static discharges • Induction heating systems • Radar devices • Fluorescent lights. Radio frequency interference and electromagnetic interference can cause unpredictable performance in instrumentation. These types of interference can often be non-repeatable, making it hard to detect, isolate and rectify the problem. RFI and EMI can also degrade an instrument's performance and possibly cause the instrument to fail completely. AMAZON multi-meters discounts AMAZON oscilloscope discounts Any of these problems can result in reduced production rates, process inefficiency, plant shutdowns and possibly even create dangerous safety hazards. There are two basic approaches to protecting instrumentation systems from the harmful effects of RFI and EMI. 1. The first is to keep the interference from entering the system by: • Shielding • Proper grounding • Terminal filters. 2. The second is to design the system so that it’s unaffected by RFI and EMI. Noise reduction techniques: Some of the more common techniques for reducing or even eliminating electrically induced noise are: • Use of transmitters, i.e. for thermocouples: The signal is more robust to noise over long distances. Typically 4-20 mA. • Shielded/twisted pair cable: Twisting is done to decouple the wires from induced currents from varying electric and magnetic fields that may exist. The principle of twisting is that equal voltages are induced in each loop of the twisted wires, but of opposite phase which makes them cancel. • AC-inductive load circuits: For AC-inductive loads, use a properly rated MOV across the load in parallel with a series RC snubber. An effective RC snubber circuit would consist of a 0.1 µF capacitor of suitable voltage rating, and a 47 ohm, 0.5 W resistor. • DC-Inductive load circuits: For DC-inductive loads, use of a diode across the load is effective, provided the polarity is correct. Use of an RC snubber circuit can be added as an enhancement. Materials of construction Often when selecting measurement or control equipment, options are available for the various materials of construction. The primary concern is that the process material won’t cause deterioration or damage to the device. Below is a brief list of other qualities or characteristics that assist in the selection of the material of construction. • 316SS • Hastelloy C-276 • Monel • Carbon steel • Beryllium copper; good elastic qualities • Ni-Span C; very low temperature coefficient of elasticity • Inconel; extreme operating temperatures and corrosive process • Stainless steel; extreme operating temperatures and corrosive process • Quartz; minimum hysteresis and drift. Signal linearization When the output of a device responds at a proportional rate to changes in the input, then the device is linear and there is a constant gain (output / input) over the full range of operation and the resolution remains constant. If the response or reaction of some device in a system is not linear then it may need to be made linear because there are two main problems, when the device is not linear: 1. The gain changes 2. The resolution and accuracy change. In a control system there are three ways to account for non-linear equipment: 1. Base application on the highest gain 2. Measure the gain at a number of points 3. Modify the gain as a function of the process variable. The simpler way to overcome any non-linearity is to linearize the signal before the control system calculations. Selection criteria + considerations Reasons for selecting one type of measuring equipment over another vary, but typically the decisions are based on the perceived advantages and disadvantages of the range of devices available. A comprehensive list would take into account the following: • Accuracy • Reliability • Purchase price • Installed cost • Cost of ownership • Ease of use • Process medium, liquid/stem/gas • Degree of smartness • Repeatability • Intrusiveness • Sizes available • Maintenance • Sensitivity to vibration. In addition particular requirements for flow would include: • Capability of measuring liquid, steam and gas • Rangeability • Turndown • Pressure drop • Reynolds number • Up and downstream piping requirements. A more systematic approach to selection process measurement equipment would cover the following steps. Application These are the requirement and purpose of the measurement. • Monitor • Control • Indicate • Point or continuous • Alarm. Processed material properties Many process-measuring devices are limited by the process material that they can measure.
Performance This relates to the performance required in the application.
Installation Mounting is one of the main concerns, but the installation does involve the access and other environmental concerns.
Economics The associated costs determine whether the device is within the budget for the application.
Processed material properties Many process-measuring devices are limited by the process material that they can measure.
Performance This relates to the performance required in the application.
Installation Mounting is one of the main concerns, but the installation does involve the access and other environmental concerns.
Economics The associated costs determine whether the device is within the budget for the application.
Failure detect; To thermocouple; Polynomial table (E2 Prom); Error; Cold junction compensation; Range Zero. - ve -- Span Processor Output 4-20mA; 0-10V Output Linear response; Thermocouple response 20 mA/10 V 4mA/0V mA/V/°C ++++Microprocessor-based thermocouple measuring system with ranged and linear output. This then gives the availability to use a thermocouple of a type with a total range of 0-650°C, to be ranged, or set for 100-300 °C and this, with a 4-20 mA transmitter output, gives an output = 12.5 °C/mA, linearly through the required or selected range. In normal use, using the entire range of the thermocouple, we would have an output sensitivity of 650 °C/20-4 mA or 40.625 °C/mA giving a sensitivity ratio of 3.25:1. This concept can be applied to most types of measurement transducers, conceptually saying that the output of these devices can be ranged and made linear before being introduced to the controllers inputs itself. NEXT: Control valves and actuators: Basic principles |
Updated:
Thursday, March 21, 2013 5:01
PST