LEVEL MEASUREMENT SYSTEMS Flow Reference Section

is designed to provide high pulses and a transmit gate signal INTRODUCTION repeatability, minimizing the effects that are routed through the analog Level measurement is an integral of shock, vibration and pressure. signal processor to the sensor. part of process control, and may be Float switches can be used in a The sensor transmits an ultrasonic used in a wide variety of industries. variety of media, as they are beam to the surface level, and the Level measurement may be divided available with various materials of returned echo from the surface is into two categories, point level construction. Also, minimum detected by the sensor and routed measurement and continuous level maintenance is required, and to the microprocessor, which measurement. Point level sensors installation is simple. The processes the signal into a digital are used to mark a single discrete reed switch incorporated in representation of the distance liquid height, a preset level a float switch is a hermetically between the sensor and the surface condition. Generally, this type of sealed, magnetically actuated, level. The microprocessor stores sensor is used as a high alarm, make-and-break type. For various the distance value and, by means to signal the existence of an overfill models, the switches may be either of a moving average technique built condition, or as a marker for a low single pole-single throw (SPST), into the software program, alarm condition. The more or single pole-double throw (SPDT). compares it to the value of others sophisticated continuous level stored in memory. If the value does sensors can provide complete not correspond to prior signals or level monitoring of a system. NON-CONTACT new signals being received, it is A continuous level sensor, as the ULTRASONIC SENSORS rejected. Incorporation of a moving name implies, measures the fluid average technique and a non-linear level within a measurement range, The non-contact ultrasonic level digital filter ensures rejection of rather than at a specific, single sensors consist of the following spurious signals and noise. The point. The continuous level sensor elements: sensor, analog signal microprocessor constantly updates provides an analog output that processor, microprocessor, and reviews the signals received, directly correlates to the level within binary coded decimal (BCD) creating new averaged values which the containing vessel. This analog range switches, and an output are indications of the actual fluid signal from the sensor may be driver circuit. The microprocessor level in the tank. directly linked to a visual indicator generates a series of transmit or to a process control loop, forming Continuous Sensors a level management system. For continuous sensors, the averaged value from the microprocessor is converted into FLOAT SWITCHES an analog 4 to 20 mA signal which The basic float switch is a is linear with the liquid level. In an simple point level sensor. empty pipe or low level condition, A magnet-equipped float, which when the echo from the level does moves directly with the liquid not return to the sensor for more surface, actuates a hermetically than 8 seconds, the output signal sealed switch within the stem. The from the system drops below the rugged construction of the reed minimum 4 mA. switch design provides long, trouble free service. The float switch Point Sensors When the averaged value from the microprocessor coincides with the BCD switch setting, the microcomputer energizes an output relay, for either high or low level indication. A momentary loss of signal from the sensor will not FLOAT PERMANENT MAGNET change the output status; the microprocessor’s memory retains the last valid value. However, a signal loss for more than 8 seconds will cause the relays to HERMETICALLY de-energize and return to original SEALED state. A fixed half-second time MAGNETIC REED SWITCH delay is incorporated into the electronics to minimize the Non-contact ultrasonic sensor effects of surface turbulence Reed-relay float switch and agitation on the unit’s output.

K-3 droplets, foam or vapor. Liquids the material level changes. CONTACT which are highly aerated, or so These changes are electronically ULTRASONIC SENSORS highly viscous as to cling in the conditioned into retrospective sensor gap, however, may present capacitive and resistive values, and The contact ultrasonic liquid level operating difficulties. are further processed and converted switch detects and controls the into an analog signal. The probe level of liquids, at a point, by means and the vessel wall form the two of a low energy ultrasonic device CAPACITANCE plates of a capacitor, with the liquid having no moving parts. The LEVEL SENSORS medium serving as the dielectric. system consists of a field-mounted The signal is generated only from sensor with an integral solid state As with ultrasonic sensors, this level true level changes, thus rejecting amplifier. Terminal blocks are measurement technology can be the unwanted effects of material provided for connecting the system used for either point or continuous built-up on the probe. Note that for to a power source and to external level measurement. A probe a non-conductive fluid vessel, control devices. No calibration or inserted into the fluid tank senses special systems utilizing either adjustment is required for dual probes or an external automatic, trouble-free operation. conducting strip may be required.

1 The sensing probe may be The sensor contains a ⁄2" gap constructed from different materials, across which an inaudible, high LVU-230S, $489, shown with either a rigid or flexible design. frequency ultrasonic signal is smaller than The most common design is a generated. When liquid is in the actual size, conducting wire insulated with gap, the ultrasonic signal easily see page K-34. PTFE A stainless steel probe travels across the gap, and the is also available for applications control relay switches. The sensing where greater probe sensitivity level is approximately midway in the is necessary. For example, low gap for horizontally mounted dielectric or non-conductive sensors, and is at the top face of fluids (dielectric constant under 4) the gap for vertically mounted or granular fluids require the sensors. As the liquid falls below stainless steel probe. The flexible this level, the ultrasonic signal is probes are used when there is not attenuated and the relay switches enough overhead clearance for the back to its previous state. This type rigid probe, or in applications of level switch can be used in requiring the longer length. The vessels or pipes to automatically rigid probe provides higher stability, operate pumps, solenoid valves especially in turbulent systems. and/or audible or visual high and Contact ultrasonic sensor If the probe sways to and from the low alarms. Two units can control vessel wall, the signal from the tank filling (or emptying), or probe may fluctuate. metering of a specific volume of liquid. The switch will perform satisfactorily in most liquids, and is K unaffected by coatings, clinging

Switch Ratings Max Resistive Load VA Volts Amps ac Amps dc 0-50 0.2 0.13 10 120 0.08 0.05 240 0.04 0.02 0-30 0.4 0.3 20 120 0.17 0.13 240 0.08 0.06 0-50 0.5 0.5 50 120 0.4 0.4 240 0.2 0.2 120 0.8 N.A. 100 240 0.4 N.A. Ultrasonic interface detector

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