Measurement Technologies Adapted by ISG Devices

Fluid Level

The Fluid Level Sensor detects and reports the presence or absence of fluid at the tip by immersion.  

These sensors:

• have no moving parts
• are sealed against leakage
• can be mounted in any orientation
• have a brass body that is electrically grounded
• incorporate a guard tip cover to protect the sensor element
• are ruggedly constructed to withstand harsh operating environments

Theory of Operation

These sensors incorporate a solid state temperature sensitive element (not a bimetallic switch contact) that is in a high impedance state while the sensor is immersed in fluid.  That element changes to a low impedance state when the sensor is out of fluid.  The sensor generates heat at its tip, which is removed by the surrounding fluid as rapidly as it is created, thus allowing the device to remain in the high impedance state.  The thicker and more viscous the liquid in contact with the sensor tip, the less heat can be removed, so the sensor trip point will be reduced. 

Once the sensor tip is removed from contact with the surrounding fluid, the element heats up and drives the sensor impedance down.  When the sensor is used as part of a voltage divider, the change in state is reflected as a change in the voltage drop across either the sensor or series load. The series load may be either a resistance or a tungsten lamp.  The final low impedance value of the sensor is a function of the ambient temperature, the heatsinking ability of the surrounding fluid, the quality of the thermal path and the magnitude of the current flowing through the sensor.

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Measurement technology for acoustics

IMPEDANCE TUBE WITH TWO MICROPHONES

The impedance tube is the most often used to determine the acoustic absorption coefficient of material samples. There is a sound source at one end of the tube, and a testing sample at the other end of the tube.  The absorption measurement is based on the measurement of the interaction between acoustic pressure of the incident sound wave and the reflected sound wave of the plane-wave at the locations of the microphones [3]. There are the two-, three- or four-microphone methods according to the number of microphones used in the measurement. Here we have an impedance tube with two microphones.  Scheme of impedance tube with two microphones are according to ISO 10534-2 Standard, the impedance tube must be sufficiently long to generate plane sound waves between the source and the sample. The tube must have a uniform cross -section along its entire length and smooth walls without pores and notches. For a tube with a circular cross-section, it is recommended to choose a length greater than 3 times the diameter of the tube [4]. Internal diameter of the tube used in the measurement is d = 0.088 m, and the tube length (i.e., the distance of the sample from the source) was l = 0.758 m. The standard recommends using microphones of the same type designed for free sound fields, and diameter less than 20% of the distance between them for impedance tubes of circular cross-section. Microphones of type 4189-A-031 (1/2-inch microphone for open-field with pre-amplifier type 2671, 20 Hz to 20 kHz, pre-polarized) are used for the tests; their mutual distance is s = 0.065 m. The benefits of using the microphones include: improved stability and accuracy, ease of use and TEDS technology. The range of the work frequency f is selected in the range (fl, fu), where fl and fu are the lower and upper frequency level. The lower frequency level is limited by the accuracy of the signal processing equipment. The upper frequency level depends on the inner diameter of the tube d and the sound velocity c0. Its value is recommended by the following equation [1]:
fud£0.58c0.   (1) 
Acoustic properties of the selected materials are tested using the constructed impedance tube in the frequency range from 100 Hz to 1600 Hz.