Transmitter

For very hard-to-handle process fluids nuclear radiation gauges are used to detect interfaces and levels. 

As you can tell from the above discussion, it is very easy to be fooled by a diflerential pressure measurement of level. As one who has been bitten many times by these problems, I highly recommend redundant sensors and judicious skeptism about the validity of instrument readings. 

The transmitter is the interface between the process and its control system. The job of the transmitter is to convert the sensor signal (millivolts, mechanical movement, pressure diflerential, etc.) into a control signal (4 to 20 mA, for example). 

The dynamic response of most transmitters is usually much faster than the process and the control valves. Consequently we can normally consider the transmitter as a simple gain (a step change in the input to the transmitter gives an instantaneous step change in the output). The gain of the pressure transmitter considered above would be 

Thus the transmitter is just a transducer that converts the process variable into an equivalent control signal. 

The frequency required to observe a particular nucleus is the first consideration. This frequency was formerly synthesised by analogue methods. Frequencies below 200 MHz 

The pulse from the modulator, which is typically at the V level, is then amplified to a level that depends on the type of experiment. The power of a transmitter can be determined by examining the pulsed voltage on an oscilloscope (but NEVER plug a high power transmitter directly into an oscilloscope). Attenuation is measured in decibels (dB) and in terms of voltage  

Hence 3 dB represents a factor of 1.4 in voltage while 20 dB is a factor of 10. If Vpp is the measured peak-to-peak voltage after taking into account the attenuation, the power is 

It can be immediately seen from Table 3.1 that a probe cannot be designed with all the properties optimised because of their differing Q-dependencies. This means that most probes are a compromise, focussing on the most important aspects for a specific application. Several probe designs exist which have advantages for different applications. 

As frequency increases, d decreases, causing the wire to present a smaller cross-sectional area. This increases the ac resistance and consequently lowers the circuit Q. NMR coils are often made of flat wire rather than normal round wire, the former also having the advantage of better rf homogeneity. The magnitude of the induced B i field is related to the power applied, Q, and the coil volume, V, by 

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If the sample is placed in the path of the infrared beam, usually between the source and the monochromator, it will absorb a part of the photon energy having the same frequency as the vibrations of the sample molecule s atoms. The comparison of the source s emission spectrum with that obtained by transmission through the sample is the sample s transmittance spectrum. 

The sonic tool measures the time taken for a sound wave to pass through the formation. Sound waves travel in high density (i.e. low porosity) formation faster than in low density (high porosity) formation. The porosity can be determined by measuring the transit time for the sound wave to travel between a transmitter and receiver, provided the rock matrix and fluid are known. 

Sensor for Monitr the Light Transm through the Fluid (Transmittance)... 

The probe for this application contains two pairs of transducers (transmitters S and receivers E), see figure 7. 

A mechanics-free airborne sound location system is used in order to record the probe movement and the rotation direction of the probe relative to the weld. Two airborne sound transmitters are arranged on the probe holder and two receivers are fixed on a 50 cm long rail equipped with two magnetic pads The exact probe position and rotation direction is continuously determined by system. 

The ultrasound system should have more independent channels and allow the transmitter pulse to be individually adjustable in width and amplitude, and an increased frequency range for the logarithmic amplifier was desired. The digitization should be improved both with respect to sampling rate and resolution. 

The use of Lamb waves offers the possibility of rapid long-range in-service inspection. Receiver and transmitter probes are positioned single sided - access is only required from one side of the specimen - in a pitch-catch-arrangement, the receiver being outside tbe field of the specular reflection. 

In order to obtain a high signal-to-noise ratio sufficient acoustical power is necessary. For special applications a programmable pulser (transmitter) is required in order to optimize the frequency spectrum. 

Additional requirements have to be met, such as norm DIN 25450 for manual testing. This norm describes the requirements of transmitters, receivers and other parts of the system. 

In order to get an extremely high resolution and a small dead zone" (after the transmitter pulse) single amplifier states must have a bandwidth up to 90 MHz ( ), and a total bandwidth of 35 MHz (-3 dB) can be reached (HILL-SCAN 3010HF). High- and low-pass filters can be combined to band-passes and provide optimal A-scans. All parameters are controlled by software. 

In testing materials with high sound damping, the burst transmitter increases the signal-to-noise ratio to, typically, 12 dB. Typical applications honeycomb and concrete components, and air-coupled testing. 

For special applications such as air coupled testing a special programmable transmitter board was developed. This transmitter generates rectangular and burst signals, which increase the acoustical power in an optimized frequency range, and provides a superior signal-to-noise ratio. 

The screen shows the predefined area to be scanned and filled to ensure maximum coverage The transmitter operates with a frequency of 40 kHz and with a pulse repetition frequency of 200 Hz. This gives the system an accuracy in positioning of better than 1 mm. 

The maximum distance between transmitter and receiver is 350 mm. The transmitter on the probe can be rotated making it possible to perform a sequence of scans on a perticular weld and on neighbouring welds without moving the microphone collar. 

Willey R R 1976 Fourier transform infrared spectrophotometer for transmittance and diffuse reflectance measurements Appl. Spectrosc. 30 593-601... 

The term transmittance (T) at a given wave length is defined by... 

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