2. Single or multichannel analysis
3. Dynamic range
4. Accuracy of measurements necessary
5. Speed at which analyses are required to be made
6. System
portability， especially if the analysis system is required for both lab and field use
7. Ease of integration with the host computer system
.u.iliary System .onitoring
Fuel System
Since the reliability of gas turbines in the power industry has been lowerthan desired in recent years because of hot-corrosion problems， techniques have been developed to detect and control the parameters that cause these problems. By monitoring the water content and corrosive contaminant inthe fuelline， any changes in fuel quality can be noted and corrective meas-ures initiated. The concept here is that .a contaminants in the fuel arecaused from external sources such as seawater;thus， by monitoring watercontent， .a content is automatically being monitored. This on-line tech-nique is adequate for lighter distillate fuels. For heavierfuels， a more complete analysis of the fuel should be carried out at least once a month using the batch-type system. The data should be input directly to the computer. The water and corrosion detecting systems also operate in con-junction with the batch analysis for the heavier fuels.
A Btu meter may be used in the fuel-quality system as an aid in determin-ing turbine system efficiency. A water capacitance probe is used for detection of water in the fuel line. A water-detecting device can be incorporated into the corrosion monitoring system. This monitoring device is based on detec-tion of changes in the dielectric constant of unknown fluid components passing through the probe. This device provides continuous and instanta-neous monitoring of the percentage of water suitable for quality or process control.
The sensor itself is based on a balanced capacitance bridge detection prin-ciple， utilizing a high-frequency oscillator with a closed-loop servo-amplitude control to assure that loading or variation in supply voltage does not affect the stability and accuracy of this instrument. .utput from the bridge is dir-ectly coupled to a preamplifier to step up the detected signal to a desired leveland，also， to correct for nonlinear characteristics of the water measurement. This measurement is achieved through a nonlinear feedback loop.
The corrected and amplified output is then directly coupled to a constant-current amplifier， which can provide 0-5 mA or 4-20 mA output. This type of signal termination allows the detector system to be located at a distance from the measuring point for ease of usage. This water detection systemoffers: (1) an accurate means of water measurement， (2) easy installation andminimum maintenance， (3) a simple two-step calibration procedure， and (4) long-term stability and dependable service.
A corrosion probe is used to monitor the corrosive condition of the fuel. This can be accomplished with a special probe which can detect metal in the lubricant.
A Btu meter is used to determine the fuel heating rate. The Btu meter is a capacitance device ideally suited to real-time on-line Btu measurement of gasturbine liquidfuel， such as naphtha， that is a valuable asset in determining turbine efficiency.
Tor.ue .easurement
This measurement can be accomplished by using a mechanical system or various types of electronic systems. All of these systems are expensive and in many cases require repeated calibration. The mechanical system (Figure19-14) is a three-gear， phase-related system which measures the displacement between two gears and the proportionate shaft twist. A third gear is situated so that any variations other than shaft twist will occur in the first two gears. This signal is used to eliminate errors caused by these variations.
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