Milliamp output

If two temperature readings from the same point in the process disagree, the chances are that a temperature indication is more accurate than a temperature-control signal. The temperature signal used for control has usually been converted from its direct milliamp output to operate a control station. The temperature indication is generated right from the junction of the thermocouple, and hence there is less chance for error. 

One of the most common safety trips is the automatic fuel-gas shutoff. We have this at home on our furnaces. We ignite the pilot light manually. The pilot light heats a thermocouple. The milliamp output from this thermocouple opens the fuel-gas valve to the main burner. The gas in the main burner is ignited from the pilot-light flame. Should the heat from the pilot light diminish below a certain point, the fuel gas to the main burner, as well as to the pilot light, will be shut off. 

A less common type of fuel-gas trip to a heater is a low-pressure trip. A pressure transducer generates a milliamp output from a boiler feed-water pump. Should this milliamp output fall below a certain level, the... 

Fig. 4.8 shows how to connect a current (milliamp) output from a monitor to a millivolt input on a recorder. The positive side of the monitor output is usually connected to the positive side of the recorder input but in some cases better results are obtained with the polarity reversed. 

The same sort of problem arises in a level-trol, which measures and transmits a process vessel liquid level to the control center. As shown in Fig. 10.3, the level-trol operates by means of two pressure transducers, devices for converting a pressure signal into a small electric current. The difference between the two pressure transducers shown in Fig. 10.3 is called the milliamp (milliampere) output. This milliamp output is proportional to the pressure difference between the bottom and top taps in the level-trol. To convert the milliamp output signal from the level-trol into a level indication, the instrument technician must assume a specific gravity ... 

Milliamp output The small (milliampere-range) electrical output from a thermocouple or pressure transducer that is used for control purposes. 

You need 3 miiliamperes of current for each sample that you have in place. If you have four tubes, then you need 12 milliamps. Go to the black output adjust control knob and slowly turn it clockwise to adjust the current to the value you need. 

Manual controls or another man-machine interface allows the user to operate the analyzer, manipulate analysis parameters, and input the concentration values for calibration materials. The detector output is integrated to produce a value that is proportional to the total amount of sulfur that flowed through the detection chamber during the analysis. This proportional value is compared to the value produced by a standard to determine its concentration. This value can be corrected for the density of the standard or sample being analyzed. The resultant concentration can then be locally displayed, written to a register for use on a serial bus, and/or used to set a representative value on a 4-20 milliamp (mA) current loop. 

Instrumentation equipment can fail in different ways. We call these "failure modes." Consider a two-wire pressure transmitter. This instrument is designed to provide a 4 - 20 milliamp signal in proportion to the pressure input. Detailed failure modes, effects, and diagnostic analyses of several of these devices reveal a number of failure modes frozen output, current to upper limit, current to lower limit, diagnostic failure, communications failure, and drifting/erratic output among perhaps others. These instrument failures can be classified into failure mode categories when the application is known. 

A monitoring device will often use an electronic comparison function but it cannot provide any magnitude information the preset threshold could have been exceeded by a few milliamps or many amps and yet the output activation would be identical. Multiple thresholds can be used, to determine an overcurrent situation vs. a short-circuit situation, but due to the tolerances of typical monitoring circuits, the magnitudes of these currents must be sufficiently separated. 

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