【正文】 data is in graphical format, they allow you to easily spot trends. Once set up, most recorders have sufficient internal intelligence to run unattended — without the aid of either an operator or a puter. Drawbacks include a lack of flexibility and relatively low accuracy, which is often constrained to a few percentage points. You can typically perceive only small changes in the pen plots. While recorders perform well when monitoring a few channels over a long period of time, their value can be limited. For example, they are unable to turn another device on or off. Other concerns include pen and paper maintenance, paper supply and data storage, all of which translate into paper overuse and waste. Still, recorders are fairly easy to set up and operate, and offer a permanent record of the data for quick and simple analysis. 4. Scanning digital multimeter Some bench top DMMs offer an optional scanning capability. A slot in the rear of the instrument accepts a scanner card that can multiplex between multiple inputs, with 8 to 10 channels of mux being fairly mon. DMM accuracy and the functionality inherent in the instrument39。t perceive small variations with the input signal. Furthermore, the electrical environment inside a PC tends to be noisy, with highspeed clocks and bus noise radiated throughout. Often, this electrical interference limits the accuracy of the PC plugin card to that of a handheld DMM .These cards also measure a fairly limited range of dc voltage. To measure other input signals, such as ac voltage, temperature or resistance, you may need some sort of external signal conditioning. Additional concerns include problematic calibration and overall system cost, especially if you need to purchase additional signal conditioning accessories or a PC to acmodate the cards. Taking that into consideration, PC plugin cards offer an attractive approach to data acquisition if your requirements fall within the capabilities and limitations of the card. 7. Data loggers Data loggers are typically standalone instruments that, once they are setup, can measure, record and display data without operator or puter intervention. They can handle multiple inputs, in some instances up to 120 channels. Accuracy rivals that found in standalone bench DMMs, with performance in the 22bit, accuracy range. Some data loggers have the ability to scale measurements, check results against userdefined limits, and output signals for control. One advantage of using data loggers is their builtin signal conditioning. Most are able to directly measure a number of different inputs without the need for additional signal conditioning accessories. One channel could be monitoring a thermocouple, another a resistive temperature device (RTD) and still another could be looking at voltage. Thermocouple reference pensation for accurate temperature measurement is typically built into the multiplexer cards. A data logger39。s superior measurement accuracy, multiple readings and averaging are not necessary, as they often are in PC plugin solutions. 8. Data acquisition front ends Data acquisition front ends are often modular and are typically connected to a PC or controller. They are used in aut