TECHNICAL DETAILS Sensors and Measurement The recorder has an external plug-in temperature/pH sensor. A ten-foot remote sensor cable is provided to allow remote sensing of both temperature and pH. Unlike many circular recorders, the CT485pH utilizes electronic sensors to measure both fluid temperature and pH. The temperature sensor is a 1000 ohm RTD, which when excited with constant current develops a voltage closely proportional to temperature. The pH sensor consists of two components, the glass bulb and the reference component. These two compartments develop a voltage depending upon the pH of the solution in which the electrode is immersed. Applied to a preamplifier, this voltage can be send over standard wire for virtually unlimited length. The electronically-conditioned output of both sensors is fed to a microprocessor via an analog-to-digital converter. The processor linearizes and temperature compensates the output of the pH circuit employing a ôlook-up table" approach. The results of the processor calculations are then used to update the front panel display and to position the pen arms on the chart paper. Pen Arm Drive This recorder utilizes a motor-driven lead screw pen arm drive located in the door of the unit. This provides several advantages over conventional pivot arm mechanisms. First, the recorder pens move along straight, radial lines as the temperature and pH change. Thus, the time scales on the chart paper are straight lines and are much easier for the user to interpret than the swooping arcs produced by a conventional recorder. Second, the placement of the recorder pen arms directly opposite one another allows the pen tips to be placed very close together. This minimizes time offset between information recorded by the temperature and pH pens, making the charts easier to interpret. Finally, the location of the drive mechanism in the door provides an automatic pen lift when changing the chart paper. The position of the pen arms on the lead screw mechanism is controlled by optical encoders. On power up or system reset, the pen arms move to the zero position. This is detected by a photo interrupter, to provide absolute positional information to the microprocessor. From this point, a count wheel mounted on the lead screw provides positional information relative to the zero position. There are 32 counts for each division on the chart paper. As with all positioning mechanisms, an error may appear in the tracking over a period of time. To minimize this error, several steps have been taken. First, a deadband for pen movement of 1 ┌2 chart division has been programmed in to minimize pen motion without affecting usable chart resolution. Second, whenever you change chart paper, a magnetic switch detects the opening of the door and initiates a system reset, thus re-zeroing the pens and eliminating any accumulated errors. Chart Paper Drive Mechanism The recorder incorporates a stepper motor to impart rotational motion to the chart paper. A novel feature of the unit is its ability to utilize 1, 7 or 32 day chart paper simply through front panel switching. This capability is provided by a special digital timing circuit which precisely controls the stepping rate of the magnetic armature in the motor. Power Supply Supervision The recorder contains power supply monitoring and switching circuitry which enables it to be used in a variety of circumstances. Under ordinary circumstances, the unit will be powered by 110 or 220 Vac with the ac adaptor provided. This is the preferred mode of operation. The recorder can also operate via 4 ôDö size batteries installed in the rear of the unit which will power the unit up to 30 days. The system may be restarted by changing the batteries or by the re-initiation of ac power. If ac power fails, the circuitry will attempt to automatically switch to battery backup. If viable battery power is available, the system will operate on battery power until the batteries fail or ac power returns. When ac power returns, the recorder will automatically reset itself (refer to Section 8.5). Typically, the unit will operate with fresh alkaline batteries for about 1 month. It should be noted that to conserve battery power, the system timing turns the sensors on and updates the display and pen position only once every 0.5, 3.5 or 16 minutes while in the 1, 7 or 32 day modes respectively. However, the display remains in continuous operation during battery operation. You should be aware, then, that during battery operation it is possible for the display and pens to lag behind actual ambient conditions by up to 0.5, 3.5, or 16 minutes to save power. Only when using the ac adaptor do the pens constantly update for changes in ambient conditions.

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