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 An Outcome-Based Calibration System Specify your calibration system based on what you need and not on what happens to be available. Buying features that you don't need now will probably cost you money later. Irwin Bluestein, RdF Corp. Sensor calibration is generally carried out either by removing the unit under test (UUT) from its installation and taking it into the lab, or by downloading performance data into a hand-held device and feeding the information to a computer. There is now a third option, thanks to the dwindling size of computers: take the computer into the plant with the calibrator.
Analog process instruments accept inputs from a variety of sensors and convert the data into a voltage or current signal that can be used for process control. To calibrate these signal conditioners, it is necessary to simulate the process variable at the input and measure the output for the appropriate response. Instrument Service Organizer software from Martel Electronics, Inc., works with any hand-held calibrator even though it was designed to control three of the company's calibrators via the computer's serial port. PPC-9001 Pressure Calibrator. This pressure sensor calibrator (see Photo 1) incorporates a small, hand-operated air pump, usually mounted on the calibrator, that feeds air pressure into the UUT. Once the proper amount of pressure is applied, the operator may press the F3 function key and the computer will read the pressure and the corresponding voltage or current output from the UUT as measured by the calibrator via the serial interface. The RS-232 interface is a feature offered by Martel calibrators; users of other calibrators need only read the digital display and enter the data via the computer keyboard. PTC-9002 Temperature and PSC-9003 Process Signal Calibrators. These devices (see Photos 2 and 3) apply electrical inputs to the UUT and are therefore capable of being controlled by the computer. The temperature calibrator PTC-9002 simulates resistance for 120
To set up the proper input from the calibrator via the serial interface, the program causes it to step through the available configurations. Communication is accomplished at 2400 baud, so it is often much faster to punch the buttons on the calibrator. The program begins by listing the installed instruments in the order of calibration due date (see Screen 1): The user may choose instead to switch the display to tag number order by pressing function key F2 (see Screen 2): The bar at the top of these screens is used to select and indicates by blinking what function is being performed. "Installed Instruments" permits entering and editing instrument identification and calibration requirements (see Screen 3): A prompt bar across the top of the instrument data form prompts the user as to what information is sought as each item on the form is completed. The program handles from 2 to 5 data points and calculates them at 0%, 25%, 50%, 75%, and 100% of span. User-specified accuracy determines the calculated PASS/FAIL criteria. The allowable input variance is computed as follows:
The prompt bar at the top of the screen offers a set of acceptable engineering units for all inputs in addition to pressure (see Screen 4). A second prompt screen offering eight choices of engineering units of pressure is presented when the Pressure Unit Menu is selected (see Screen 5). The program allows the user to save the information entered and to exit the UUT entering and editing function at any time, even if incomplete. If an attempt is made to enter the calibration function for a UUT that is lacking essential data, the program advises that more information is required and returns to the UUT data entry screen. When the calibration function is selected, the calibration screen for the particular UUT showing the required calibration points, as calculated by the program, is presented. If the UUT is a pressure input device, the user is prompted to set zero pressure and to zero the calibrator before beginning. For other units, the user is prompted to set the calibrator for the desired sensor type and set the range. If the serial port is used, pressing function key F3 will set up the calibrator. When performing calibrations, it is not always possible to set the input to the ideal value. This is frequently the case with a pressure transmitter and a hand pump. The program will require the input to be set to within the accuracy tolerance of the calibration point, and it calculates the high and low limits. More important, it displays the ideal output for the actual input. Suppose, for example, that the closest the user could get to setting an input pressure of 500 pressure units is 502 units. The calibration screen will display the output limits as 3.90–4.22 mA instead of 3.84– 4.16 mA, which are the limits for an input of exactly 500 units. It also displays the ideal output that corresponds to this not quite ideal input as 4.06 mA. To adjust this particular transmitter for the correct output with the input set to 502, the adjustment would be made to 4.06 mA (see Screen 6).
The program then leads the user through the calibration. If a Martel calibrator is connected to the computer's serial port, the user can press function key F3 and cause the computer to read the input pressure and output current directly from the calibrator.
The user is prompted to record the "As Found" data and store them without adjustment. The data are read again as the UUT is adjusted as necessary. Upon saving the "As Left" data, the user is offered the opportunity to enter a single line remark. The program stores the calibration data in As Found and As Left format. When the Date File function is selected, the calibration data for a selected UUT are presented with the most recent record first (see Screen 7). The program has extensive help screens as well as independent text files that chronologically record the day's work and provide an audit trail. The UUT information is stored in a text file, UNITS.TXT, and in an indexed data file, UNITS. DAT. Because all entries are recorded in the text file, it contains a record of all changes in the order in which they were made. Calibration data are stored in a indexed data file, CAL.DAT, and in a separate text file, CAL.TXT. There are three index files: CALDUE____________.INX Finds data in UNITS.DAT by calibration due date. TAGS_______________.INX Finds data in UNITS.DAT by tag number. CTAGS_____________.INX Finds calibration data in CAL.DAT by searching out the tag number. A new text file is generated each day that calibrations are performed and it is named Cyymmdd.TXT where yymmdd are read from the computer's clock. Data backup requires only that the user copy all files with DAT, INX, and TXT extensions. This is done with a DOS program that will run on any compatible PC. All you need is the most inexpensive model (laptop, if you are carrying it into the plant) with one serial port (if you are using a Martel calibrator) and one parallel port.
Irwin Bluestein is National Sales Manager, RdF Corp., 23 Elm Ave., Hudson, NH 03051-0490; 603-882-5195 or 800-445-8367, fax 603-882-6925, sensor@rdfcorp.com or www.rdf corp.com For more information on Martel products, contact Tracy Johnson, Martel Electronics Corp., PO Box 897, Windham, NH 03087; 603-893-0886 or 800-821-0023, fax 603-898-6820, sales@martelcorp.com or www. martelcorp .com
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