Campbell Scientific CR510 CR510 Basic Datalogger - Page 162

SECTION 13. CR510 MEASUREMENTS table WILL be overrun by the automatic calibration. If an overrun occurs every time calibration is executed, then 1 execution is skipped for every 512 times that the program table is executed. If the measurements are being averaged, the effect of the overrun is negligible. Program table overruns are indicated by the appearance of two decimals on either side of the sixth digit on the CR10KD and are also stored in memory (Section 1.7). INSTRUCTION 24 CALIBRATION The alternative to automatic calibration is the use of Instruction 24, the calibration instruction. Instruction 24 implements a complete calibration which occurs ONLY when EXECUTED by a program table. Instruction 24 calibration is the average of 10 calibrations, and takes approximately 2.8 seconds to complete. Automatic calibration is disabled when a program is compiled that contains Instruction 24. Instruction 24 calibration, as opposed to automatic calibrations, may be advantageous in applications where: 1) the CR510 is exposed to extreme thermal gradients, or 2) automatic calibration would interfere with the desired sampling rate, and the ambient temperature is stable enough to allow calibration at specific points during program execution. Calibration coefficients are replaced each time that Instruction 24 is executed. Unlike automatic calibration, there is no time constant for the coefficients to respond in changes to calibration. Instruction 24 calibration ensures that the coefficients are optimum at the time that the instruction is executed. For example, consider a CR510 mounted under the dash of an automobile, where temperature could easily change 50 degrees. Temperature changes affect the measurement circuitry which must be compensated for by calculating new coefficients. Each time Instruction 24 is executed a new set of calibration coefficients is calculated based on the measurements made at that time. Calibration at a certain point during program execution may be advantageous for some applications. For example, suppose Table 2 has an execution time of 15.6 ms, but only executes when flag 1 is set. Table 1 has a 5 minute execution time which makes a temperature measurement, and sets flag 1 if the temperature exceeds a fixed value. To prevent overrun errors which would occur in Table 2 if the automatic calibration was used, Instruction 24 could be executed before the temperature measurement was made by Table 1. Instruction 24 also has an option to store the results of the automatic calibration in Input Storage. This can be used to detect hardware problems. If -99999 appears in any of the 19 input locations, the CR510 has a hardware problem or needs factory calibration. 13-18