Campbell Scientific CR1000KD CR1000 Measurement and Control System - Page 179

Section 7. Installation Table 26. SDI12Recorder() Commands SDIRecorder() Instruction SDICommand Entry Actions Internal to CR1000 and Sensor 2Use variable replacement in program to use same instance of SDI12Recorder() as issued aCV! (see the CRBasic example Using SDI12Recorder() C Command ). Alternate Start Measurement Command (Cv) The SDIRecorder() aCv (not C!) command facilitates using the SDI-12 standard Start Concurrent command (aCv!) without the back-to-back measurement sequence normal to the CR1000 implementation of aCv!. Consider an application wherein four SDI-12 temperature sensors need to be nearsimultaneously measured at a 5 minute interval within a program that scans every 5 seconds. The sensors requires 95 seconds to respond with data after a measurement request. Complicating the application is the need for minimum power usage, so the sensors must power down after each measurement. This application provides a focal point for considering several measurement strategies. The simplest measurement is to issue a M! measurement command to each sensor as follows: Public BatteryVolt Public Temp1, Temp2, Temp3, Temp4 BeginProg Scan(5,Sec,0,0) 'Non-SDI-12 measurements here SDI12Recorder(Temp1,1,0,"M!",1.0,0) SDI12Recorder(Temp2,1,1,"M!",1.0,0) SDI12Recorder(Temp3,1,2,"M!",1.0,0) SDI12Recorder(Temp4,1,3,"M!",1.0,0) NextScan EndProg However, the code sequence has three problems: 1. It does not allow measurement of non-SDI-12 sensors at the required frequency. 2. It does not achieve required 5-minute sample rate because each SDI12Recorder() instruction will take about 95 s to complete before the next SDI12Recorder() instruction begins, resulting is a real scan rate of about 6.5 minutes. 3. There is a 95-second time skew between each sensor measurements. Problem 1 can be remedied by putting the SDI-12 measurements in a SlowSequence scan. Doing so allows the SDI-12 routine to run its course without affecting measurement of other sensors, as follows: Public BatteryVolt Public Temp(4) BeginProg 179