Campbell Scientific CR6 CR6 Measurement and Control System - Page 322

Section 8. Operation Summary In-Depth 322 • An option to detect an open-input, such as a broken sensor or loose connection, is available in the CR6. • The option is selected by appending a C to the Range code. • Using this option, the result of a measurement on an open connection will be NAN (not a number). A useful option available to single-ended and differential measurements is the detection of open inputs due to a broken or disconnected sensor wire. This prevents otherwise undetectable measurement errors. Range codes appended with C enable open-input detect for all input ranges (see table Analog Voltage Input Ranges with CMN / OID (p. 311) ). Appending the Range code with a C results in a 50 µs internal connection of the V+ input of the PGIA to an over-voltage. The V- input is connected to ground. Upon disconnecting the inputs, the true input signal is allowed to settle and the measurement is made normally. If the associated sensor is connected, the signal voltage is measured. If the input is open (floating), the measurement will overrange since the injected over-voltage will still be present on the input, with NAN as the result. Range codes and applicable over-voltage magnitudes are found in the table Range-Code Option C Over-Voltages (p. 322). The C option may not work, or may not work well, in the following applications: • If the input is not a truly open circuit, such as might occur on a wet cut cable end, the open circuit may not be detected because the input capacitor discharges through external leakage to ground to a normal voltage within the settling time of the measurement. This problem is worse when a long settling time is selected, as more time is given for the input capacitors to discharge to a "normal" level. • If the open circuit is at the end of a very long cable, the test pulse (7.6 V) may not charge the cable (with its high capacitance) up to a voltage that generates NAN or a distinct error voltage. The cable may even act as an aerial and inject noise which also might not read as an error voltage. • The sensor may "object" to the test pulse being connected to its output, even for 100 µs. There is little or no risk of damage, but the sensor output may be caused to temporarily oscillate. Programming a longer settling time in the CRBasic measurement instruction to allow oscillations to decay before the Ato-D conversion may mitigate the problem. Table 68. Range-Code Option C Over-Voltages Input Range Over-Voltage ±200 mV ±1000 mV 1250 mV ±5000 mV 8000 mV