I’m calibrating the DP15 with CD 280 demodulator. I leave both pressure ports in the air (zero pressure) and adjust the CD280 ‘Zero’ to 0. Then, I applied 3000Pa water column to one pressure port (maximum stimulus) and leave the other still in the air. And I adjust the the CD280 ‘Span’ to 5Vdc, because the maximum input of my third-party datalogger is 5V. I also bleed the transducer.
My problem is that when I applied the maximum stimulus, the output from the demodulator is unstable. I adjusted the ‘Span’ to 5V and waited some time. The output for my four sensors either increased or decreased by as large as 0.2V. The applied pressure is constant.
The thing to do here is to disconnect the transducer cable at the DP15 sensor and observe the signal output of the CD280. It won’t be 0 Vdc, but it will be some + or – voltage (representing the sensor balance) and it should be steady with no drift. If the signal drifts, it is a CD280 problem.
Re-connect the transducer at zero applied pressure and record the CD280 output to see if it drifts. If so, then check the DP15 for isolation by measuring the DC resistance between pin A and the metal case. If this is below 100K Ohms, the transducer may be corroded and loss of isolation is a cause for drift. If that is the case, the sensor must be replaced.
If the drift only occurs when the transducer is pressured, the tubing and pressure connections should be checked to see if there is any chance for leaks. We recommend two layers of teflon tape on the threaded connections and a dab of vac grease on any hose barbs for plastic tubing. There have been cases where this was not done and small leaks can easily form.