Hi
I work on an intercomparison of tides measurements. The measures come from different tides gauges whose two campbell sensors : CS455 (pressure sensor) and CS476 (radar sensor).
In starting to process the data, I realized that the radar sensor CS476 had a problem. It's shifted in time. it's a shift of several minutes : 6 at 7. It always 6 minutes ahead of the other sensors.
The 2 sensors are connected at the data logger CR1000, but the problem not come from to the data logger, otherwise the CS455 would also shift.
The sensor could it have its own internal clock? I do not see other solutions.
Thanks for your help. And sorry for my English!
Audrey
Can you please post your program.
My program :
'CR1000
'Declare Variables and Units
Public BattV
Public PTemp_C
Public CS450(2)
Public CS475(3)
Alias CS450(1)=Level
Alias CS450(2)=Temp_C
Alias CS475(1)=Stage
Alias CS475(2)=Distance
Alias CS475(3)=Error_Code
Units BattV=Volts
Units PTemp_C=Deg C
Units Level=metre
Units Temp_C=deg C
Units Stage=metre
Units Distance=metre
Units Error_Code=Units
'Define Data Tables
DataTable(CR1000,True,-1)
DataInterval(0,6,Sec,10)
'CS455'
Sample (1,Level,FP2)
Sample (1,Temp_C,FP2)
'CS476'
Sample (1,Distance,FP2)
Sample (1,Stage,FP2)
Sample (1,Error_Code,UINT2)
EndTable
'Main Program
BeginProg
'Main Scan
Scan(6,Sec,1,0)
'Default Datalogger Battery Voltage measurement 'BattV'
Battery(BattV)
'Default Wiring Panel Temperature measurement 'PTemp_C'
PanelTemp(PTemp_C,_50Hz)
'CS450/CS455 Pressure Transducer measurements 'Level' and 'Temp_C'
SDI12Recorder(CS450,7,0,"M!",1,0)
'Generic SDI-12 Sensor measurements 'CS475'
SDI12Recorder(CS475,5,0,"M!",1,0)
'Call Data Tables and Store Data
CallTable(CR1000)
NextScan
EndProg
Are you seeing a 6 minute time shift or perhaps a 6 second shift (your storage interval) please?
Do you know if the CS476 data appears to be ahead of real time or more likely the CS450 data is behind realtime.
I'm seeing a 6 or 7 minute time shift. But I'm sure than the shift is more important than 5 minutes.
The CS476 data are really ahead of real time. I compare several sensors and the CS476 is the only shifted. The others sensors are at the same level.
It seems hard to comprehend how the sensor could be reporting measurements 6 minutes into the future, unless the datalogger clock is wrong and the other sensors are responding slowly! Are you sure there are no factors that might be causing the CS450 to become sluggish to respond.
Two common factors can affect pressure sensors to cause strange results. One is blockage of the holes in the nose cone caused by fouling from wildlife and algae - this would need to be checked for by retrieving the probe and looking at it.
The other factor could be the vent tube is vented to atmosphere via a restricted opening or via a desiccant tube that has wet desiccant in it that is restricting the movement of air.
An extension to the last problem is that there is serious condensation in the vent tube such that the tube is blocked by liquid water. When this happens you can get very slow responding measurements as the water moves up and down the vent tube. As sensor with liquid water in the vent tube can be difficult to recover if the water is far inside the cable.
Hi Andrew and thanks for your answer.
I understand what you're saying, however lets detail my setup here :
-My CS455 pressure sensor is clean and new and the vent tube has been checked dry and open. Moreover as said in my previous mail, I have in addition to the Campbell tide gauge another gauge of a different manufacturer and it's giving exactly the same measurement as the CS455 (almost to the millimeter of water height).
-The other gauge has its time synchronised by GPS and the CR1000 as well so the timing given by the data acquisition system is good. Timestamp of the data has been checked and is correct.
-Now when I plot the data of CS475 vs CS476 vs other tide gauge, I see very clearly that the curve given by the CS476 data is shifted to the right. ALSO this data is much more smooth than the other gauges data (the pressure sensor and another radar in the same conditions). This is not normal as I thought that my programm (mail above)was getting the same kind of sample for the two gauges, i.e 1 measurement every 6 sec. Obviously what I have from the CS476 is not a raw single measurement but an average water height computed by the sensor.
My question now is how is this average made and how is it timestamped by the sensor ? I suspect that the sensor is somehow sending to the CR100 a smoothed out measurement with a 5-min delay...
This is not very clear in the Campbell CS476 manual, in the Vega Technical Reference Manual for SDI-12 Radar Water Level Sensor I can read "The sensors make multiple distance measurements. The results of those measurements are averaged and converted to read Height or Stage in units of feet or meters."
There doesn't seem to be any way to setup the sensor parameters such as an averaging window for instance (only parameter is "set water condition" but it's not of great help), I definitely need to know what does the sensor output corresponds to as at the moment my high and low tides are 5 minutes late...
Thanks again for your help
Yes there is some filtering in the CS475/476. I will get the details of this and come back to you. This will delay the response of the sensor and put the response behind the other sensors, not ahead of them though.
Ok thanks for the answer.
In the meantime I managed to connect to the sensor via a Vega Hart interface and the Pactware software vith Vega PS62 DTM.
Then you can actually view and change all the sensor parameters, I have thus modified the following parameters:
-Integration time was 100 sec. I moved it to 1 sec.
-Special parameter n°24 "Time for opening the focus range" was 600 sec., I moved it to 1 sec.
-Special parameter n°9 "smoothing" was set to yes, I set it up "no".
This should sort things out.
Not sure if those parameters were factory default or set by campbell before shipment...One thing though, SDI-12 communication doesn't allow for what Hart can do, it's therefore handy to be able to communicate with Hart as well on this kind of sensor