The main sequence in the ATEM timing program assures a stable execution interval according to the sampling frequency [Hz] set in ATEM setup by relying on the
LabView ® “wait for multiple ms” function.
The timestamp (time_string) is generated from the computer clock reading and is rounded to the nearest full 100 ms multiple. Any delays in the sequence start are recorded as "interval delay" in the error code. The latest sonic and open path data are retrieved from the global variables in ATEM joint control every 100 ms and stored in an array, which is cycled by on position during each interval.
As correct timing is the most crucial aspect in REA sampling, special care was taken to all aspects related to the transfer of data from the sensor to the data in memory and data in the file. See flow charts below and refer to the file ATEM timing diagram v7.xls for more detail.
The largest uncertainty (+/- 100 ms at 10 Hz sampling frequecy) results from the uncertainty related to the time between actual measurement of air and wind properties and the data availability at the RS232 ports. This is a result of different clocks being used in each sensor and the computer and can only be avoided, if either sensors send exact time stamps together with the data or all clocks are exactly synchronized or measurements are polled.
Time shifts between the data can easily be corrected in raw data for EC flux calculations. REA sampling requires fast online evaluation of the data. In order to assure the best possible synchronization in REA, a time delay experiment
1. inlet tube, REA system up to the valves plus the short connection tube and CO2
Analyzer and
has to be performed before the actual REA measurements. The REA-system design (tube, valve and pump inner volumes) and flow rates need to be controlled and kept
constant at the same rates during the ‘time delay experiment’ and during the actual REA measurements. Time delay of the sample in the tube from the sampling inlet up to the valves can be determined by cross correlation of the vertical wind velocity (z, w) and a scalar (e.g. CO2). As any CO2 Analyzer will add internal volume and therefore time delay to the system, a differential measurement must be performed.
This means measuring the time delay of
2. short connection tube and CO2 Analyzer only. (see foto in Appendix)
16 1. Time delay in
inlet tube, REA system up to valves + short connection tube and CO2Analyzer
Pump 1 1. Time delay in
inlet tube, REA system up to valves + short connection tube and CO2Analyzer
Pump 1
2. Time delay in
Short connection tube and CO2Analyzer
CO2 analyzer
2. Time delay in
Short connection tube and CO2Analyzer
CO2 analyzer e.g. LI-7500
Pump 1
Pump 2
During the first part of the time delay experiment both pumps in the sampling line (pump 1) and in the bypass (pump 2) have to be running at the same flow rate that will be used later during REA measurements. In order to determine the
corresponding time delay of the connection tube and CO2 Analyzer, in the second part of the experiment only the flow rate of the sample line provided by pump 1 is needed.
In both parts of the experiment the time shift yielding the maximum cross correlation gives the corresponding time delay. The time delay in the REA system up to the valves and in the inlet tube including additional flow through the bypass with pump 2 is then determined as the difference in time delay measured in both parts of this experiment. By measuring the time delay with this kind of experiment we are able to correct for any delay resulting from the actual experiment setup like it will be used for the measurements in the field. The precision of the measured time delay is only limited by the time resolution of the data record (e.g. 0.1 s corresponding to 10 Hz measurements) and very short delays in data transfer (< 0.1 s). During REA measurements additional delays for valve switching should be kept as short as possible (< 0.1 s).
For all REA measurements it is essential that flow rates through the sample line and the bypass will be kept at the same rate. In our system both flows are monitored by low pressure drop mass flow meters (MFM) and pumps are kept at very steady flow rates using a DC pulse width power supply unit. Tests with mass flow controllers (MFC) were not able to keep flows as constant. The reason was that bigger membrane pumps needed to provide a higher pressure drop at the control valve, introduced more fluctuation in the flow rate.
Slightly reduced covariance between vertical wind velocity (z, w), the scalar of interest (air sample, e.g. 13C, s) and the proxy scalar (CO2, s_proxy) can result from
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uncorrected small time shifts. However, in whole-air REA the measurement of multiple gases in the REA updraft and downdraft samples allows to measure the proxy scalar (CO2) additionally to the scalar of interest (13C) in the updraft and downdraft air samples. Any reduction in covariance between the air sample and the vertical wind velocity would influence updraft and downdraft values for both scalars in the same way as long as we observe scalar similarity. Therefore, when calculating individual b-factors for each REA measurement from the proxy scalar EC flux
results, slightly reduced covariance due to small time shifts during the REA sampling are compensated correctly.
3.1
Procedures within one sampling interval
ATEM setup:sampling frequency = 10 Hz =100 ms intervals open path delay = 2 intervals
start filter calculation = 4 intervals tube delay = 8 intervals
Tim ing ... Wait for multiple ms stopped Tim ing generate new timestamp
Valves set Digital output according to V1, V2 raw data generate error and event code
cycle raw data arrays
insert latest sonic data in x,y,z,ta(0)
insert latest open path data in co2(2), h2o(2) insert error(0),event,valve,mfm1,mfm2(0) REA data cycle REA data arrays
planar fit rotate current z data and insert into w (0) insert current co2 data and instert into s(2) calculate mean, std
calculate w p(4) from w (4), sp(4) from s(4) set V1, V2 according to w p(8-1) and sp(8-1) data rec w rite raw data from arrays(4)
w rite REA data from arrays(4) Tim ing w ait for multiple ms...
see also ATEM timing diagram v7.xls
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3.2 ATEM timing data handling and storage
tube
signal processing: +100 ms
write raw data, calculate and write REA data: +400 ms
(start_filter_calculation= 4 intervals)
reservoirs write raw data, calculate and write REA data: +400 ms (start_filter_calculation= 4 intervals)
reservoirs
write raw data, calculate and write REA data: +400 ms (start_filter_calculation= 4 intervals) reservoirs
76543218 0
76543218
tube7654wp sp8 CO276543218 0w 76543218 076543218 010
5437data file
621083V1,V2 event4210valves +300 ms
reservoirsup down
tube
7654wp sp8 CO276543218 0w76543218 076543218 0105437data file621083V1,V2 event4210valves+300 ms
reservoirsup down
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write REA sampling data:
+1200 ms
write REA sampling data:
+1200 ms
reservoirs up down
3.3 Data storage
ATEM timing is transferring raw data and REA data as well as all information on the REA sampling process (valve switching, ….) to the .atd raw data file continuously.
According to the settings on the ATEM timing front panel, a new data file is started e.g. every 30 min (= when the time_string section containing mm:ss.ss matches 00:00.00 or 30:00.00). The file number contained in the filename is increased by 1.
.atd files are comma separated ASCII files using ten digits (5 before “.” 4 after =
%10.4f LabView® number format). They can be looked at with a text editor.
Compression using e.g. Zip software reduces the size of files by ~ 90%.
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Example of an .atd raw data file (not all columns included):
3.4 Error code
ATEM timing converts the binary code shown below (0 = false, 1 = true, x = false OR true) to a decimal number, which then is stored in the .atd raw data file.
xxxxxxxxx1 = incorrect data from sonic
xxxxxxxx1x = error during sonic data conversion xxxxxxx1xx = error during OP data conversion xxxxxx1xxx = warning from OP diagnostic flag xxxxx1xxxx = interval delay
xxxx1xxxxx = op_agc > 70%
xxx1xxxxxx = analog in reading error xx1xxxxxxx = digital out error x1xxxxxxxx = <not used>
1xxxxxxxxx = user flag
3.5 Event code
xxxxxxxxx1 = valve 1 set on xxxxxxxx1x = valve 2 set on xxxxxxx1xx = flush bags and flasks xxxxxx1xxx = Eddy Sampling
xxxxx1xxxx = transfer bag 1 <not used>
xxxx1xxxxx = transfer bags 1+2 to flask 1+2 xxx1xxxxxx = ES system locked
xx1xxxxxxx = bag 1 samples up/down x1xxxxxxxx = manual set
1xxxxxxxxx = event 10 <not used>
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