High resolution BPR and frequency counters for cabled ocean observatories

Introduction

Loggers with frequency counter channels can achieve a very high measurement resolution (10 ppb). In order to maximize the resolution some additional considerations are required on the part of the user. This chapter presents the main points to follow to ensure the best performance for loggers using frequencies counters (and/or BPR channels). When sampling at a rate of 1Hz or slower, the frequency counter board will integrate the signal over around 800 ms; with faster sampling speeds, the whole period of time between samples is used as the integration time.

Deploying a frequency counter/BPR logger running continuously at 1Hz and streaming over serial

To avoid any loss of resolution, calibrated output formats caltext03 and caltext04 should be used (see the outputformat command). The other calibrated output formats output 6 significant digits, which is insufficient to represent the full potential sensor resolution.

We should then first make sure the output format is in caltext04 (or caltext03) and we can enable streaming while logging:

>> outputformat type = caltext04
<< outputformat type = caltext04
>> streamserial state = on
<< streamserial state = on

We can then setup the deployment in continuous mode at 1Hz, and define the starttime and the endtime (here between the 2017-12-01 and the 2017-12-02):

>> sampling mode = continuous, period = 1000
<< sampling mode = continuous, period = 1000
>> deployment starttime = 20171201000000, endtime = 20171202000000
<< deployment starttime = 20171201000000, endtime = 20171202000000

High-resolution data must not be used in conjunction with the EasyParse memory format. The EasyParse memory format stores readings as single-precision (32-bits) floating-point numbers, which limits the achievable resolution.

We can now enable logging:

>> enable erasememory = true
<< enable status = logging, warning = none
>> deployment status
<< deployment status = logging

The instrument now continuously outputs the measurements:

<< 2017-12-01 18:37:48.000, 30.39588279090822e+006, 5.825177871156484e+006, 17.1028520e+000, 22.4525380e+00

BPR channels

High-resolution RBR BPR instruments are configured to provide 2 frequency counter channels and 2 BPR channels per BPR sensor: pressure signal period, temperature signal period, calculated pressure, calculated temperature. The instrument stores only the frequency measurements (BPR channels being derived).

The channel command gives back:

>> channel 1
<< channel 1 type = peri00, module = 96, status = on, settlingtime = 900, readtime = 858, equation = lin, userunits = ps, label = none
>> channel 2
<< channel 2 type = peri01, module = 97, status = on, settlingtime = 900, readtime = 858, equation = lin, userunits = ps, label = none
>> channel 3
<< channel 3 type = bpr_08, module = 243, status = on, settlingtime = 0, readtime = 0, equation = lin, userunits = dbar, label = none
>> channel 4
<< channel 4 type = bpr_09, module = 244, status = on, settlingtime = 0, readtime = 0, equation = lin, userunits = C, label = none

And the calibration command returns:

>> calibration 1
<< calibration 1 label = none, type = peri00, datetime = 20170401000000, c0 = 20.000000e+006, c1 = 10.000000e+006
>> calibration 2
<< calibration 2 label = none, type = peri01, datetime = 20170401000000, c0 = 5.0000000e+006, c1 = 2.5000000e+006
>> calibration 3
<< calibration 3 label = none, type = bpr_08, datetime = 20171123120721, x0 = 5.8310300e+000, x1 = -24.514030e+003, x2 = -573.64115e+000, x3 = 76.129280e+003, x4 = 35.688000e-003, x5 = 0.0000000e+000, x6 = 30.413170e+000, x7 = 664.14899e-003, x8 = 58.803408e+000, x9 = 180.91160e+000, x10 = 0.0000000e+000, n0 = 1, n1 = 2
>> calibration 4
<< calibration 4 label = none, type = bpr_09, datetime = 20171123120722, x0 = 5.8310300e+000, x1 = -3.8981210e+003, x2 = -10.493120e+003, x3 = 0.0000000e+000, n0 = 2

If the Paroscientific, Inc. transducer attached to the RBR BPR logger is changed, please refer to Example 15: deri_bprpres and deri_bprtemp, BPR channels in order to change the calibration.

Operating an RBRquartz³ BPR|zero instrument

RBRquartz³ BPR|zero instruments are able to compensate the drift of the pressure sensor by regularly measuring the internal pressure of the instrument body with the same pressure sensor and comparing it to a reference barometer placed inside the instrument.
In order to do so, the instrument operates a valve to switch the pressure sensor between being exposed to the sea pressure (Marine) and being exposed to the instrument housing pressure (Atmospheric).
In order to set up the deployment so that the RBRquartz³ BPR|zero instrument regularly operates the valve, the following steps are required prior to enabling the deployment (see details of command valve ) .

If one wants to perform a measurement of the internal pressure every month for 5 minutes:

>> valve scheduled = true
<< valve scheduled = true
>> valve interval = 2678400000
<< valve interval = 2678400000
>> valve duration = 300000
<< valve duration = 300000

Then enable the deployment (see details above in the case of a BPR instrument):

>> enable erasememory = true
<< enable status = logging, warning = none
>> deployment status
<< deployment status = logging

If one wants to trigger a measurement of the internal pressure (Atmospheric) manually:

>> valve command = setpositionA
<< valve status = positionA

And setting the instrument back to the sea pressure (Marine) measuring position:

>> valve command = setpositionM
<< valve status = positionM

If the last step is omitted, the valve will return to the Marine position (positionM) at the end of the next scheduled episode.