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AN95: Setting Up Altimeter with Sea-Bird Profiling CTD

Document Number: 
Publication Date: 
Monday, November 14, 2016
PDF icon AppNote95Nov16.pdf842.49 KB


This application note describes the use of the following 0 - 5 volt output altimeters when integrated with a Sea-Bird CTD (SBE 9plus, 19, 19plus, 19plus V2, 25, or 25plus):

  • Teledyne Benthos PSA-916
  • Valeport VA500

The altimeter is cabled to an A/D channel on the CTD end cap; a y-cable is required if you are connecting multiple sensors to one connector.

Seasoft calculates the altimeter height as follows:

altimeter height = [300 * voltage / scale factor] + offset


  • scale factor = full scale voltage * 300/full scale range
  • full scale range is dependent on the sensor (e.g., 50m, 100m, etc.)
  • full scale voltage is nominally 5V
  • offset is typically 0, but can be used to adjust the calculated height based on in-field testing

Example Calculation in Sea-Bird Software —
For a 100 m sensor with 5 V full scale voltage, calculate the scale factor for input in the software:
scale factor = full scale voltage * 300/full scale range = 5 * 300 / 100 = 15
If the altimeter is outputting 2.45 V, calculated height = [300 * voltage / scale factor] + offset = 300 * 2.45 / 15 = 49 m
Note for the Teledyne Benthos PSA-916: If no echo is received (altimeter is out of range), the altimeter actually outputs 4.98 V (not 5 V),
so the maximum calculated height in Seasoft for this example = 300 * 4.98 / 15 = 99.6 m


Calculating and Entering Calibration Coefficients for Altimeter in Seasave V7

If Sea-Bird integrates the altimeter at the factory, we send it to you with a configuration file (.xmlcon or .con) that matches the factory setup. If you are integrating the altimeter yourself, you must update your existing configuration file to include the altimeter and to enter its calibration coefficients.

  1. In Seasave V7, click Configure Inputs.
  2. In the Configure Inputs dialog box, click Open. In the Select Instrument Configuration File dialog, browse to the desired configuration file, and click Open.
  3. In the Configure Inputs dialog box, click Modify.
  4. (SBE 19, 19plus, 19plus V2, 25, 25plus) Verify that External voltage channels corresponds to the number of voltage channels you are using for auxiliary sensors.
    (SBE 9plus) Verify that Voltage words suppressed is correct for the number of voltage channels you are using for auxiliary sensors.
  5. To add the altimeter to the sensor list: Click on the voltage channel corresponding to the altimeter (see the CTD manual for details) and click Select. The Select New Voltage Sensor dialog appears; double click Altimeter. The Altimeter dialog box appears; enter the serial number, calibration date, scale factor, and offset, and click OK.
  6. Make other changes to the configuration as needed, and click Save or Save As to save the configuration file.
  7. (SBE 19, 19plus, 19plus V2, 25, 25plus) Using a terminal program, enable the voltage channel in the CTD by sending the appropriate command to the CTD; see the CTD manual for details. If you do not enable the channel, the CTD will not output the sensor data.

Note: You can create or modify the configuration file in Seasave V7 (real-time data acquisition software) or SBE Data Processing (post-processing software); both programs use the same configuration file.


Viewing Altimeter Data and Setting Up Altimeter Alarms in Seasave V7

Viewing Altimeter Data

You can view the following data related to the altimeter output in Seasave’s fixed, scrolled, and/or plot displays.

  • altimeter height (distance from the seafloor, m or ft)
  • seafloor depth (m or ft):
    Seafloor depth = depth (calculated from CTD’s pressure sensor output) + altimeter output
    Note: For Seafloor depth, you must select salt water or fresh water. The depth calculation for salt water is dependent on gravity (which is dependent on latitude). Seasave uses the latitude from a NMEA navigation device, if NMEA is enabled in the .con or .xmlcon configuration file. If your system does not have NMEA, enter the desired latitude on the Miscellaneous tab in Configure Inputs.

See the Seasave manual and/or Help files for details on setting up the displays.

Setting Up Altimeter Alarms

You can activate an audible alarm in the personal computer (PC) that is running Seasave, based on an altimeter, minimum and maximum pressures, and/or a bottom contact switch (pressure and bottom contact switch alarms are not covered in this application note; see the Seasave manual and/or Help files). To enable and set up the altimeter alarm:

  1. In Seasave V7, click Configure Outputs.
  2. In the Configure Outputs dialog box, click the PC Alarms tab.
  3. Click Enable altimeter alarm, and enter the alarm set point, hysteresis, and minimum pressure. Click OK.

Note: You can set up similar alarms in an SBE 11plus Deck Unit (for SBE 9plus CTD) or SBE 14 Remote Display.

After you enable one or more audible alarms on Configure Output’s PC Alarms tab, you can also enable a visual alarm in Seasave.

In the Display menu, select Alarms. The Alarms display appears.
If no alarm is on when Seasave is collecting data or playing back archived data, the Alarms display looks like this:
If an alarm is on when Seasave is collecting data or playing back archived data, the display looks like this, with the two icons alternating every 0.5 seconds:  


Cleaning and Testing Altimeter Prior to Deployment

Note: See the altimeter manual for details on cleaning and testing.


A major source of problems with acoustic equipment is that the transducer is not clean, resulting in masking of acoustic signals. Both Teledyne and Valeport recommend that the user clean the exterior with fresh water prior to each deployment. Teledyne also recommends the use of a mild detergent. Do not use glass cleaner.


Note for the Teledyne Benthos PSA-916: The analog output ranges from 0 - 4.98 V (which represents 0.0 - 99.6 m for a 100 m altimeter).

If no echo is received, the output will go to full scale (4.98 V for Teledyne; 5.00 V for Valeport). Perform the following tests prior to deployment:

  1. In-air test.
  2. Short range in-water test ‑ Make sure the transducer is well below the keel of the boat and away from prop wash and noise. A test tank or barrel filled with water can be used if the water depth is at least 1 m.

These tests can be performed with the altimeter on the CTD, by observing the altimeter channel voltage output in a Seasave display.


Application Note Revision History

Date Description
January 2014 Initial release.
November 2016

- Include Valeport VA500 altimeter.
- New template.


Applies To