Calibration; Pressure Sensor; Pressure/Depth Relationship - Valeport TideMaster Operating Manual

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Section 7 | Calibration

7 Calibration

When supplied with a pressure sensor, the TideMaster is supplied with a factory calibration, which provides

an output of pressure in decibars and takes out any non-linearity in the transducer characteristic. Note that

the pressure calibration is specifically for a combination of TideMaster and transducer, and the unit and

transducer serial numbers are held in the logger memory. If the transducer is changed, the pressure

calibration will be slightly in error, but the system can still be used provided an in-situ calibration of depth

against pressure output is made.

The user can add to this calibration to provide output in metres [or feet] above datum. This secondary

calibration takes the form of a Gain factor [distance per dBar] and Offset [transducer position with

respect to datum]. In order to determine this secondary calibration, some on site measurements will need

to be taken to establish the relationship between decibars and depth with respect to a datum for the

particular site. The exact calibration is effected by transducer offset, water density and gravity.

The transducer is temperature compensated, but this compensation does rely on the temperature of all parts

of the transducer being the same, so when the transducer is first put into water, time must be allowed for the

temperature to stabilize before any calibration measurements are taken. Five minutes should be adequate.

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The relationship between pressure and depth in shallow water is

a straight-line function:

Height above datum = (Gain x Pressure) + Offset

Where Offset = Transducer position with respect to a Datum

[+ve if above datum, -ve below].

e.g. if the transducer is 1.15 metres below datum, then the

offset is –1.15 metres.

Gain = (H2 – H1)/(P2 – P1)

Offset = H1 – (P1 * (H2 – H1)/(P2 – P1))

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