4 Introduction
The pressure probe OTT PLS 500 is used for precisely measuring water level of
ground and surface waters. The pressure probe uses the hydrostatic pressure of
the water column above a relative pressure measuring cell. A pressure compensa-
tion capillary in the pressure probe cable gives the measuring cell the current
ambient air pressure as a reference. Erroneous measurement results due to atmo-
spheric air pressure fluctuations are thus eliminated.
The OTT PLS 500 can be supplied with various measuring ranges:
▶ 0 … 10 m water column (0 ... 1 bar)
▶ 0 … 20 m water column (0 ... 2 bar)
▶ 0 … 40 m water column (0 ... 4 bar)
▶ 0 … 100 m water column (0 ... 10 bar)
The pressure probe has two interfaces that can be used in parallel:
▶ SDI-12 interface
▶ EIA-485 (RS-485) interface (SDI-12 or Modbus (RTU) protocol)
The OTT PLS 500 can be configured in many ways via the SDI-12 transparent
mode of a data logger or with the aid of the "OTT USB/SDI-12 Adapter" inter-
face converter. For example, a reference or offset value can be entered upon ini-
tial setup
A particular feature is that the pressure probe measures the water temperature as
well as the hydrostatic pressure of the water column and thus yields highly precise
and reproduceable measurement results by compensating effects of temperature,
specific density or salinity of the water and the local gravitational acceleration at
the specific station. (For this, the specific density or the salinity and the local gravi-
tational acceleration must be entered upon the initial setup.)
The OTT PLS 500 performs four individual measurements within one second;
duration 250 milliseconds each. An OTT PLS 500 measurement interval is defined
as the arithmetic mean of individual measurements over an adjustable averaging
time. The averaging time is 0.5 … 59.5 seconds (corresponding to 2 … 238 indi-
vidual measurements; factory setting: 1.5 seconds).
At the SDI-12 and RS-485 interfaces, the OTT PLS 500 outputs either the water
level (compensated) or the hydrostatic pressure as well as the water temperature.
Measurement units can be set with SDI-12 commands as m · cm · mm · ft · inch
(water level measurement), bar · mbar · kPa · psi (pressure measurement) and
°C · °F · K (temperature measurement). The pressure probe is available with preset
metric or imperial units by using a variant code.
Two different humidity absorbing systems are available as accesories for drying
the ambient air entering the pressure compensation capillary.
The pressure probe is installed either in an individual protective device adapted to
the respecive installation site and to be provided by the customer or suspended
from the pressure probe cable. A cable suspension specially adapted to the pres-
sure probe cable is available as accessory.
In addition to the actual measured values water level and temperture, the OTT PLS
500 records additional operating parameters and makes them available as meta
data. This enables comprehensive remote monitoring of the pressure probe. Also,
statistical data within each measurement interval will be determined.
The OTT PLS 500 can also calculate the current discharge of a water course from
the measured water level. This is a special function that can be activated as an
option. To use this function, a rating table or the factors of the exponential formu-
la according to ISO 1100-2-standard must be entered upon the initial setup.
7