METER HYDROS 21 Water Level User Guide

METER HYDROS 21 Water Level User Guide

SENSOR DESCRIPTION

The HYDROS 21 is a low-cost, accurate tool for monitoring water level, electrical conductivity (EC), and temperature in both groundwater and surface water. The sensor employs a precision pressure transducer to sense water levels between 0 and 10 m for the CTD-10. The sensor cable is vented to remove the effects of
barometric pressure changes. The integrated 4-probe electrical conductivity transducer accurately senses EC up to 120 mS/cm. The sensor also features a precision thermistor to measure temperature. The HYDROS 21 has a compact 3.4 -cm-diameter body made of rugged Delrin® resin. The electronic circuitry is encapsulated in a marine-grade epoxy to protect the sensor in corrosive environments.

For a more detailed description of how this sensor makes measurements, refer to the HYDROS 21 User Manual

APPLICATIONS

• Aquifer recharge and recovery
• Saltwater intrusion, desalination, and wastewater
• Wetland monitoring
• Groundwater contamination monitoring
• Surface water monitoring

ADVANTAGES

• Precision pressure transducer for water depth measurements
• Accurate 4-probe EC measurement
• Robust thermistor for accurate temperature measurements
• Differential pressure measurement referenced to atmospheric pressure so no external pressure sensor is needed
• Robust marine-grade epoxy encapsulation to resist corrosive environments
• Stainless steel cover improves durability
• Three-wire sensor interface: power, ground, and data
• Digital sensor communicates multiple measurements over a serial interface
• Low-input voltage requirements
• Low-power design supports battery-operated data loggers
• Supports SDI-12 or DDI serial communications protocols
• Modern design optimized for low-cost sensing

PURPOSE OF THIS GUIDE

METER provides the information in this integrator guide to help HYDROS 21 customers establish communication between these sensors and their data acquisition equipment or field data loggers. Customers using data loggers that support SDI-12 sensor communications should consult the data logger user manual. METER sensors are fully integrated into the METER system of plug-and- play sensors, cellular enabled data loggers, and data analysis software.

COMPATIBLE FIRMWARE VERSIONS

This guide is compatible with firmware versions 3.99 or newer for the HYDROS 21 Gen 1.

SPECIFICATIONS

MEASUREMENT SPECIFICATIONS

Bulk Electrical Conductivity (EC)

• Range 0−120 dS/m
• Resolution 0.001 dS/m
• Accuracy ±0.01 dS/m or ±10%, whichever is greater

NOTE: The EC measurement is corrected to a standard temperature of 25 °C.

Temperature

• Range −11 to +49 °C
• Resolution 0.1 °C
• Accuracy ±1 °C

Water Depth

• Range 0−10,000 mm
• Resolution 1 mm
• Accuracy ±0.5% of full scale at 20 °C

NOTE: Depth measurement accuracy assumes no abrupt temperature variations.

COMMUNICATION SPECIFICATIONS

Output

• DDI serial or
• SDI-12 communication protocol

Data Logger Compatibility

METER ZL6 data loggers and any data acquisition system capable of 3.6- to 15-VDC power and serial or SDI-12 communication.

PHYSICAL SPECIFICATIONS

Dimensions

• Length 9.0 cm (3.5 in)
• Width 3.4 cm (1.3 in)

Operating Temperature Range

• Minimum 0 °C
• Maximum 50 °C

NOTE: Sensors may be used at higher temperatures under certain conditions; contact Customer Support for assistance.

Cable Length

• 10 m (standard)
• 20 m
• 40 m (maximum cable length)

Cable Diameter

• 6 mm

Connector Types

3.5-mm stereo plug connector or stripped and tinned wires

ELECTRICAL AND TIMING CHARACTERISTICS

Supply Voltage (VCC to GND)

• Minimum 3.6 V
• Typical NA
• Maximum 15.0 V

Digital Input Voltage (logic high)

• Minimum 2.8 V
• Typical 3.6 V
• Maximum 5.0 V

Digital Input Voltage (logic low)

• Minimum –0.3 V
• Typical 0.0 V
• Maximum 0.8 V

Digital Output Voltage (logic high)

• Minimum NA
• Typical 3.6 V
• Maximum NA

Power Line Slew Rate

• Minimum 1.0 V/ms
• Typical NA
• Maximum NA

Current Drain (during measurement)

• Minimum 0.5 mA
• Typical 0.5 mA
• Maximum 1.0 mA

Current Drain (while asleep)

• Minimum NA
• Typical 0.3 mA
• Maximum NA

Power Up Time (DDI serial)

• Minimum NA
• Typical 475 ms
• Maximum 500 ms

Power Up Time (SDI-12)

• Minimum 300 ms
• Typical 475 ms
• Maximum 500 ms

Measurement Duration

• Minimum NA
• Typical 350 ms
• Maximum 500 ms

COMPLIANCE

• Manufactured under ISO 9001:2015
• EM ISO/IEC 17050:2010 (CE Mark)

EQUIVALENT CIRCUIT AND CONNECTION TYPES

Refer to Figure 2 and Figure 3 to connect the HYDROS 21 to a data logger. Figure 2 provides a low-impedance variant of the recommended SDI-12 specification.

Figure 2 Equivalent circuit diagram

• PIGTAIL CABLE
  
  NOTE: Early HYDROS 21 (CTD) units may have the older Decagon wiring scheme where the power supply is white, the digital out is red, and the black and bare wires are ground.

• STEREO CABLE
  
  Figure 3 Connection types

PRECAUTIONS

METER sensors are built to the highest standards, but misuse, improper protection, or improper installation may damage the sensor and possibly void the warranty. Before integrating sensors into a sensor network, follow the recommended installation instructions and implement safeguards to protect the sensor from damaging interference.

SURGE CONDITIONS

Sensors have built-in circuitry that protects them against common surge conditions. Installations in lightning-prone areas, however, require special precautions, especially when sensors are connected to a well-grounded third- party logger.

Read the application note Lightning surge and grounding practices on the METER website for more information.

POWER AND GROUNDING

Ensure there is sufficient power to simultaneously support the maximum sensor current drain for all the sensors on the bus. The sensor protection circuitry may be insufficient if the data logger is improperly powered or grounded. Refer to the data logger installation instructions. Improper grounding may affect the sensor output as well as sensor performance.

Read the application note Lightning surge and grounding practices on the METER website for more information.

CABLES

Improperly protected cables can lead to severed cables or disconnected sensors. Cabling issues can be caused by many factors, including rodent damage, driving over sensor cables, tripping over the cable, not leaving enough cable slack during installation, or poor sensor wiring connections. To relieve strain on the connections and prevent loose cabling from being inadvertently snagged, gather and secure the cable travelling between the HYDROS 21 and the data acquisition device to the mounting mast in one or more places. Install cables in conduit or plastic cladding when near the ground to avoid rodent damage. Tie excess cable to the data logger mast to ensure cable weight does not cause sensor to unplug.

SENSOR COMMUNICATIONS

METER digital sensors feature a serial interface with shared receive and transmit signals for communicating sensor measurements on the data wire (Figure 3). The sensor supports two different protocols: SDI-12 and DDI serial. Each protocol has implementation advantages and challenges. Please contact Customer Support if the protocol choice for the desired application is not obvious.

SDI-12 INTRODUCTION

SDI-12 is a standards-based protocol for interfacing sensors to data loggers and data acquisition equipment. Multiple sensors with unique addresses can share a common 3-wire bus (power, ground, and data). Two-way communication between the sensor and logger is possible by sharing the data line for transmit and receive as defined by the standard. Sensor measurements are triggered by protocol command. The SDI-12 protocol requires a unique alphanumeric sensor address for each sensor on the bus so that a data logger can send commands to and receive readings from specific sensors.

Download the SDI-12 Specification v1.3 to learn more about the SDI-12 protocol.

DDI SERIAL INTRODUCTION

The DDI serial protocol is the method used by the METER data loggers for collecting data from the sensor. This protocol uses the data line configured to transmit data from the sensor to the receiver only (simplex). Typically, the receive side is a microprocessor UART or a general-purpose I/O pin using a bitbang method to receive data. Sensor measurements are triggered by applying power to the sensor.

INTERFACING THE SENSOR TO A COMPUTER

The serial signals and protocols supported by the sensor require some type of interface hardware to be compatible with the serial port found on most computers (or USB-to-serial adapters). There are several SDI-12 interface adapters available in the marketplace; however, METER has not tested any of these interfaces and cannot make a recommendation as to which adapters work with METER sensors. METER data loggers and and the ZSC and PROCHECK handheld devices can operate as a computer-to-sensor interface for making on-demand sensor measurements. For more information, please contact Customer Support.

METER SDI-12 IMPLEMENTATION

METER sensors use a low-impedance variant of the SDI-12 standard sensor circuit (Figure 2). During the power-up time, sensors output some sensor diagnostic information and should not be communicated with until the power-up time has passed. After the power-up time, the sensors are fully compatible with all commands listed in the SDI-12 Specification v1.3 except for the continuous measurement commands (aR3 and aRC3). M, R, and C command implementations are found. The aR3 commands are used by METER systems and as a result uses a space delimiter, instead of a sign delimiter as required by the SDI-12 standard.

Out of the factory, all METER sensors start with SDI-12 address 0 and print out the DDI serial startup string during the power-up time. This can be interpreted by non-METER SDI-12 sensors as a pseudo-break condition followed by a random series of bits.

The HYDROS 21 will omit the DDI serial startup string when the SDI-12 address is nonzero. Changing the address to a nonzero address is recommended for this reason.

SENSOR BUS CONSIDERATIONS

SDI-12 sensor buses require regular checking, sensor upkeep, and sensor troubleshooting. If one sensor goes down, that may take down the whole bus even if the remaining sensors are functioning normally. Power cycling the SDI-12 bus when a sensor is failing is acceptable, but METER does not recommend scheduling power cycling events on an SDI-12 bus more than once or twice per day. Many factors influence the effectiveness of the bus configuration. Visit metergroup.com for articles and virtual seminars containing more information.

SDI-12 CONFIGURATION

Table 1 lists the SDI-12 communication configuration.

Table 1 SDI-12 communication configuration

Baud Rate

| 1,200 bps
---|---
Start Bits|

1

Data Bits

| 7 (LSB first)
Parity Bits|

1 (even)

Stop Bits

| 1
Logic|

Inverted (active low)

SDI-12 TIMING

All SDI-12 commands and responses must adhere to the format in Figure 4 on the data line. Both the command and response are preceded by an address and terminated by a carriage return and line feed combination () and follow the timing shown in Figure 5.

Figure 4 Example SDI-12 transmission of the character 1 (0x31)

Figure 5 Example data logger and sensor communication

COMMON SDI-12 COMMANDS

This section includes tables of common SDI-12 commands that are often used in an SDI-12 system and the corresponding responses from METER sensors.

IDENTIFICATION COMMAND (aI!)

The Identification command can be used to obtain a variety of detailed information about the connected sensor. An example of the command and response is shown in Example 1, where the command is in bold and the response follows the command.

Example 1 1I!113DECAGON␣CTD␣␣␣389631800001

Parameter

| Fixed Character Length| Description
---|---|---
1I!| 3|

Data logger command. Request to the sensor for information from sensor address

1

| 1| Sensor address. Prepended on all responses, this indicates which sensor on the bus is returning the following information.
13| 2|

Indicates that the target sensor supports SDI-12 Specification v1.3.

DECAGON␣

| 8| Vendor identification string. (DECAGON and one space ␣)
CTD␣␣␣| 6|

Sensor model string. This string is specific to the sensor type. For the HYDROS 21, the string is CTD␣␣␣.

389

| 3| Sensor version. This number divided by 100 is the METER sensor version (e.g., 389 is version 3.89).
631800001| ≤13, variable|

Sensor serial number. This is a variable length field. It may be omitted for older sensors.

CHANGE ADDRESS COMMAND (aAB!)

The Change Address command is used to change the sensor address to a new address. All other commands support the wildcard character as the target sensor address except for this command. All METER sensors have a default address of 0 (zero) out of the factory. Supported addresses are alphanumeric (i.e., a–z, A–Z, and 0–9). An example output from a METER sensor is shown in Example 2, where the command is in bold and the response follows the command.

Example 2 1A0!0

Parameter| Fixed Character Length|

Description

---|---|---

1A0!

| 4| Data logger command.
Request to the sensor to change its address from 1 to a new address of 0.
0| 1|

New sensor address.
For all subsequent commands, this new address will be used by the target sensor.

ADDRESS QUERY COMMAND (?!)

While disconnected from a bus, the Address Query command can be used to determine which sensors are currently being communicated with. Sending this command over a bus will cause a bus contention where all the sensors will respond simultaneously and corrupt the data line. This command is helpful when trying to isolate a failed sensor. Example 3 shows an example of the command and response, where the command is in bold and the response follows the command. The question mark (?) is a wildcard character that can be used in place of the address with any command except the Change Address command.

Example 3 ?!0

Parameter

| Fixed Character Length|

Description

---|---|---

?!

| 2| Data logger command.
Request for a response from any sensor listening on the data line.
0| 1|

Sensor address.
Returns the sensor address to the currently connected sensor.

COMMAND IMPLEMENTATION

The following tables list the relevant Measurement (M), Continuous (R), and Concurrent (C) commands and subsequent Data (D) commands, when necessary

MEASUREMENT COMMANDS IMPLEMENTATION

Measurement (M) commands are sent to a single sensor on the SDI-12 bus and require that subsequent Data (D) commands are sent to that sensor to retrieve the sensor output data before initiating communication with another sensor on the bus.

Please refer to Table 2 and for an explanation of the command sequence and to Table 6 for an explanation of response parameters.

Table 2 aM! command sequence

Command

|

Response

---|---

aM!

| atttn
aD0!|

a+±+

NOTE: The measurement and corresponding data commands are intended to be used back to back. After a measurement command is processed by the sensor, a service request a is sent from the sensor signaling the measurement is ready. Either wait until ttt seconds have passed or wait until the service request is received before sending the data commands. See the SDI-12 Specifications v1.3 document for more information.

CONCURRENT MEASUREMENT COMMANDS IMPLEMENTATION
Concurrent Measurement (C) commands are typically used with sensors connected to a bus. C commands for this sensor deviate from the standard C command implementation. First, send the C command, wait the specified amount of time detailed in the C command response, and then use D commands to read its response prior to communicating with another sensor.

Please refer to Table 3 for an explanation of the command sequence and to Table 6 for an explanation of response parameters.

Table 3 aC! measurement command sequence

Command

|

Response

---|---

aC!

| atttnn
aD0!|

a+±+

NOTE: Please see the SDI-12 Specifications v1.3 document for more information.

CONTINUOUS MEASUREMENT COMMANDS IMPLEMENTATION

Continuous Measurement (R) commands trigger a sensor measurement and return the data automatically after the readings are completed without needing to send a D command.

Please refer to Table 4 through Table 5 for an explanation of the command sequence and see Table 6 for an explanation of response parameters.

Table 4 aR0! measurement command sequence

Command     Response

aR0!|

a+±

Table 5 aR3! measurement command sequence

Command     Response

aR3!|

a

NOTE: This command does not adhere to the SDI-12 response format or timing. See METER SDI-12 Implementation for more information. The values in this command are space delimited. As such, a + sign is not assigned between values and a – sign is only present if the value is negative.

PARAMETERS

Table 6 l ists the parameters, unit measurement, and a description of the parameters returned in command responses for HYDROS 21.

Table 6 Parameter Descriptions

Parameter

| Unit| Description
---|---|---
±| —|

Positive or negative sign denoting sign of the next value

a

| —| SDI-12 address
n| —|

Number of measurements (fixed width of 1)

nn

| —| Number of measurements with leading zero if necessary (fixed width of 2)
ttt| s|

Maximum time measurement will take (fixed width of 3)