Stevens Smart PT Manuel utilisateur

Digital Pressure and Temperature Sensor

Introduction

Stevens Smart PT is an advanced digital pressure and temperature sensor ideal for water

level measurements as well as many other types of pressure and fluid level monitoring

applications. The SDI-12 communications interface provides universal compatibility with

industry standard data loggers.

In addition to simple instantaneous pressure, level, and temperature measurements, Smart

PT includes the ability to automatically record peak crest levels, calculate average level as

well as standard deviation all without complex datalogger configurations. Other advanced

features include adjustable fluid density, automatic water temperature density

compensation, and adjustable local gravity compensation.

An M14-1 threaded sensor head allows for easy mounting to pipes. An included threaded

cap offers a loop hole which can be used to mount weights or pull the sensor through pipes

or other small areas. The Smart PT is built to last for years in the field with fully sealed and

potted components, a robust ceramic membrane, stainless-steel housing, and an industrial-

quality cable.

Table of Contents

Introduction...............................................................................................................1

Order numbers...........................................................................................................3

Performance Characteristics.........................................................................................3

Environmental Conditions............................................................................................3

Warranty...................................................................................................................4

Installing the Smart PT probe.......................................................................................4

Wiring....................................................................................................................4

Vent Tube...............................................................................................................4

Ice ........................................................................................................................4

Packaging for calibration and repair ..............................................................................5

SDI-12 Commands and Responses ...............................................................................6

Command quick reference ........................................................................................6

Variation in the last digits of pressure readings ...........................................................6

Basic SDI-12 Commands..........................................................................................6

Advanced SDI-12 Commands....................................................................................9

Configuring pressure, depth, and temperature units .................................................9

Configuring gravity compensation.........................................................................10

Configuring density compensation ........................................................................11

Configuring the Smart PT for top-of-casing or reference-relative measurements.........11

Restoring the Smart PT to the default configuration................................................13

Using the Smart Sampling features and digital crest gage mode.....................................14

Example configurations for average and daily maximum.............................................14

Using Excel to extract timestamped crest values from a data set.................................15

Appendix A - Metadata Commands in SDI-12 version 1.4 ..............................................16

Appendix B – Calculation of depth ..............................................................................16

Order numbers

Ordering

Number

Range in bar

Water depth

in meters

Water depth

in feet

Overpressure in

meters

vented

51168-201

0.2

6.6

51168-202

0.4

51168-203

100

51168-204

150

51168-205

130

250

51168-206

100

330

400

51168-207

200

660

400

non-vented (note: non-vented depth specs are assumed at sea level. Actual max depth

depends on elevation and atmospheric pressure.)

51168-303

1.4

51168-304

140

51168-305

100

240

51168-306

300

390

51168-307

190

630

390

32142

Recommended Yearly Calibration

Performance Characteristics

Parameter

Unit

Pressure accuracy

0.1%

Full scale

Long term stability

max 0.15% per year

Full scale

Temperature accuracy

±0.25

°C

Average current consumption, SDI-12

0.9

Average current consumption, Modbus

1.5

Environmental Conditions

Parameter

Min

Max

Unit

Supply voltage during operation

Temperature during operation

-20

°C

Warranty

The Smart PT has internal surge protection components for lightning protection. However,

damage due to lightning is not covered under the warranty.

Except for the 0.2 bar vented sensor, the Smart PT is warrantied to withstand freezing

conditions without damage if the black cap is removed. The 0.2 bar vented sensor is more

sensitive to overpressure and isn’t warrantied for freezing conditions.

Installing the Smart PT probe

Wiring

Wire Color

Signal

Black

Ground

Red

+12Vdc

Blue

SDI-12 Data

White

RS485+ / B

Green

RS485- / A

Vent Tube

The vented version of the Smart PT has a

tube running the length of the cable. This

allows the water pressure on the front of

the transducer to reference against

barometric pressure.

The Smart PT ships with a black cap over

end of the vent tube to prevent moisture

ingress, and with a separate desiccant

capsule. Before installation the dessicant

capsule needs to be connected and the

yellow cap removed.

Ice

The Smart PT ships with an engineered resin cap designed to

protect the ceramic membrane. It’s important to remove the cap if

the Smart PT is expected to freeze. If the cap isn’t removed,

expanding ice trapped under the cap will damage the ceramic

membrane.

Note: RS485 is used for

firmware updates and future

interface expansion.

Packaging for calibration and repair

To correct for long term drift, the Smart PT should be calibrated every year.

Before returning the sensor for calibration or repair, navigate to the ‘Support’ page at

http://www.stevenswater.com/ and fill out the RMA form.

If the sensor was used in contaminated water, the sensor must be cleaned before shipping.

Coil and zip-tie the sensor cable before shipping.

SDI-12 Commands and Responses

Command quick reference

M: pressure, temperature M1: minimum, maximum M2: average, standard deviation

Variation in the last digits of pressure readings

Smart PT reports pressure results to a precision of 0.0001 bars or better. This ensures that

each pressure range of the Smart PT will return results with the same number of significant

figures. Because this level of precision is higher than either the accuracy or the inter-reading

stability of the Smart PT, it’s normal to see variation in the last few digits of the pressure

reading.

Basic SDI-12 Commands

Command

Response

Description

a<CR><LF>

Acknowledge active

a – sensor address

aI!

a14ccccccccmmmmmmvvvxxx...xx<CR>

<LF>

Example:

Vented:

014STEVENSW_SVP01_VT_1234567890

Non-vented:

014STEVENSW_SVP01_NV_1234567890

Send identification

a – sensor address

14 – SDI-12 protocol version

ccc… – manufacturer identification

mmm… – sensor identification

vvv – sensor version

xxx… – serial number

aAb

b<CR><LF>

Example: b

Change address

b – new address

a<CR><LF>

Example: a

Address query

a – sensor address

aM!

atttn<CR><LF>

Example: a0002

Request a single pressure and temperature

reading

t - seconds until the measurement is ready

(always zero)

n – number of data fields in the measurement

(always two for this command)

Command

Response

Description

aD0!

a<value1><value2><CR><LF>

Example: a+1.0+25.6

Send a single pressure and temperature

reading

a – sensor address

value1 – depth or pressure

value2 – temperature

aM1!

atttn<CR><LF>

Example: a0004

Request min and max (crest and trough)

since the last M1 command.

Smart PT takes a sample every second and

stores min and max in nonvolatile memory.

Min and max are reset when the M1

command is received.

aD0!

a<min><max><tmin><tmax><CR><LF>

Example: a+1.0+1.4+48+67

Send min and max since the last M1

command

a – sensor address

min – lowest pressure encountered since last

M1 reading

max – highest pressure encountered since

last M1 reading

tmin – seconds elapsed since the minimum

reported in <min>

tmax – seconds elapsed since the maximum

reported in <max>

See the section, “Using Excel to extract

timestamped crest values from a data set” for

more information on using tmin and tmax

aM2!

atttn<CR><LF>

Example: a0003

Request average and standard deviation of

pressure since the last M2 command

Smart PT takes a sample every second and

maintains a cumulative average and standard

deviation. Average and standard deviation are

reset when the M2 command is received.

Command

Response

Description

aD0!

a<avg><stddev><nsamples><CR><LF>

Example: a+1.2+0.01+129

Send average and standard deviation since

the last M2 command

a – sensor address

avg – average of all pressure samples taken

since the last M2 reading

stddev – standard deviation of all pressure

samples taken since the last M2 reading

nsamples – number of samples taken since

the last M2 reading

Advanced SDI-12 Commands

Configuring pressure, depth, and temperature units

The Smart PT can be configured to report in various units of pressure and temperature.

To compensate for the density-temperature curve in water, the Smart PT Sensor uses Kell’s

formulation, as described in the publication ITS-90 Density of Water Formulation for

Volumetric Standards Calibration (Jones 1992). This, and the gravity parameter, are applied

to all measurements returned in units of depth.

When the Smart PT is configured to report in units of pressure, rather than depth, no

temperature compensation will be applied.

The non-vented Smart PT only reports in units of pressure. Because it measures the weight

of the water column plus the atmosphere, it wouldn’t be useful to report units of depth.

Command

Response

Description

aXR_PUNITS!

aPUNITS=’UUU’<CR><LF>

Example:

aXR_PUNITS!

aPUNITS=’M’

Query pressure units

UUU… – pressure units

aXW_PUNITS_UUU!

aPUNITS=’UUU’<CR><LF>

Example:

aXW_PUNITS_M!

aPUNITS=‘M’

Configure pressure units

uuu… – pressure units

* meters

* centimeters

* millimeters

* feet

* inches

bars

BAR

millibars

MBAR

kilopascals

KPA

pounds per square inch

PSI

default

BAR

* Only allowed for vented

aXR_TUNITS!

aTUNITS=’UU’<CR><LF>

Example:

aXR_TUNITS!

aTUNITS=‘DC’

Query temperature units

UU… – temperature units

aXW_TUNITS_UU!

aTUNITS=’UU’<CR><LF>

aXW_TUNITS_DC!

aTUNITS=‘DC’

Configure temperature units

degrees centigrade

degrees fahrenheit

kelvin

default

Configuring gravity compensation

Gravity on the surface of the earth can vary by 0.7%, from a minimum of 9.7639 m/s2in

Peru, to a peak of 9.8337 m/s2on the surface of the arctic ocean.

The Smart PT can be configured to compensate for local gravitational acceleration.

Wolfram Alpha provides a convenient tool to find your local gravitational acceleration:

https://www.wolframalpha.com/input/?i=gravity+portland+oregon

When the Smart PT is configured to report in units of pressure, rather than depth, no

gravity compensation will be applied.

Command

Response

Description

aXR_GRAVITY!

aGRAVITY=’vvv’<CR><LF>

Example:

aXR_GRAVITY!

aGRAVITY=’9.80665’

Query gravity

a – sensor address

vvv… – gravitational acceleration

aXW_GRAVITY_vvv!

aGRAVITY=’vvv’

Example:

aXW_GRAVITY_9.80665!

aGRAVITY=’9.80665’

Configure gravity

a – sensor address

vvv… – gravitational acceleration

Default: 9.80665 m/s2

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