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 adjustab le 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 industrialquality cable.
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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 ga ge 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
2
Order numbers
Ordering
Range in bar
Water depth
Water depth
Overpressure in
vented
51168-201
0.2
2
6.6
50
51168-202
0.4
4
13
60
51168-203
1
10
33
100
51168-204
2
20
66
150
51168-205
4
40
130
250
51168-206
10
100
330
400
51168-207
20
200
660
400
non-vented (note: non-vented depth specs are assumed at sea level. Actual max depth
depends on elevation and atmospheric press ure.)
51168-303
1.4
4
13
90
51168-304
2
10
33
140
51168-305
4
30
100
240
51168-306
10
90
300
390
51168-307
20
190
630
390
32142
Recommended Yearly Calibration
Parameter
Unit
Pressure accuracy
0.1%
Full scale
Long term stability
max 0.15% per year
Full scale
Temperature accuracy
±0.25
°C
Average curren t con sumption , SDI-12
0.9
mA
Average current consumption, Modbus
1.5
mA
Parameter
Min
Max
Unit
Supply voltage during operation
6
18
V
Temperature during operation
-20
80
°C
Number
Performance Characteristics
in meters
in feet
meters
Environmental Conditions
3
Wire Color
Signal
Black
Ground
Red
+12Vdc
Blue
SDI-12 Data
White
RS485+ / B
Green
RS485- / A
Note: RS485 is used for
Warranty
The Smart PT has internal surge protection components for lightning protec tion. However,
damage due to lightning is not covered under the warr a nty.
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
firmware updates and future
interface expan sion.
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 r emoved,
expanding ice trapped under the cap will damage the ceramic
membrane.
4
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/
If the sensor was used in contaminated water, the sensor must be cleaned before shipping.
Coil and zip-tie the sensor cable before shipping.
and fill out the RMA form.
5
Command
Response
Description
a!
a<CR><LF>
Acknowledge active
aI!
a14ccccccccmmmmmmvvvxxx...xx<CR>
Send identification
aAb
b<CR><LF>
Change address
?!
a<CR><LF>
Address query
aM!
atttn<CR><LF>
Request a single pressure and temperature
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
a – sensor address
<LF>
Example:
Vented:
014STEVENSW_SVP01_VT_1234567890
Non-vented:
014STEVENSW_SVP01_NV_1234567890
a – sensor address
14 – SDI-12 protocol version
ccc… – manufacturer identification
mmm… – sensor identification
Example: b
Example: a
Example: a0002
vvv – sensor version
xxx… – serial number
b – new address
a – sensor address
reading
t - seconds until the measurement is ready
(always zero)
n – number of data fields in the measurement
(always two for this command)
6
Command
Response
Description
aD0!
a<value1><value2><CR><LF>
Send a single pressure and temperature
aM1!
atttn<CR><LF>
Request min and max (crest and trough)
aD0!
a<min><max><tmin><tmax><CR><LF>
Send min and max since the last M1
aM2!
atttn<CR><LF>
Request average and standard deviation of
Example: a+1.0+25.6
Example: a0004
Example: a+1.0+1.4+48+67
reading
a – sensor address
value1 – depth or pressure
value2 – temperature
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.
command
a – sensor address
min – lowest pressure encountered since last
M1 reading
max – highest pressure encountered since
last M1 reading
Example: a0003
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
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.
7
Command
Response
Description
aD0!
a<avg><stddev><nsamples><CR><LF>
Send average and standard deviation since
Example: a+1.2+0.01+129
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
8
Command
Response
Description
aXR_PUNITS!
aPUNITS=’UUU’<CR><LF>
Query pressure units
aXW_PUNITS_UUU!
aPUNITS=’UUU’<CR><LF>
Configure pressure units
* meters
M
* centimeters
CM
* millimeters
MM
* feet
FT
* inches
IN
bars
BAR
millibars
MBAR
kilopascals
KPA
pounds per square inch
PSI
default
BAR
* Only allowed for vented
aXR_TUNITS!
aTUNITS=’UU’<CR><LF>
Query temperature units
aXW_TUNITS_UU!
aTUNITS=’UU’<CR><LF>
Configure temperature units
degrees cent igrade
DC
degrees fahr enheit
DF
kelvin
DK
default
DC
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 des cribed 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.
Example:
aXR_PUNITS!
aPUNITS=’M’
UUU… – pressure units
Example:
aXW_PUNITS_M!
aPUNITS=‘M’
Example:
aXR_TUNITS!
aTUNITS=‘DC’
aXW_TUNITS_DC!
aTUNITS=‘DC’
uuu… – pressure units
UU… – temperature units
9
Command
Response
Description
aXR_GRAVITY!
aGRAVITY=’vvv’<CR><LF>
Query gravity
aXW_GRAVITY_vvv!
aGRAVITY=’vvv’
Configure gravity
Configuring gravity compensation
Gravity on the surface of the earth can vary by 0.7%, from a minimum of 9.7639 m/s2 in
Peru, to a peak of 9.8337 m/s
The Smart PT can be configured to compensate for local gravitational acceleration.
Wolfram Alpha p rovides a convenient tool to find your local gravitational acceleration:
2
on the surface of the arctic ocean.
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.
Example:
aXR_GRAVITY!
aGRAVITY=’9.80665’
Example:
aXW_GRAVITY_9.80665!
aGRAVITY=’9.80665’
a – sensor address
vvv… – gravitational acceleration
a – sensor address
vvv… – gravitational acceleration
Default: 9.80665 m/s
2
10
Command
Response
Description
aXR_DENSITY!
aDENSITY=’vvv’<CR><LF>
Query density
aXW_DENSITY_vvv!
aDENSITY=’vvv’
Configure density
Command
Response
Description
aXR_TOC_vvv!
aTOC=’vvv’<CR><LF>
Query top of casing
aXW_TOC_vvv!
aTOC=’vvv’
If non-zero, depth will be
aXR_OFFSET_vvv!
aOFFSET=’vvv’<CR><LF>
Query offset
aXW_OFFSET_vvv!
aOFFSET=’vvv’
This value will be added to depth
Configuring density compensation
The density of water can vary due to salinity, aeration, or suspended sediment.
The Smart PT can be configured to compensate for working fluid density.
Because the built-in temperature density curve is only valid for fresh water, temperature
compensation will be disabled when the density parameter is modified.
Example:
aXR_DENSITY!
aDENSITY=’1’
Example:
aXW_DENSITY_1.1!
aDENSITY=’1.1’
a – sensor address
vvv… – density
a – sensor address
vvv… – density
Default: 1 g/mL
Configuring the Smart PT for top-of-casing or reference-relative measurements
The Smart PT can be configured to report depth measurements from actual or surveyed top
of casing. Following this command table is an example.
Example:
aXW_TOC_1!
aTOC=’1’
Example:
aXW_OFFSET_1!
aOFFSET=’1’
subtracted from the TOC value.
See the section, “Calculation of
depth with corrections”
Default: 0
after all other corrections have
been applied. See the section,
“Calculation of depth with
corrections”
Default: 0
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In this example, a Smart PT is installed in a 100 foot borewell, 75 feet from the top of
casing. The bottom of the well is 10 feet below sea level.
Without any special configuration the Smart PT will report the sensed depth, 55 feet.
To report feet above sea level, set the “offset” parameter to 15. The Smart PT will report the
sensed depth plus the offset, for a reported value of 70.
aXW_OFFSET_15!
To report distance from water to top of casing, set “toc” to 75. The Smart PT will return the
“TOC” value minus sensed depth, for a reported value of 20.
aXW_TOC_75!
To report distance from the water surface to bottom of well, set “offset” to 25. The Smart PT
will return the sensed depth plus the offset, for a reported value of 80.
aXW_OFFSET_25!
12
Command
Response
Description
aXD_*!
arestore fact ory configuration...
Restore the sensor to a
Restoring the Smart PT to the default configuration
It may be useful to restore the Smart PT to the factory default configuration.
factory default state. Stored
data will be lost. Sensor
retains factory calibration.
13
Using the Smart Sampling features and digital crest gage mode
A conventional pressure sensor only samples data when requested by the logger. As seen in
the chart below, if the sampling interval is set too long, there’s a risk of missing critical
events.
The Smart PT takes a sample once per second and can report relevant statistics on demand,
including crest events. As you can see in the chart above, the Smart PT was able to
accurately capture crest events that a conventional sensor would have missed.
The Smart PT is also able to report average and standard deviation over the logging
interval. This may be useful for integrating data from rough water and quantifying surface
roughness. Instead of a sliding window, the Smart PT uses a numerically stable online
variance algorithm (Welford 1962) to maintain the mean and standard deviation since the
last time those values were queried.
Example configurations for average and daily maximum
To record the ten-minute average, configure the data logger to sample the M2 command
once every ten minutes.
To record the daily maximum, configure the data logger to sample the M1 command once
every 24 hours.
Minimum and maximum values are backed-up to flash and will persist if the sensor loses
power.
14
Logger timestamp
Min
Max tMi n tMax Time of crest
A3-TIME(0,0,E3)
8:30:00 AM 1004 4996 33 340 8:24:20 AM
8:40:00 AM 1004 4995 23 324 8:34:36 AM
8:50:00 AM 1003 4999 47 310 8:44:50 AM
9:00:00 AM 1001 4991 16 339 8:54:21 AM
Using Excel to extract timestamped crest values from a data set
The Smart PT reports the time at which a min or max event occurred in the 3rd and 4th fields
of the M1 response.
These values, tMin and tMax, show how many seconds ago the event occurred.
In the example below, a crest event occurred at 8:24:20 AM. The sensor was polled by a
datalogger at 8:30.00 AM, at which time the sensor repor ted the crest event as happening
340 seconds in the past.
15
Appendix A - Metadata Commands in SDI-12 version 1.4
Revision 1.4 of the SDI-12 specification, released in May of 2017, adds a set of commands
to access metadata – descriptions of the returned data including SHEF codes and units. The
Smart PT sensor implements the 1.4 specification.
aIM0! a00002
aIM0_001! 0,PW,BAR,pressure;
aIM0_002! 0,TW,DC,temperature;
Appendix B – Calculation of depth
Depth is computed as follows
depth = depth * (9 .80665 / gravity) * (1000 / density)
If density is set to 1, the Smart PT will apply a temperature correction:
density = (999.83952 + 16.945176*t - .0079870401*t*t - 0.000046170461*t*t*t +
0.00000010556302*t*t*t*t - 0.00000000008054253*t*t*t*t*t) / (1 + .016897850*t);
If density is set to a value other than 1, that value will override the temperature correction.
If top of casing is greater than zero, depth is subtracted from top of casing:
depth = top of cas ing - depth
Finally, offset is added to the depth value:
depth = depth + of fset
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