U.S. Department of the Interior

U.S. Geological Survey

Open-File Report 2015–1139

Hydraulic Laboratory Testing of SonTek-IQ Plus
Cover photographs. Left: SonTek-IQ Plus on office table without power and communication cable. Right: Three SonTek-IQ Plus sensors 

mounted side by side on Hydraulic Laboratory tilting flume bottom.
Hydraulic Laboratory Testing of SonTek-IQ Plus

By Janice M. Fulford and Scott Kimball

Open-File Report 2015–1139

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2015

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Suggested citation:

Fulford, J.M., and Kimball, Scott, 2015, Hydraulic laboratory testing of SonTek-IQ Plus: U.S. Geological Survey 

Open-File Report 2015–1139, 16 p., http://dx.doi.org/10.3133/ofr20151139.

ISSN 2331-1258 (online)
iii

Contents

Abstract...........................................................................................................................................................1

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

Description of Instrument.............................................................................................................................1

Test Procedures .............................................................................................................................................2

Depth test...............................................................................................................................................2

Discharge and velocity tests ..............................................................................................................3

SDI-12 compliance testing ..................................................................................................................5

Test Results.....................................................................................................................................................5

Depth testing..........................................................................................................................................5

Discharge and Velocity Measurements with Firmware 1.52.........................................................7

Discharge and Velocity Measurements with Firmware 1.60.......................................................11

SDI-12 compliance..............................................................................................................................14

Operating Observations..............................................................................................................................14

Summary........................................................................................................................................................15

References Cited..........................................................................................................................................16

Figures

1. Photo of SonTek-IQ Plus on office table without power and communication cable.........2

2. Photo of custom angle mount on instrument arm with SonTek-IQ Plus installed 

on mount.........................................................................................................................................3

3. Photo of SonTek-IQ Plus sensor mounted to Hydraulic Laboratory tilting flume 

bottom and three SonTek-IQ Plus sensors mounted side by side on Hydraulic 

Laboratory tilting flume bottom ..................................................................................................4

4. Graph showing difference between depth readings measured by SonTek-IQ Plus 

and laboratory reference for 0 and ±5 degrees pitch-and-roll angles for depth 

below water surface from 0 to 7 feet ........................................................................................5

5. Graph showing difference between depth readings measured by SonTek-IQ Plus 

and laboratory reference for 0 and ±5 degrees pitch-and-roll angles for depth 

below water surface from 0 to 7 feet ........................................................................................6

6. Graph showing difference between depth readings measured by SonTek-IQ Plus 

and laboratory reference for 0 and ±5 degrees pitch-and-roll angles for depth 

below water surface from 0 to 7 feet ........................................................................................6

7. Graph showing difference between depth readings measured by SonTek-IQ Plus 

and laboratory reference for 0 and ±10 degrees pitch-and-roll angles for depth 

below water surface from 0 to 7 feet ........................................................................................7

8. Graph showing percent difference in discharge between SonTek-IQ Plus using 

firmware 1.52 and SonTek’s “theoretical” computation method and FlowTracker 

using midsection computation method for discharges from approximately 7 to 

36 cubic feet per second .............................................................................................................8

9. Graphs showing velocity profiles measured by SonTek-IQ Plus and FlowTracker 

for average stream-wise velocities of 0.5, 1.1, and 2.0 feet per second .............................9

10. Graphs showing velocity profiles measured by SonTek-IQ plus using firmware 1.52 

and FlowTracker for average stream-wise velocities of 0.7, 1.4, and 3.2 feet 

per second .....................................................................................................................................9
iv

11. Graph showing percent velocity difference between the SonTek-IQ Plus using 

firmware 1.52 and the FlowTracker for velocity profiles measured for average 

stream-wise velocities of 0.5, 1.1, and 2.0 feet per second plotted with the 

IQ Plus accuracy.........................................................................................................................10

12. Graphs showing percent velocity difference between the SonTek-IQ Plus using 

firmware 1.52 and the FlowTracker for velocity profiles measured for average 

stream-wise velocities of 0.7, 1.4, and 3.2 feet per second plotted with the 

IQ Plus accuracy.........................................................................................................................10

13. Graph showing percent difference between SonTek-IQ Plus using firmware 1.52 

and FlowTracker mean-profile velocity at various mean-flow velocities from 0.5 to 

about 3.25 feet per second........................................................................................................11

14. Graph showing percent difference in discharge between SonTek-IQ Plus using 

firmware 1.60 and SonTek’s “theoretical” computation method and FlowTracker 

using midsection method for discharges from approximately 7 to 36 cubic feet 

per second ...................................................................................................................................12

15. Graphs showing velocity profiles measured by SonTek-IQ Plus using 

firmware 1.60 and FlowTracker for average stream-wise velocities of 0.7, 1.6, 

and 2.7 feet per second .............................................................................................................13

16. Graphs showing percent velocity difference between the SonTek-IQ Plus using 

firmware 1.60 and the FlowTracker for velocity profiles measured for average 

stream-wise velocities of 0.7, 1.6, and 2.7 feet per second plotted with the 

IQ Plus accuracy.........................................................................................................................13

17. Graph showing percent difference between SonTek-IQ Plus using firmware 1.60 

and FlowTracker mean-profile velocity at various mean-flow velocities from 0.7 to 

about 2.7 feet per second..........................................................................................................14

18. Screen shot of SonTek-IQ Plus software showing the setup menu...................................15

19. Screen shot of tilt warning given when SonTek-IQ Plus is installed with a tilt 

greater than 5 degrees ..............................................................................................................15

Tables

1. Selected manufacturer’s specifications for the SonTek-IQ Plus..........................................3

2. Discharge measured with SonTek-IQ Plus with firmware 1.52 using SonTek’s 

“theoretical” computation method compared with traditional discharge 

measurement made with FlowTracker using the midsection computation method .........7

3. Mean-profile velocity measured with SonTek-IQ Plus using firmware 1.52 and 

with FlowTracker.........................................................................................................................11

4. Mean-profile velocity measured with SonTek-IQ Plus using firmware 1.60 and 

with FlowTracker.........................................................................................................................12

5. Mean-profile velocity measured with SonTek-IQ Plus using firmware 1.60 and 

with FlowTracker.........................................................................................................................14
v

Conversion Factors

Inch/Pound to SI

Multiply By To obtain

Length

inches (in.) 2.54 centimeter (cm)

foot (ft) 0.3048 meter (m)

Area

square foot (ft2

) 929.0 square centimeter (cm2)

square foot (ft2

) 0.09290 square meter (m2)

Volume

cubic foot (ft3

) 0.02832 cubic meter (m3) 

Flow rate

foot per second (ft/s) 0.3048 meter per second (m/s)

cubic foot per second (ft3

/s) 0.02832 cubic meter per second (m3/s)

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows:

°F=(1.8×°C)+32

Hydraulic Laboratory Testing of SonTek-IQ Plus

By Janice M. Fulford and Scott Kimball

Abstract

The SonTek-IQ Plus (IQ Plus) is a bottom-mounted 

Doppler instrument used for the measurement of water depth 

and velocity. Evaluation testing of the IQ Plus was performed 

to assess the accuracy of water depth, discharge, and velocity 

measurements. The IQ Plus met the manufacturer’s specifications and the U.S. Geological Survey (USGS) standard for 

depth accuracy measurement when the unit was installed, 

according to the manufacturer’s instructions, at 0 degrees 

pitch and roll. However, because of the limited depth testing conducted, the depth measurement is not recommended 

as a primary stage measurement. The IQ Plus was tested in a 

large indoor tilting flume in a 5-foot (ft) wide, approximately 

2.3-ft deep section with mean velocities of 0.5, 1, 2, and 

3 ft per second. Four IQ Plus instruments using firmware 1.52 

tested for water-discharge accuracy using SonTek’s “theoretical” discharge method had a negative bias of -2.4 to -11.6 percent

when compared with discharge measured with a SonTek 

FlowTracker and the midsection discharge method. The IQ 

Pluses with firmware 1.52 did not meet the manufacturer’s 

specification of +/-1 percent for measuring velocity. Three 

IQ Pluses using firmware 1.60 and SonTek’s “theoretical” 

method had a difference of -1.6 to -7.9 percent when compared with discharge measured with a SonTek FlowTracker 

and the midsection method. Mean-velocity measurements 

with firmware 1.60 met the manufacturer’s specification and 

Price Type AA meter accuracy requirements when compared 

with FlowTracker measurements. Because of the instrument’s 

velocity accuracy, the SonTek-IQ Plus with firmware 1.60 is 

considered acceptable for use as an index velocity instrument 

for the USGS. The discharge computed by the SonTek-IQ Plus

during the tests had a substantial negative bias and will not be 

as accurate as a discharge computed with the index velocity 

method. The USGS does not recommend the use of undocumented computation methods, such as SonTek’s “theoretical” 

method for computing discharge.

Introduction

The U.S. Geological Survey (USGS) Hydrologic 

Instrumentation Facility (HIF) evaluates the performance of 

instruments and equipment that are used to directly measure 

hydrologic data. Instrument and equipment evaluations are 

conducted primarily to determine if particular sensors would 

be suitable for use by USGS personnel for hydrologic data 

collection. Sensors are evaluated against the manufacturer’s 

stated specifications for accuracy and resolution and any relevant USGS accuracy requirements.

The USGS uses several types of in-situ, fixed-mounted 

acoustic Doppler velocity meters (ADVMs) for measurement 

of water velocities. Fixed mounted ADVMs are mounted 

either on the side or bottom of the stream or canal. These 

instruments are used to measure a representative stream 

velocity that is used by the index velocity method (Levesque 

and Oberg, 2012) to compute discharge. This report documents testing of SonTek-IQ Plus (IQ Plus) by the USGS HIF 

for compliance with the manufacturer’s depth, velocity, and 

discharge accuracy specifications and with the Serial Digital 

Interface at 1200 baud (SDI-12) communication standard 

(SDI-12 Support Group, 2013). Initial discharge and velocity testing used SonTek-IQ Plus firmware 1.52. After discharge and velocity testing was completed, firmware 1.60 

was released in June 2013 and testing was repeated with 

firmware 1.60. Results for both discharge and velocity tests 

are presented in the report. The results are applicable only to 

the instrument versions tested and may or may not be representative of the results obtained with future versions. 

Description of Instrument

The SonTek-IQ Plus (fig. 1) is designed for use as a lowprofile, bottom-mounted velocity and water-discharge measuring system. The instrument was designed in collaboration with 

California Polytechnic State University and the University of 

Illinois to compute discharge in irrigation canals (Cook and 

others, 2012). Using the velocities and depths measured by the 

instrument and a known cross section input by the user, the IQ 

Plus software can calculate discharge using either SonTek’s 

“theoretical” model of the velocity distribution or the index 

velocity method preferred by the USGS (Levesque and Oberg, 

2012). The USGS does not recommend the use of undocumented computation methods, such as SonTek’s “theoretical” 
2 Hydraulic Laboratory Testing of SonTek-IQ Plus

method for computing discharge. The IQ Plus measures water 

depth using two sensors, an upward-looking acoustic beam 

and a nonvented pressure sensor. The nonvented pressure 

sensor is used to measure depth when no data are available 

from the vertical acoustic beam. Because the pressure sensor 

is not vented to the atmosphere, it will need to be adjusted for 

atmospheric pressure at periodic intervals by the user. Alternatively, the instrument can be configured to use the acoustic 

vertical beam to perform regular, automatic calibrations of the 

pressure sensor. 

The IQ Plus uses monostatic, acoustic transducers with 

the Doppler shift principal to measure water velocity. Doppler 

shift occurs when an acoustic signal reflecting off particles 

moving with the water shifts the frequency of the transmitted 

signal. The reflected signal is shifted by a frequency that is 

proportional to the speed of the acoustic scatters in the water. 

The frequency is shifted higher for particles moving towards 

the transducer and lower for particles moving away from the 

transducer. Because the Doppler shift is also a function of the 

sound speed of the water, knowledge of water temperature 

and salinity are important to obtain accurate velocity measurements. A detailed explanation of acoustic Doppler velocity 

profile measurement principals is presented by Mueller and 

others (2013). 

The IQ Plus measures velocity with four acoustic transducers with profiling beams that are angled upstream, downstream, and at right and left skew angles. The upstream- and 

downstream-facing beams are oriented along the stream axis 

and are angled 25 degrees from the vertical. The skew beams 

are oriented 60 degrees from the center-stream axis and are 

angled 60 degrees from the vertical. For each pair of profiling 

beams, the sensor automatically selects between three types 

of acoustic pulses or pings (coherent, incoherent, and broadband), depending on the flow conditions (Xylem, 2012). The 

automatic selection of the acoustic pulsing scheme is marketed 

as “SmartPulseHD” in the manufacturer’s sales brochures 

(SonTek, 2011). The IQ Plus manual does not have any option 

to turn off the “SmartPulseHD” or to change 

acoustic operation modes.

The velocity is profiled by the 

IQ Plus over the flow depth in multiple 

depth cells by measuring the Doppler 

shift of the transmitted acoustic signal 

in each depth cell. The velocity profiling 

depth is dynamically adjusted based on 

the water depth measured by the vertical 

acoustic beam. The profiling range is also 

automatically limited to the cell farthest 

from the transducer with an acceptable 

signal-to-noise ratio (SNR) if the next 

farthest cell has a SNR that is too low for 

a reliable velocity measurement. The limit 

of the profiling range is recorded for each 

sample. The velocity measurement is automatically compensated for water-density 

changes due to temperature in fresh water. 

Water salinity must be defined by the user. Selected specifications from the manufacturer’s literature are listed in table 1 

(Xylem Incorporated, 2012). 

Test Procedures

The IQ Plus was tested for the ability to accurately 

measure stage, discharge, velocity, and for compliance with 

version SDI-12 1.3 (SDI-12 Support Group, 2013). Four different instruments were tested—one instrument, loaned by the 

vendor, and three subsequentially purchased instruments. All 

instruments were configured to use the default units of feet 

(ft). SonTek’s “theoretical” discharge measurement method 

was tested to access the accuracy of the method. This method 

is not used by the USGS because it is not well documented 

and its accuracy is unknown. Because the index velocity 

discharge method is well documented and relies on the mean 

velocity, it was not tested.

Depth test

The depth accuracy test was performed in the jet tank 

at the USGS HIF Hydraulics Laboratory to test the effect of 

pitch-and-roll angles on the depth measurement. This facility 

has a mechanical arm that can be accurately positioned (raised 

and lowered) to depths ranging from 0 to 7 ft with a measurement resolution of 0.01 ft. A custom angle mount (fig. 2) 

was made for the IQ Plus that allowed the device to be tilted 

forward and backward (pitch) or side to side (roll). The angle 

mount was used to test the IQ Plus’ ability to measure depth 

correctly for various pitch-and-roll angles. The depth accuracy test did not test either the pressure sensor or the vertical 

acoustic depth sensor over their entire operating temperature 

and depth range. 

Depth testing was initially conducted using the 

loaned instrument (serial number (sn) IQ1151134) using 

Figure 1. Photo of SonTek-IQ Plus on office table without power and 

communication cable.
Test Procedures 3

firmware 1.52. Readings were recorded at pitch angles of -20, 

-10, -5, 0, 5, +10, and 20 degrees for 0-degree roll angle and 

at roll angles of -20, -10, -5, 0, 5, +10, and 20 degrees for 

0-degree pitch angle at depth intervals of 1 ft from 1 to 7 ft. 

Further testing was performed on three additional instruments 

(sns IQ1211002, IQ1211003, and IQ1211004) using firmware 

1.60 at level and at pitch angles of 0, 5, and -5 degrees, and at 

roll angles of 0, 5, and -5 degrees, at intervals of 1 ft from 1 to 

7 ft in depth. IQ Plus software instrument “standard settings” 

for depth testing were set to 0 parts per thousand (ppt) water 

salinity, 10-second (s) sample duration, 10-s sample interval, 

and profile recording.

The angle mount was positioned to a test angle prior to 

attaching the IQ Plus. The IQ Plus was attached to the mount 

by using the mounting locations provided on the IQ Plus. A 

Pro 360 Digital Protractor (Mitutoyo Inc., 2003) with a stated 

accuracy of ±0.1 degree was used to set the mount’s pitch 

or roll angle and to verify the final pitch or roll angle of the 

mounted instrument. The protractor reading was compared 

to the pitch-and-roll angle given by the IQ Plus. For 0-degree 

angles of pitch and roll, the IQ Plus was leveled in the mount 

and then lowered until it reported a depth of 1 ft. An engineer’s

measuring tape was then used to verify that 1 ft of water was 

over the transducer. For all other angles of pitch and roll, getting an accurate tape down was impractical and the unit was 

lowered until the IQ Plus gave a reading of 1 ft. At the initial 

1-ft depth, measurements were recorded at 10-s intervals for 

about 2.5 minutes or until 15 readings were obtained. The 

sensor continued to record measurements as it was lowered 

another foot to the next depth. Fifteen more readings were 

then recorded at the next depth. This process was repeated 

until the maximum depth of 7 ft was reached. The procedure 

was then repeated as the unit was raised back to the surface.

Discharge and velocity tests

The discharge and velocity accuracy tests were performed 

in the tilting flume at the USGS Hydraulics Laboratory at the 

HIF. The tilting flume is 6 ft wide, 3 ft deep, and 250 ft long 

and can be tilted to slopes of ≤1 percent. The discharge capacity is a maximum of 50 cubic feet per second (ft3

/s), partially 

or wholly supplied by the laboratory’s constant head tank. 

All testing was performed with the flume in a level position. 

The loaned sensor (sn IQ1151134) was the first sensor tested. 

Table 1. Selected manufacturer’s specifications for the 

SonTek-IQ Plus. 

Feature Specification

Velocity Measurement

Sampling range 0.3 to16 feet

Number of cells Up to 100

Cell size 0.8 to 4 inches

Range 16 feet per second

Resolution 0.0003 feet per second

Accuracy 1 percent of reading, 0.0167 feet 

per second

Water Level

Vertical acoustic beam range 0.2 to 16 feet

Vertical acoustic beam accuracy 0.1 percent of reading or 0.01 

feet, whichever is greater

Pressure sensor range 98 feet of water, 42 pounds per 

square inch

Pressure sensor accuracy 0.1 percent full scale

Acoustics

Frequency 3.0 megahertz

Blanking distance from transducers0.8 inches

Beam width 1.4 degrees

Temperature

Operating/storage -5 to 60 degrees Celsius 

Sensor accuracy ± 0.2 degrees Celsius

Sensor/resolution ± 0.01 degrees Celsius

Other

Tilt sensor accuracy ± 1.0 degrees

Communications RS232, SDI-12, Modbus, Analog (via optional flow display)

Data storage 4 gigabytes

Power input 8–15 voltage direct current

Power consumption 0.5–1.0 watts (0.02 watts 

when idle)

Figure 2. Custom angle mount on instrument arm with SonTek-IQ 

Plus installed on mount.
4 Hydraulic Laboratory Testing of SonTek-IQ Plus

It was mounted to an aluminum plate on the bottom of the 

channel (fig. 3) in a 5-ft-wide, contracted section of the tilting 

flume. Subsequently, three additional sensors (sns IQ1211002, 

IQ1211003, and IQ1211004) were tested as a group in the 

same section of the tilting flume. The three sensors were 

mounted 8 inches apart (fig. 3) and were operated one at a 

time to prevent them from acoustically interfering with each 

other’s measurement. The three sensors were initially tested 

using SonTek-IQ Plus firmware 1.52 and subsequently retested 

using firmware 1.60. Regardless of the firmware version, 

IQ Plus software instrument “standard settings” for discharge 

and velocity testing were set to 0-ppt water salinity, 10-s 

sample duration, 10-s sample interval, and profile recording. 

The IQ Plus software was configured to compute the discharge 

from its velocity measurements by entering the flume crosssection dimensions into the Smart Page channel shape section 

and using the defaults in the Flow Page section: equation type 

of “Theoretical,” velocity type of “Main Vx,” velocity offset 

of 0.00, velocity coefficient of 1.00, and stage coefficient of 

0.00. The software defaults for the “total volume” settings 

were used. 

IQ Plus equation type indicates the method used to determine discharge. However, discharge computation methods are 

only vaguely described in the IQ Plus manual. According to 

the manual, the “Theoretical” equation type uses a power-law 

velocity profile model with a 1/6 exponent, the user-entered 

channel geometry data, and the sensor’s measured velocities 

and depths to compute discharge. 

Four flume discharges were used to compare the 

IQ Plus with discharges and velocities measured by a SonTek 

FlowTracker (sn P1515). For the initial test with the loaner 

instrument, tested discharge regimes had mean velocities of 

0.5, 1.1, and 2.0 ft per second (ft/s) and flow depths of 2.24, 

2.48, and 2.16 ft. For the three sensors tested as a group, the 

tested discharge regimes had target mean velocities of 0.5, 1.5 

and 3.0 ft/s and a target flow depth of 2.25 ft. Discharge and 

velocity profiles were measured for each flume discharge. 

Reference discharges and velocities were measured 

with the SonTek FlowTracker. These measurements were 

made with FlowTracker firmware version 3.7 and software 

version 2.30. SonTek FlowTrackers have a manufacturer’s 

specified velocity accuracy of ±1 percent of measured velocity plus ±0.25 centimeter per second (SonTek/YSI, 2007). 

The FlowTracker measurement cross section was located 2 ft 

upstream of the SonTek-IQ Plus instruments. The SonTek 

FlowTracker discharge was computed from velocities measured at the 0.2 and 0.8 depth sampling locations at 0.25-ft 

intervals and 0.5-ft intervals across the flume width using 

the midsection method (Turnipseed and Sauer, 2010) for the 

initial test and for the group tests, respectively. A FlowTracker 

discharge measurement was made before and after the measurement of the velocity profile with the FlowTracker and the 

discharge measurement by the IQ Plus. The two FlowTracker 

discharge measurements were averaged for comparison with 

the discharge calculated by the IQ Plus.

The FlowTracker was also used to measure reference 

velocity profiles in front of each IQ Plus. At each observation 

depth, 40 1-second velocities were made by the FlowTracker 

and averaged. Velocities were measured at nine depths, equally 

spaced over the total flow depth and 2 ft in front of each IQ 

Plus. The velocity profile data collected by the IQ Plus for 

each depth cell included the “center” and “skew” cell velocities and cell depth, as well as information on the “center” and 

“skew” cell ping method, pulse length, and blanking distance. 

Because the IQ Plus measured 10-s samples, the FlowTracker 

profile velocities were compared to the time average of four 

IQ Plus “center” cell velocities measured at the corresponding 

FlowTracker measurement depth and sample time. 

Figure 3. SonTek-IQ Plus sensor (serial number (sn) IQ1151134) (left photo) mounted to Hydraulic Laboratory tilting flume bottom and 

three SonTek-IQ Plus sensors (sns IQ1211002, IQ1211003, and IQ1211004) (right photo) mounted side by side on Hydraulic Laboratory 

tilting flume bottom.
Test Results 5

SDI-12 compliance testing

The SDI-12 verification test was performed using an NR 

Systems SDI-12 verifier and running the standard SDI-12 

verifier test for version 1.3 (SDI-12 SupportGroup, 2013). The 

USGS commonly deploys sensors that use SDI-12 communication protocol because of the protocol’s low power consumption and standard commands. The NR Systems verifier tests 

for compliance to timing and commands used by the standard 

and does not test for electrical compliance to the standard. No 

testing for electrical compliance to the standard was done.

Test Results

Test results are presented in tables and charts for depth 

testing, discharge, and velocity testing with firmware 1.52, discharge and velocity testing for firmware 1.60, and for SDI-12 

testing. Values measured by the IQ Plus instruments and the 

reference instruments and percent differences between the 

IQ Plus and references are plotted for the depth, discharge, and 

velocity testing. Some plots include either the manufacturer’s 

accuracy specifications or USGS criteria. 

Depth testing

 Figures 4, 5, 6, and 7 show the absolute differences 

plus 0.0001 between each IQ Plus and the laboratory stage 

reference for various pitch-and-roll angles over the measured 

depths. A value of 0.0001 was added to each absolute difference to allow plotting of all differences on the semilogarithm 

plot. Also plotted in figures 4, 5, 6, and 7 is the manufacturer’s 

accuracy to a resolution of 0.01 ft. Depth differences are plotted in figures 4 through 6 for pitch-and-roll angles between 

±5 degrees for sns IQ1211002, IQ1211003, and IQ1211004. 

Figure 7 has depth differences plotted for pitch-and-roll angles 

between ±10 degrees for sn IQ1151134. Depths were grossly 

in error for angles exceeding ±10 degrees. The IQ Plus depth 

readings were within the ±0.01 ft of the laboratory depth reference for pitch-and-roll angles of 0 degrees. All depth measurements for pitch-and-roll angles of 0 degrees met the manufacturer’s accuracy specification and USGS requirements for 

stage measurements for the 0-to-7-ft range tested. Sensitivity 

to pitch-and-roll angles varied across the instruments tested. 

One instrument, sn IQ1211002, was more frequently outside 

±0.01 ft of the laboratory depth reference for pitch-and-roll 

angles of 5 degrees than the other tested sensors. The USGS 

requirements state that stage (or depth) measurements are ±0.2 

percent of reading or 0.01 ft, whichever is larger (U.S. Geological Survey, 1996). When the instrument was tilted more 

than 5 degrees during the depth tests, warnings were issued by 

the IQ Plus software. Depth values that were returned when 

the instruments were tilted more than 5 degrees were either 

“0” or very high and obviously erroneous.

The IQ Plus depth measurement may be adequate as a 

primary stage measurement at a USGS continuous-discharge 

site, providing that the water is well mixed and does not 

experience density stratification or changing water density 

due to changing salinity or sediment. However, because of 

0.0001

0.0010

0.0100

0.1000

1.0000

10.0000

100.0000

0 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00

Difference between IQ Plus (SN IQ1211002)

and reference depth, in feet 

Depth below water surface in feet 

0 degree pitch and roll

+5 roll

+5 pitch

Manufacturer accuracy

–5 roll

–5 pitch

EXPLANATION

Figure 4. Difference between depth readings measured by SonTek-IQ Plus (serial number 

IQ1211002) and laboratory reference for 0 and ±5 degrees pitch-and-roll angles for depth below 

water surface from 0 to 7 feet.
6 Hydraulic Laboratory Testing of SonTek-IQ Plus

0.0001

0.0010

0.0100

0.1000

1.0000

10.0000

100.0000

0 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00

Difference between IQ Plus (SN IQ1211003)

and reference depth, in feet 

Depth below water surface in feet 

0 degree pitch and roll

+5 roll

+5 pitch

Manufacturer accuracy

–5 roll

–5 pitch

EXPLANATION

Figure 5. Difference between depth readings measured by SonTek-IQ Plus (serial number 

IQ1211003) and laboratory reference for 0 and ±5 degrees pitch-and-roll angles for depth below 

water surface from 0 to 7 feet.

0.0001

0.0010

0.0100

0.1000

1.0000

10.0000

100.0000

0 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00

Difference between IQ Plus (SN IQ1211004)

and reference depth, in feet 

Depth below water surface in feet 

0 degree pitch and roll

+5 roll

+5 pitch

Manufacturer accuracy

–5 roll

–5 pitch

EXPLANATION

Figure 6. Difference between depth readings measured by SonTek-IQ Plus (serial number 

IQ1211004) and laboratory reference for 0 and ±5 degrees pitch-and-roll angles for depth below 

water surface from 0 to 7 feet.
Test Results 7

the limited depth testing conducted, the depth measurement 

is not recommended as a primary stage measurement unless 

the accuracy of the depth measurement is confirmed over 

the expected range of conditions at the deployment site with 

another sensor meeting USGS accuracy requirements. 

Discharge and Velocity Measurements with 

Firmware 1.52

The discharge-measurement comparisons are shown in 

table 2 for the loaner instrument (sn IQ11151134) and for the 

three side-by-side instruments (sns IQ1211002, IQ1211003, 

and IQ1211004) using firmware 1.52 and the “theoretical” 

discharge method. The expanded uncertainty of discharge 

measurements by the FlowTracker was estimated based 

on estimated uncertainties for the steadiness of the flow 

(±0.5 percent flume discharge), manufacturer’s stated accuracy (±1 percent), and number of velocity measurements 

made throughout the flow area (±0.64 percent) as ±1.9 percent 

( 22 2 1.9 2 0.5 0.5 0.64 =× + + ), using a simplified analysis based on the Guide to the Expression of Uncertainty in 

Measurement (International Standards Organization, 1993). 

The standard uncertainty due to the number of velocity measurements in the flow area was estimated by computing the 

average difference between the total discharge computed using 

all and half (every other subarea measurement) the subarea 

Table 2. Discharge measured with SonTek-IQ Plus with 

firmware 1.52 using SonTek’s “theoretical” computation method 

compared with traditional discharge measurement made with 

FlowTracker using the midsection computation method. Percent 

differences are computed as 100 x (IQ Plus – FlowTracker)/

FlowTracker.

[ft3/s, cubic feet per second]

IQ Plus FlowTracker

Percent 

Serial number difference Discharge, 

in ft3

/s

Discharge in 

ft3/s

IQ11151134 6.37 6.82 -6.6

12.9 13.4 -3.7

20.2 21.7 -6.9

IQ1211002 7.21 7.68 -6.1

14.5 16.4 -11.6

36.2 37.6 -3.7

IQ1211003 7.30 7.68 -4.9

16.0 16.4 -2.4

34.5 37.6 -8.2

IQ1211004 7.18 7.68 -6.5

15.9 16.4 -3.0

33.6 37.6 -10.6

Figure 7. Difference between depth readings measured by SonTek-IQ Plus (serial number 

IQ1151134) and laboratory reference for 0 and ±10 degrees pitch-and-roll angles for depth below 

water surface from 0 to 7 feet.

0.0001

0.0010

0.0100

0.1000

1.0000

10.0000

100.0000

0 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00

Difference between IQ Plus (SN IQ1151134)

and reference depth, in feet 

Depth below water surface in feet 

0 degree pitch and roll

+5 roll

+5 pitch

Manufacturer accuracy

–5 roll

–5 pitch

–10 pitch

+10 roll

+10 pitch

–10 roll

EXPLANATION
8 Hydraulic Laboratory Testing of SonTek-IQ Plus

velocity and depth measurements for discharge measurements 

made at mean velocities of 1 and 2 ft/s. The manufacturer’s 

accuracy was also assumed to be equal to the expanded 

uncertainty (twice the standard uncertainty) of the instrument’s velocity measurement. The percent differences shown 

in table 2 were computed as 100 x(IQ Plus - FlowTracker)/

FlowTracker. All IQ Plus instruments tested under-registered 

discharge when compared to the FlowTracker. The discharge 

bias ranged from -2.4 to -11.6 percent and averaged -6.2 percent

for the test discharges. As the mean velocity increased to 3 ft/s,

the IQ Plus began to return “#N/A” values for velocity in several cell locations, even though good SNR numbers (generally 

in the 50-dB range, much larger than the minimum required 

3 decibels) were measured. The “#N/A” values for velocity 

required the IQ Plus software to interpolate more of the velocity data in the vertical, possibly contributing to some of the 

discharge bias. There does not appear to be a trend to the bias 

with increasing or decreasing discharge (fig. 8). 

Figure 8 also plots the IQ Plus “theoretical” discharges 

according to USGS criteria and includes vertical error bars of 

±1.9%, the estimated uncertainty of the FlowTracker discharge 

measurement. The USGS rates discharge measurements as 

excellent (≤2% error), good (>2% and ≤5% error), fair (>5% 

and ≤8% error), or poor (>8% error) (Nolan and Shields, 

2000). IQ Plus measurements were rated based on the difference between the discharges measured by the FlowTracker 

and IQ Plus. None of the IQ Plus measured discharges using 

the “theoretical” method were rated excellent, in spite of the 

excellent measurement section and steadiness of the flow. 

Most IQ Plus discharge measurements were rated as either 

good (5) or fair (4). Several were rated as poor (3). The IQ Plus

“theoretical” discharge measurements with firmware 1.52 have 

a negative bias compared to the reference FlowTracker measurement and none of the tested instruments using the “theoretical” method were within the manufacturer’s suggested 

accuracy of 3 to 5 percent (Xylem, 2012, p. 97).

The velocity profiles measured by the IQ Plus instruments and the FlowTracker are shown in figures 9 and 10. 

In these figures, the center-cell velocity measurements of the 

IQ Plus instruments are plotted with the nearest FlowTracker 

measurements. Velocity profiles measured by the first instrument tested are in figure 9 and velocity profiles for the 

instruments tested as a group are in figure 10. Except at a few 

locations, the velocity profiles measured by the IQ Plus instruments generally underregistered velocity when compared to 

the FlowTracker. Horizontal error bars in figure 9 represent the 

manufacturer’s stated accuracy for the instrument. 

The percent velocity differences between the velocity 

measured by the IQ Plus and the FlowTracker are plotted in 

figures 11 and 12, with the IQ Plus accuracy specification. The 

percent velocity differences were computed as 100 x (IQ Plus –

FlowTracker)/FlowTracker. The percent velocity differences 

for the first instrument tested are in figure 11. The red error 

bars represent the accuracy of the reference velocity measurement (FlowTracker). The percent velocity differences for the 

instruments tested in a group are in figure 12. The general negative bias of the velocity is clearly seen in all of these plots and 

shows that the velocity measured by the IQ Plus frequently 

does not meet the manufacturer’s accuracy specification.

–15

–13

–11

–9

–7

–5

–3

–1

0 10 20 30 40

Percent difference between IQ Plus and flow

tracker discharge, in cubic feet per second 

Discharge, in cubic feet per second

IQ1151134

IQ1211002

IQ1211003

IQ1211004

Poor

Fair

Good

Excellent

EXPLANATION

Figure 8. Percent difference in discharge between SonTek-IQ Plus using firmware 1.52 and 

SonTek’s “theoretical” computation method and FlowTracker using midsection computation 

method for discharges from approximately 7 to 36 cubic feet per second.
Test Results 9

0

0.5

1.0

1.5

2.0

2.5

0.4 0.5 0.6 0.7

Distance from bottom, in feet

0

0.5

1.0

1.5

2.0

2.5

0.8 1 1.2 1.4

Distance from bottom, in feet

Velocity, in feet per second Velocity, in feet per second Velocity, in feet per second

FlowTracker

IQ Plus (IQ1151134)

0

0.5

1.0

1.5

2.0

2.5

1.0 1.5 2.0 2.5 3.0

Distance from bottom, in feet

EXPLANATION

Figure 9. Velocity profiles measured by SonTek-IQ Plus (serial number IQ1151134) and FlowTracker for average 

stream-wise velocities of 0.5, 1.1, and 2.0 feet per second (as measured by the FlowTracker).

0

0.5

1.0

1.5

2.0

2.5

0.5 0.6 0.7 0.8

Distance from bottom, in feet

0

0.5

1.0

1.5

2.0

2.5

1.0 1.2 1.4 1.6

Distance from bottom, in feet

Velocity, in feet per second Velocity, in feet per second Velocity, in feet per second

0

0.5

1.0

1.5

2.0

2.5

2.0 2.5 3.0 3.5 4.0

Distance from bottom, in feet

EXPLANATION

IQ Plus (IQ1211004)

IQ Plus (IQ1211003)

IQ Plus (IQ1211002)

FlowTracker East

FlowTracker Center

FlowTracker West

Figure 10. Velocity profiles measured by SonTek-IQ plus using firmware 1.52 and FlowTracker for average 

stream-wise velocities of 0.7, 1.4, and 3.2 feet per second (as measured by the FlowTracker).
10 Hydraulic Laboratory Testing of SonTek-IQ Plus

0

0.5

1.0

1.5

2.0

2.5

–40 –20–30 0–10 2010 –40 –20–30 0–10 2010 –40 –20–30 0–10 2010

Distance from bottom, in feet

0

0.5

1.0

1.5

2.0

2.5

Distance from bottom, in feet

Difference between IQ Plus and

FlowTracker, in percent

Difference between IQ Plus and

FlowTracker, in percent

Difference between IQ Plus and

FlowTracker, in percent

Accuracy

IQ Plus (IQ1151134)

0

0.5

1.0

1.5

2.0

2.5

Distance from bottom, in feet

EXPLANATION

Figure 11. Percent velocity difference between the SonTek-IQ Plus using firmware 1.52 and the 

FlowTracker for velocity profiles measured for average stream-wise velocities of 0.5, 1.1, and 2.0 feet per 

second (as measured by the FlowTracker) plotted with the IQ Plus accuracy (red lines).

0

0.5

1.0

1.5

2.0

2.5

–40 –20–30 0–10 2010 –40 –20–30 0–10 2010 –40 –20–30 0–10 2010

Distance from bottom, in feet

0

0.5

1.0

1.5

2.0

2.5

Distance from bottom, in feet

Difference between IQ Plus and

FlowTracker, in percent

Difference between IQ Plus and

FlowTracker, in percent

Difference between IQ Plus and

FlowTracker, in percent

Accuracy

IQ Plus (IQ1211004)

IQ Plus (IQ1211003)

IQ Plus (IQ1211002)

0

0.5

1.0

1.5

2.0

2.5

Distance from bottom, in feet

EXPLANATION

Figure 12. Percent velocity difference between the SonTek-IQ Plus using firmware 1.52 and the 

FlowTracker for velocity profiles measured for average stream-wise velocities of 0.7, 1.4, and 3.2 feet per 

second (as measured by the FlowTracker) plotted with the IQ Plus accuracy (red lines).
Test Results 11

Table 3 lists the mean-profile velocity measured by each 

IQ Plus and the FlowTracker, along with the computed percent 

mean-velocity difference. The mean-profile velocity at a 

discharge was computed by averaging all the individual cell 

velocities over the flow depth. Figure 13 is a plot of percent 

difference between IQ Plus and FlowTracker mean-profile 

velocities over the range of mean-flume velocities tested. The 

manufacturer’s and Price Type AA USGS accuracy criteria 

(Hubbard and others, 1999) are also plotted in figure 13. The 

difference was computed as 100 x (IQ Plus – FlowTracker)/

FlowTracker. The differences are between -5 and -12 percent 

and vary more by sn than by velocity. 

The IQ Plus differences are larger than those found by 

a test of a standard IQ by the Bureau of Reclamation (BOR) 

(Heiner and Vermeyen, 2012). The BOR test had positive 

bias for mean velocities less than 1 ft/s and negative bias for 

mean velocities greater than 1 ft/s. The bias for the BOR test 

was 12.9 percent at a mean velocity of 0.56 ft/s, -1.2 percent 

at 1.12 ft/s, and -6.1 percent at 1.52 ft/s. The BOR test was in 

a slightly smaller flume (4 ft wide instead of 5 ft), tested only 

one IQ instrument, and had a larger range of flow depths over 

similar mean velocities—2.24, 3.34, and 4.11 ft—in comparison to depths of 2.24, 2.28, and 2.26 ft. The IQ is similar to the 

IQ Plus, except it does not measure velocity profiles, lacks the 

capability to compute discharges in a “natural” channel section, and can be used only in water depths up to 5 ft.

Discharge and Velocity Measurements with 

Firmware 1.60

Discharge measurement comparisons are shown in table 

4 for the three instruments tested side by side (sns IQ1211002, 

IQ1211003, and IQ1211004) using firmware 1.60 and SonTek’s

“theoretical” discharge method. As in the previous test, the 

FlowTracker was used as the reference measurement and the 

uncertainty of the reference measurement was estimated as 

1.9 percent. The percent differences shown in table 4 were 

Table 3. Mean-profile velocity measured with SonTek-IQ Plus 

using firmware 1.52 and with FlowTracker. Percent differences 

are computed as 100 x (IQ Plus – FlowTracker)/FlowTracker.

[ft/s, feet per second]

IQ Plus FlowTracker

Percent 

Serial number difference Mean velocity, 

in ft/s

Mean velocity, 

in ft/s

IQ11151134 0.560 0.635 -11.8

1.01 1.13 -10.5

1.76 1.99 -10.8

IQ1211002 0.612 0.686 -10.9

1.28 1.43 -10.3

2.98 3.32 -11.8

IQ1211003 0.631 0.676 -6.57

1.36 1.44 -5.53

2.97 3.14 -5.41

IQ1211004 0.625 0.681 -8.23

1.30 1.43 -9.52

2.92 3.15 -7.50

EXPLANATION

–15

–10

–5

0

5

10

0 0.5 1 1.5 2 2.5 3 3.5

Percent difference between IQ Plus 

and FlowTracker mean velocity, 

in feet per second

Mean velocity, in feet per second

IQ1151134 IQ1211002

IQ1211003 IQ1211004

AA Price accuracy Manufacturer accuracy

Figure 13. Percent difference between SonTek-IQ Plus using firmware 1.52 and FlowTracker 

mean-profile velocity at various mean-flow velocities from 0.5 to about 3.25 feet per second. 

Vertical error bars represent the uncertainty of the FlowTracker velocity.
12 Hydraulic Laboratory Testing of SonTek-IQ Plus

computed as for table 2. The discharge bias was much smaller 

than the previous test and ranged from -1.6 to -7.3 percent 

and averaged -4.8 percent for the test discharges. For the two 

lowest test velocities, the IQ Plus measured in 30 to 33 cells. 

At the highest test velocity, the IQ Plus usually measured 

using 9 cells and pulse-incoherent-type pings, but periodically 

switched to 30 cells and pulse-coherent-type pings. All results 

at the highest test discharge are for data with nine cells. There 

is not an obvious trend to the differences with increasing 

discharge (fig. 14). Figure 14 shows plots of percent difference 

between the IQ Plus and FlowTracker discharges, with vertical 

error bars of ±1.9 percent, the estimated uncertainty of the 

FlowTracker discharge measurement. The plot also includes 

the USGS subjective discharge rating criteria. All measured 

discharges by sn IQ1211002 using firmware 1.60 were rated 

excellent. All other discharge measurements were rated as 

either good (1) or fair (4). In contrast with the earlier test, none 

were rated as poor. It was anticipated that the IQ Plus “theoretical” discharge measurements would all be excellent in the 

controlled flow and channel conditions in the laboratory flume. 

However, only one IQ Plus of the three tested with firmware 1.60

measured discharges consistently within the manufacturer’s 

suggested accuracy of 3 to 5 percent (Xylem, 2012, p. 97). 

Based on the three instruments tested, the IQ Plus “theoretical” discharge measurements with firmware 1.60 usually do 

not meet the manufacturer’s suggested discharge accuracy. 

Because the “theoretical” method used by the IQ Plus is not 

documented, the USGS does not recommend its use.

The velocity profiles measured by the IQ Plus instruments and the FlowTracker are shown in figure 15. As with 

the previous test, the center-cell velocity measurements of the 

IQ Plus instruments are plotted in figure 15 with the nearest 

FlowTracker measurements. The IQ Plus profile measurements (fig. 15) either straddle the FlowTracker profile measurements or slightly underregister the profile velocities.

The percent velocity differences between the velocity 

measured by the IQ Plus and the FlowTracker are plotted in figure 16 with the IQ Plus accuracy specification. 

The percent velocity differences were computed as 100 x 

(IQ Plus – FlowTracker)/FlowTracker). The error bars represent the accuracy of the reference velocity measurement 

Table 4. Mean-profile velocity measured with SonTek-IQ Plus 

using firmware 1.60 and with FlowTracker. Percent differences 

are computed as 100 x (IQ Plus – FlowTracker)/FlowTracker.

[ft3/s, cubic feet per second]

SonTek-IQ Plus FlowTracker

Percent 

Serial number difference Discharge, 

in ft3

/s

Discharge,

 in ft3/s

IQ1211002 7.85 8.00 -1.86

19.27 19.58 -1.58

35.52 36.11 -1.61

IQ1211003 7.74 8.00 -3.29

18.49 19.58 -5.56

33.52 36.11 -7.17

IQ1211004 7.37 8.00 -7.88

18.3 19.58 -6.54

33.47 36.11 -7.29

–12

–10

–8

–6

–4

–2

0

2

0 5 10 15 20 25 30 35 40

Percent difference between IQ Plus and flow

tracker discharge, in cubic feet per second 

Discharge, in cubic feet per second

IQ1211002

Poor

Fair

Good

Excellent

EXPLANATION

IQ1211003 IQ1211004

Figure 14. Percent difference in discharge between SonTek-IQ Plus using firmware 1.60 and 

SonTek’s “theoretical” computation method and FlowTracker using midsection method for 

discharges from approximately 7 to 36 cubic feet per second.
Test Results 13

0

0.5

1.0

1.5

2.0

2.5

0.5 0.6 0.7 0.8

Distance from bottom, in feet

Velocity, in feet per second

0

0.5

1.0

1.5

2.0

2.5

1.2 1.4 1.6 1.8

Distance from bottom in, feet

Velocity, in feet per second

IQ Plus (IQ1211004)

IQ Plus (IQ1211003)

IQ Plus (IQ1211002)

FlowTracker East

FlowTracker Center

FlowTracker West

0

0.5

1.0

1.5

2.0

2.5

2.0 2.5 3.0 3.5

Distance from bottom, in feet

Velocity, in feet per second

EXPLANATION

Figure 15. Velocity profiles measured by SonTek-IQ Plus using firmware 1.60 and FlowTracker for 

average stream-wise velocities of 0.7, 1.6, and 2.7 feet per second (as measured by the FlowTracker).

EXPLANATION

0

0.5

1.0

1.5

2.0

2.5

–20 –10 0 10 20

Distance from bottom in feet

Difference between IQ Plus

and FlowTracker in percent 

Difference between IQ Plus

and FlowTracker in percent 

Difference between IQ Plus

and FlowTracker in percent 

0

0.5

1.0

1.5

2.0

2.5

–20 –10 0 10 20

Distance from bottom in feet

Accuracy

IQ Plus (IQ1211004)

IQ Plus (IQ1211003)

IQ Plus (IQ1211002)

0

0.5

1.0

1.5

2.0

2.5

–20 –10 0 10 20

Distance from bottom in feet

Figure 16. Percent velocity difference between the SonTek-IQ Plus using firmware 1.60 and the 

FlowTracker for velocity profiles measured for average stream-wise velocities of 0.7, 1.6, and 2.7 feet per 

second (as measured by the FlowTracker) plotted with the IQ Plus accuracy (red lines).
14 Hydraulic Laboratory Testing of SonTek-IQ Plus

(FlowTracker). For a mean velocity of 1.4 ft/s, the IQ Plus 

cell velocities are generally within the manufacturer’s accuracy specification. For the other two mean velocities, 0.7 and 

3.2 ft/s, the velocities are usually within ±10 percent of the 

FlowTracker measurements.

Table 5 lists the mean-profile velocity measured by each 

IQ Plus and the FlowTracker, along with the computed percent 

mean-velocity difference. As in the previous test, the meanprofile velocity at a flume discharge was computed by averaging over depth all the individual profile velocities at a flume 

Table 5. Mean-profile velocity measured with SonTek-IQ Plus 

using firmware 1.60 and with FlowTracker. Percent differences 

are computed as 100 x (IQ Plus – FlowTracker)/FlowTracker.

[ft/s, feet per second]

SonTek-IQ Plus FlowTracker

Percent 

Serial number difference Mean velocity, 

in ft/s

Mean velocity, 

in ft/s

IQ1211002 0.690 0.715 -3.50

1.592 1.597 -0.26

2.721 2.79 -2.48

IQ1211003 0.709 0.724 -2.09

1.59 1.61 -1.24

2.722 2.786 -2.31

IQ1211004 0.734 0.735 -0.15

1.641 1.638 0.17

2.751 2.815 -2.29

discharge. Figure 17 is a plot of percent difference between IQ 

Plus and FlowTracker mean-profile velocities over the range 

of mean-flume velocities tested. The percent difference was 

computed as 100 x (IQ Plus – FlowTracker)/FlowTracker. The 

manufacturer’s and Price Type AA USGS accuracy criteria 

are also plotted in figure 17. For the flume velocities tested, 

the differences are between -2.50 and 0.17 percent. When the 

uncertainty (or accuracy) of the FlowTracker is included, all 

the mean velocities measured by the IQ Plus instruments are 

within the accuracy criteria of a Price Type AA meter and the 

manufacturer’s accuracy specification. Therefore, the IQ Plus 

with firmware 1.60 is considered to be an acceptable index 

velocity instrument for the USGS. Percent differences are 

computed as 100 x (IQ Plus – FlowTracker)/FlowTracker.

SDI-12 compliance

All four IQ Pluses were tested for SDI-12 compliance 

using the NR Systems SDI-12 verifier. Once all four IQ Pluses 

had their output type in the Real Time Data menu changed 

to SDI-12 output, they all passed the verifier test. The output 

must be changed to SDI-12 output or the IQ Plus will not 

function properly as an SDI-12 device.

Operating Observations

The IQ Plus software (fig. 18) was easy to use and made 

quick work of setting up the IQ Plus in the flume for testing. 

The software gave warnings 

during stage testing when the 

instrument was tilted beyond the 

5 degrees it needs to measure 

stage accurately (fig. 19). The 

software has many options for 

viewing and exporting the data. 

After collecting test data, it was 

relatively easy to export the data 

into tables for comparisons with 

the FlowTracker data. 

The instrument has a very 

low profile compared to the previous SonTek SW sensor and was 

easy to install for testing in the 

flume. Some provision will have 

to be made at field sites to protect 

the communication/power cable 

assembly. The manual gives good 

guidance on installing the IQ Plus 

and protecting the cable assembly. 

The manual is well written and 

easy to follow.

EXPLANATION

–8

–6

–4

–2

0

2

4

6

8

0 0.5 1 1.5 2 2.5 3 3.5

Percent difference between IQ Plus 

and FlowTracker mean velocity, 

in feet per second

Mean velocity, in feet per second

IQ1211002 IQ1211003

IQ1211004 AA Price accuracy

Manufacturer

accuracy

Figure 17. Percent difference between SonTek-IQ Plus using firmware 1.60 and FlowTracker 

mean-profile velocity at various mean-flow velocities from 0.7 to about 2.7 feet per second 

(ft/s). Vertical error bars represent the uncertainty of the FlowTracker velocity.
Summary 15

Figure 18. Screen shot of SonTek-IQ Plus software showing the setup menu.

Summary

Figure 19. Screen shot of tilt warning given 

when SonTek-IQ Plus is installed with a tilt 

greater than 5 degrees.

Evaluation testing of the SonTek-IQ Plus (IQ Plus) was 

performed to assess the accuracy of the instrument’s water 

depth, discharge, and velocity measurements. Four IQ Plus 

instruments were tested using firmware versions 1.52 and 

1.60, a loaner instrument, and three purchased instruments. 

The water-depth accuracy test was performed in the jet tank 

at the USGS HIF Hydraulics Laboratory to test the effect of 

pitch-and-roll angles on the depth measurement. Discharge 

and velocity accuracy tests were performed in a large indoor 

tilting flume in a 5-ft-wide, approximately 2.3-ft-deep section 

with mean velocities of 0.5, 1, 2, and 3 ft per second at the 

USGS Hydraulics Laboratory at the HIF.

The IQ Plus readings were within the ±0.01 ft of the laboratory depth reference for pitch-and-roll angles of 0 degrees 

in water depths up to 7 ft. The IQ Plus depth measurements 

met the USGS requirements and the manufacturer’s accuracy 

specification when the unit was installed, according to the 

manufacturer’s instructions with less than 5 degrees pitch and 

roll. However, because of the limited depth testing conducted, 

the IQ Plus depth measurement is not recommended as a primary stage measurement.

The differences between the IQ Plus “theoretical” 

discharge and the FlowTracker measured discharge ranged 

from -2.4 to -11.6 percent for instruments using firmware 

1.52. Instruments using firmware 1.60 had no discharge 

measurements that differed by more than 7.50 percent from 

the FlowTracker measurements. One instrument using firmware 1.60 had all discharge measurements within 2 percent 
16 Hydraulic Laboratory Testing of SonTek-IQ Plus

of the FlowTracker measurements. The differences with 

the FlowTracker measured discharges ranged from -1.6 

to -7.9 percent. It was anticipated that the IQ Plus “theoretical” 

discharge measurements would all be excellent in the controlled flow and channel conditions in the laboratory flume. 

The 1.60 firmware performed better than the 1.52 firmware. 

However, the IQ Plus discharges measured using SonTek’s 

“theoretical” method usually did not meet the manufacturer’s 

suggested discharge accuracy of 3 to 5 percent when compared 

with FlowTracker discharge measurements using the midsection method. The discharge computed by the SonTek-IQ Plus 

using the “theoretical” method is not expected to be as accurate as a discharge computed with the USGS-recommended 

index velocity method.

For both firmware versions, differences with the 

FlowTracker velocity measurements tended to be negative. 

Velocities measured using firmware 1.52 had a general negative bias and frequently did not meet the manufacturer’s 

accuracy specification for the IQ Plus. Mean-profile velocity 

differences ranged from -6 to -12 percent when compared to 

FlowTracker mean-profile velocity. Velocities measured using 

firmware 1.60 had a smaller negative bias than the velocities 

measured with firmware 1.52. Mean-profile velocity differences using firmware 1.60 ranged from -2.50 to 0.17 percent 

when compared to the FlowTracker. Mean-profile velocity 

measurements with firmware 1.52 did not meet either the 

manufacturer’s specifications or Price Type AA meter accuracy requirements when compared with FlowTracker measurements. Mean-velocity measurements with firmware 1.60 met 

the manufacturer’s specification and Price Type AA meter 

accuracy requirements when compared with FlowTracker 

measurements. The IQ Plus with firmware 1.60 is considered acceptable for use as an index velocity instrument for 

the USGS.

References Cited

Cook, Mike, Bird, Steven, and Corbett, Arran, 2012, Using 

the SonTek IQ Plus to measure flow in open irrigation 

channels—A case study on flow meter accuracy and 

resource management: Irrigation Australia 2012 Conference, 63d International Executive Council and 7th Asian 

Regional Conference Proceedings, June 24–30, 2012. (Also 

available at http://irrigation.org.au/documents/publicationsresources/conference_papers_2012/Using_the_SonTek_IQ_

Plus_to_measure_flow_in_open_irrigation_channels.pdf.)

Heiner, B.J., and Vermeyen, T.B., 2012, Laboratory evaluation 

of open channel area-velocity flow meters: Bureau of Reclamation Hydraulic Laboratory Report HL–2012–03, 15 p.

Hubbard, E.F., Thibodeaux, K.G., and Duong, M.N., 1999, 

Quality assurance of U.S. Geological Survey stream current meters; The meter exchange program 1988–98: U.S. 

Geological Survey Open-File Report 99–221, 15 p.

International Organization for Standardization, (1993) 

Guide to the expression of uncertainty in measurement, 

ISBN 92–67–10188–9, ISO: Geneva, Switzerland.

Levesque, V.A., and Oberg, K.A., 2012, Computing discharge 

using the index velocity method: U.S. Geological Survey 

Techniques and Methods 3–A23, 148 p. (Also available at 

http://pubs.usgs.gov/tm/3a23/.)

Mitutoyo Inc., 2003, Pro 360 manual, accessed Feb 5, 2014, at 

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Mueller, D.S., Wagner, C.R., Rehmel, M.S., Oberg, K.A,, and 

Rainville, Francois, 2013, Measuring discharge with acoustic 

Doppler current profilers from a moving boat (ver. 2.0, 

December 2013): U.S. Geological Survey Techniques and 

Methods, book 3, chap. A22, 95 p., at http://dx.doi.org/10.3133/

tm3A22.

Nolan, K.M., and Shields, R.R., 2000, Measurement of stream 

discharge by wading: U.S. Geological Survey WaterResources Investigations Report 00–4036, 34 p.

SDI-12 Support Group, 2013, SDI-12—A serial-digital interface standard for microprocessor-based sensors, version 1.3, 

27 p., at http://www.sdi-12.org/current%20specification/

SDI-12_version1_3%20January%2026,%202013.pdf.

SonTek/YSI Inc., July 2007, FlowTracker handheld ADV 

user’s manual: San Diego, Calif., SonTek/YSI Inc., at 

http://www.sontek.com.

SonTek/YSI Inc., 2011, White Paper—Product Introduction; The 

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Turnipseed, D.P., and Sauer, V.B., 2010, Discharge measurements at gaging stations: U.S. Geological Survey Techniques and Methods book 3, chap. A8, p. 2–8. (Also available at http://pubs.usgs.gov/tm/tm3-a8/.)

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manual: San Diego, Calif., Xylem Incorporated, at http://

www.sontek.com.

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stage data: U.S. Geological Survey, Office of Surface Water 

Technical Memorandum 96.05.

Fulford and Kimball—Hydraulic Laboratory Testing of SonTek-IQ Plus—Open-File Report 2015–1139

ISSN 2331-1258 (online)

http://dx.doi.org/10.3133/ofr20151139