Electronic Data Collection

Colin S. Campbell Ph.D.Research Scientist

Decagon Devices, Inc.

Outline Data collection for the early

scientist Progress toward modern field

techniques Converting electronics into

information Electrical Engineering meets the

scientist Assessing the requirements of

a project Making the right system choice

Field research: Quantify physical environment

Early pioneers in environmental biophysics Howard Penman at Rothamsted Research

Station Manual readings or strip chart/disk

recordings Sleepless nights Seminal paper on evapotranspiration

Champ Tanner at U. Wisconsin Travel trailer

Changing world of measurements There’s got to be a better way

All-nighters at research site not terribly popular

Miss fast changes or events with human sampling

No control of processes Goals

Make unattended measurements Store measurements for analysis

later Transform data into information and

understanding

Requirements for field research Possible needs

Sensors that generate electrical signals that can be correlated with environmental phenomena

System to read electronic signals and store them

Make decisions based on measurements Control external systems based on

analysis

Modern field research Sensors

No limit to parameters that can be measured Passion for instrumentation design

Only challenge is to find correlations Indirect measurements

Measuring one parameter and inferring the one of interest

Data logger Basically glorified multimeter and oscilloscope

Repository for raw sensor output Interprets electronic signals and stores them

Evolution of measurement: Temperature example

First automation by strip chart recorder Change in temperature of bimetallic strip

Deflection calibrated to known temperatures Temperature variation changed pen position Ink recorded changes over time

Data evaluated by hand Widely used

Conversion to an electrical signal: The thermocouple

Seebeck effect Two dissimilar metals jointed

together produce voltage potential when differentially heated

Potential related to temperature difference Correlation (copper-constantan

thermocouple) ~ 40 mV per oC Measurement of minute voltage

changes provides accurate temperature Assuming know the temperature

of one junction Electrical measurement is

accurate

Other measurement techniques: Temperature

Thermocouple limitations voltage accuracy

requirement reference temperature

Alternatives Thermistor, platinum

resistance thermometer Change electrical

resistance with temperature

Diode Voltage drop across a PN

junction

Data loggers and sensors

Sensor signal types Four general types of electronic sensor output

Voltage Probably the most common type

Includes thermocouples, radiation sensors, some anemometers, etc.

Current Often used over long cable distances Common to some measurement and control industries

Pulse or switch closure Rain gauge, some anemometers, some soil moisture

sensors, etc. Digital

Typical of sensors measuring more than one parameter Allows for more than one signal per input location

Data logger types Plug and play (P&P)

Decagon Em50, Em5b Onset Hobo CrossBow eKo

Measurement and Control Systems (MCS) Campbell Scientific CR1000, 3000, etc. DataTaker DT80 National Instruments LabView

Choosing a data logger: Things to consider

What electronic outputs do you need to measure? Voltage, current, pulse, digital

How many sensors are you putting at each research site?

How often will you be storing a measurement?

Choosing a data logger: Things to consider

Will some measurements need to be made more often than others? >10 Hz (i.e. eddy covariance) 1 minute (i.e. radiation)

Do you need to control anything with your system (lights, heater, valve, etc.)?

Do you have the time or resources to program and setup the system?

No

Yes

No

Yes

Choosing the right system

P&P

MCS

P&P

MCS

P&P

MCS

P&P

MCS

Choosing a data loggerPlug-and-play data logger

Built for specific sensor measurements or specific sensor types

Allow only a minimum of configuration Date/time Measurement interval Sensor type

Limited sensor inputs Low flexibility for sensor

types

Plug-and-play data loggers

Advantages Fast configuration Simple deployment No/low programming

complexity Simple data collection

and analysis Straight-forward sensor

integration Low power consumption Price

Disadvantages Limited sensor

types Limited input ports Little or no

configurability No event-based

sampling No/little external

control

Choosing a data loggerMeasurement and control systems (MCS)

Build for general purpose measurement Measure most types of voltage, current, pulse, and

digital sensors Highly configurable

Many different measurement and control option Programming allows for multiple measurement intervals On board data processing and decision making High speed measurement

Expandable Add additional sensor capacity

Accurate May utilize high resolution signal processing for

accuracy

Measurement and Control SystemsAdvantages Configurability Precision and accuracy Programmability Speed Decision making and

control Data processing

Disadvantages Programming Configuration Installation and setup Power

Characteristics to evaluate Required resolution and range

Thermocouple 0.1o C resolution = 4 mV data logger resolution 50o C range = 2000 mV data logger range

Water content sensor 0.1% VWC ~ 1 mV data logger resolution 100% VWC ~ 1000 mV data logger range

Excitation Many sensors require a voltage be provided to the

sensor Decagon EC-5 – 2.5 or 3V regulated Gill WindSonic anemometer – 12V unregulated

Excitation requirements vary mV to 10s of volts Many data loggers have limited excitation options

Characteristics to evaluate (cont.)

Analog to digital converter (ADC) Voltage and current measurements are made by an

ADC Precision of ADC defines accuracy of the

measurement Defined by bits

i.e. 12 bit ADC 0 to 4095 2.5 V range 0.61 mV/bit Obviously not good enough for the thermocouple, but good for

VWC 24 bit ADC 0 to 16777216 2.5 V range 0.15 mV/bit

Good enough for thermocouples

Noise rejection Multiple sources of ambient electrical noise

60 cycle from electricity, radio frequency

Data logger applications Making the decision

Many choices available Sometimes confusion trying to decide

which one will work the best Discuss some applications from personal

experience Caveat: Vast majority of my experience is with

Decagon and Campbell Scientific data logger Many other manufacturers that you may

consider Delta-T, Onset, DataTaker, Stevens, Unidata, etc.

Rice net carbon exchangeConditional sampling Stored 77 different outputs

CO2, H2O concentration (voltage output from IRGA) Pyranometer, quantum sensor, net radiation (mV) Water content (pulse count) Rain gauge (pulse count) 3-D sonic anemometer (digital)

Data downloaded by cell phone (2.5 h away) 5 Marine batteries charged by 6-12V solar panels 2 CR10X dataloggers, 2 MUX, Relay driver

Flexibility, control, programmability, storage, communication

Turf grass wateringTurf field with pop-up sprinklers

Control based on distributed water content sensors VWC at several

locations Threshold values

control solenoid values for sprinklers

Decision: things to consider P&P data logger

Easy to read VWC sensors Fast installation Low power requirements Data easily collected and

graphed over radio or cell phone

Often lack control capability MCS

Required for system control Large sensor input capacity

Distributed field analysis of physical and morphological interactions

Site description 37 ha research farm Large topographical

variation Goal

Investigate water, temperature, and EC variation in relation to soil morphology

42 distributed profiles Measurement at 5

depths

System choice Plug-and-play logging

system well suited for distributed networks Small number of

sensors at each site Radio or cell phone

communications Fast setup Low power use No requirement for

control or specialized sampling

Fast, simple plot measurements

Description Goal

Compare performance of drought tolerant cultivars

Requirements Soil moisture,

temperature in plots Weather station

parameter in central location

Simple deployment

Considerations P&P systems require

no programming No specialized sample

timing or control Self contained loggers

require no enclosure setup or external power

Summary Data loggers simply measure and

store electronic signals Art of instrumentation is to dream up

new ways or correlating electronics to science

Data logger choices are numerous Carefully determine all experimental

needs Evaluate system specifications