Define specifications for instrument

Goal

  • Someone can quickly, with minimal error, run large batches of soil respiration with high repeatability.

Key improvements

These are factors we need to control that we know impact repeatability, usability, and quality of measurement

  • Measure by volume is easiest for user, but weight is necessary for the calculation of pore volume and we can't have dramatically different weights as that would impact internal chamber size.
  • We can use the pore size to calculate water addition (which is unique!), and automate the water addition to reduce user error, simplify the process, and create a more accurate measurement.
  • We use a temperature controlled chamber for the 24hr storage to reduce user error, and improve repeatability.
  • We changed to a flow-thru design to measure CO2 by using consistent but slow steppers to push the plunger. This gives a more accurate real CO2 reading inside the chamber, and allows for absolute calculation of ug mineralizable carbon... and therefore could be used as part of carbon sequestration models, with less dead volume.
  • We continuously pulling air out of container holding the co2 sensor, drawing air through the intake port, to flush the sensor whenever it's not in use.
  • Using ~15g which will produce a more representative and consistent result.
  • Added check valve to each of the syringes to reduce user error in not closing off the samples before or after use.
  • Improved the user process to make it simple + repeatable + easy to use (cartridge design) and more ergonomic to use, vertical mounting
  • Have integrated equipment + standards testing as part of the normal procedure (5000ppm container)...
  • Better software system + data integration with other data systems (SoilStack, SurveyStack, farmOS).
  • (end of process... next steps)... Also using bacterial v fungal measurements by adding fungicide / bacteriacide to the water addition...

Hardware Specs

  • Teensy 3.2
  • Bluetooth or USB OTG connected
  • Separate plug for power - using standard AC-DC plug --> teensy battery input and other motor drivers, etc.
  • USB is used for communication (not power)
  • K30 (0 - 10k range), flow thru design, for CO2 measurements
  • Temperature controlled incubator (buy heating / cooling cooler box, replace lid to create larger box).
    • plugged into wall
    • provides current temp, and (if possible) 24hr min / max (if this provides to be rock solid consistent, we don't need it).
  • Barcode reader through the phone

Software Specs

  • SurveyStack.io
  • Connect via Bluetooth or USB OTG
  • Send JSON w/ device info and data...
  • Get interpreted, displayed as a graph, (with error checks) data saved as objects in the survey

Equipment / parts (received by the user) Specs

  • cassette - cassette + plungers (attached permanently) + cups (magnetically or optionally attached). Cassetes are numbered w/ qr and visible numbers 1 - 36 (1-6, 7-12, 13-18...)
  • syringe - syringe + polyester filter + check valve (combined)
    • make sure we can combine the check valve and syringe!
  • Water
    • should be 15 - 40C (otherwise thermal mass of water may impact value)
    • a difference of 10C on 2g water added to 15g soil would change soil by 5C
  • 5000ppm standard for calibration
  • Mineral soils only
    • no potting type soils without modification of the method

User Process Flow

  • dry, grind + screen to 2mm, have some kind of bag ID.
  • get a cassette
  • plug in respiration device
  • connect respiration device to Android
  • open Run Respiration survey (repeat for each sample)
    • Directions page (make sure cooler is working + at temp, etc. etc.)
    • In survey, start with calibration at 5000ppm or other standard to provide consistency between batches, and identify the offset to get an accurate CO2 measurement.
    • In survey, choose 'baseline' or '24hr' measurement
    • if baseline

      • ask for original sample ID, then sample cup ID (qr on both)

      • Tare sample cup (on device)

      • Standard procedure for filling the sample volume measuring "spoon"? Shake it level? Scrape it level? Tap it to settle it? etc.

      • Weigh (surveystack question) Standard procedure for filling the soil

      • Calculate pore volume + water addition (surveystack question, user sees result)

Consider Soil type to determine if the specific gravity needs to be changed from the default value of 2.65g/cc https://www.globalsecurity.org/military/library/policy/army/fm/5-472/ch2part2.pdf

A soil’s specific gravity largely depends on the density of the minerals making up the individual soil particles. However, as a general guide, some typical values for specific soil types are as follows:

  • The specific gravity of the solid substance of most inorganic soils varies between 2.60 and 2.80.
  • Tropical iron-rich laterite, as well as some lateritic soils, usually have a specific gravity of between 2.75 and 3.0 but could be higher.
  • Sand particles composed of quartz have a specific gravity ranging from 2.65 to 2.67.
  • Inorganic clays generally range from 2.70 to 2.80.
  • Soils with large amounts of organic matter or porous particles (such as diatomaceous earth) have specific gravities below 2.60. Some range as low as 2.00.

Example Calc. Sample volume = 15.0cc Bulk density 1.1 to 1.7 undisturbed. So we can assume that our disturbed samples will be lower. Say the 0.9 to 1.5

If the bulk density of our disturbed soil is 0.9 the sample will weigh 13.5g 1.5 the sample will weigh 22.5g

Default specific gravity of the soil = 2.65g/cc

13.5/2.65 = 5.09 cc of solid or 15.00-5.09 = 9.9cc void; 9.9*0.65 = 6.4g water to add.

22.5/2.65 = 8.50 cc of solid or 15.00-8.50 = 6.5cc void; 6.5*0.65 = 4.2g water to add.

40g/2.65g/cc = 15cc

say our sample weighs 18gm. 18g/15cc = bulk density 18g/2.65g/cc = Solid volume 15cc- solid volume = pore volume

water to add is pore volume x 0.55

Samples for the Solvita CO2 burst test were prepared as outlined by Woods End Laboratories (2013). Forty grams of dried soil was taken in a 50-mL Solvita beaker, which was perforated at the bottom. The beaker was then placed in a 200-mL jar before being rewetted. Initially, the procedure outlined in the soil CO2 emission test official Solvita guideline manual (Woods End Laboratories, 2013) was followed. In this procedure, 20 mL of deionized water was dispensed to the bottom of the glass jars supplied and allowed to move into 50-mL plastic beakers through perforations in the bottom of the beaker and wet the soil sample via capillary action. However, some soils became saturated and excessively wet while others remained relatively dry. Therefore, in an effort to modify the method, 10 mL of water was applied directly to the soil for all the Solvita analyses presented in the comparison to the GC direct headspace method.

For above 40g/2.65g/cc = 15cc solid; assume bulk density of 1.5g/cc = 10 cc so there addition of 10 cc was 100% of the pore void volume. An Automated Laboratory Method for Measuring CO2 Emissions from Soils March 2018Agricultural & Environmental Letters 3(1) DOI: 10.2134/ael2018.02.0008

The above procedure adds a lot of water. Assume that the bulk density of the dried sample is 1.33. 40g/1.33g/cc=30cc; 40g/2.75g/cc= 14.5cc. (1.33 assumes about 50% pore void volume so that makes sense) That means that the pore void volume is about 15cc. They added 10cc of water.

IF our sample size is about 10cc and it had 50% pvv there is about 5cc of void. if we want to fill half of it we should add 2.5cc.

  - Add water + reweigh (surveystack script, user sees result)
  - Put sample cup on syringe piston and set to 300ml (surveystack instruction)
  • OR if 24 hr
    • have user confirm min-max temps of cooler
    • confirm time (make sure they are 24hr +- some amount).
    • enter cassette ID (0 - 36)
    • checks DB, shows cassette ID's original sample ID, date and time run (surveystack script)
  • run the fan and check background value - average and standard deviation
  • user prompted to move sample to CO2 measure position (instuctions question type)
  • measure CO2 (surveystack script)
    • initiate a CO2 measurement
    • push up the CO2 sensor down through the plunger check valve / syringe body.
    • slowly push air (20 - 40s)
    • show result, error check
  • run the fan again and read background and print average and standard deviation, make sure it is within a standard deviation of the first fan measurement
  • if 24 hr
    • calculate final value (surveystack script).
      • Get baseline data from DB, calculate final respiration value, show to user.
      • if there were errors and things were updated, ensure this script pulls from the original locations (baseline script) so that no other scripts need to be re-run.
  • user submits survey
  • may review survey in the 'results' screen, or we may need a specialized dashboard just for this... (?) needs more UX thought here.
  • While prepping next sample, fan is running to evacuate any CO2 from the chamber.

Final testing

  • Run old method, new method, solvita, microbiometer on a varied set of soils, with several users (experienced, novice...)
    • error bars
    • number of mistakes...
  • Feedback to improve the process (notes for version 2)

Additional tests (future)

  • Impact of water temperature on results (low or high)
Edited by George Albercook