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Project background:

UCSD worked in partnership with REOTEMP Instruments to aid in the development of a moisture sensor. It is to be used in green and biosolid composting applications. Currently, the composting industries rely on gravimetric methods to measure the actual water content present in chosen samples.

Gravimetric method:

• 1. Gather a sample of green or biosolid compost

  2. Weigh the sample

  3. Put the sample in an oven to evaporate water (full evaporation will take at least one day*)

  4. Reweigh the sample

  5. Weight difference represents moisture originally in the sample

*One sample might not be accurate; moisture distribution has variance within the compost pile

Given the time consumption of the process, industry has been driven to develop quicker methods with the hope of finding a solution that has better or at minimum, equivalent accuracy. REOTEMP has developed a relative humidity sensor, whose performance is not within desired accuracy.

Objective:

• • Implement already available time-domain transmissometry technology into a standalone device

    • ensure it can measure absolute moisture in green and bio-solids compost

    • integrity of mechanical structure holding the sensor must be rugged enough to withstand field conditions

    • tabulate the effects of density and temperature on desired readings

The sensor we incorporated into our device is Environmental Sensors Inc's "Gro-Point Lite".

Final design:

Heavily inspired by the success of "REOTEMP OXYTEMP", the final design is presented below. Additional details of individual components can be found in the Final Design section. A report detailing the design process and experimental results can be found here.

Performance results:

Executive Summary

The compost industry operates within a unique business model where they charge people and waste management companies to drop of their green waste, process it into compost, then charge consumers to pick it up. As a result, compost facilities’ only recurrent costs include labor, fuel, and water. If the optimal amount of water is not reached, the composting process will be delayed and the facilities holding costs rise. The current method of measuring the water content in compost is referred to as the gravimetric method, where a sample of compost is collected, weighed, dried in an oven for 24 hours, then reweighed to determine the mass of water in the sample. This process is painfully slow and labor intensive. To keep cost low and profits high, our sponsor, ReoTemp Instruments, would like to offer compost facilities a true percent moisture meter alongside their industry leading temperature and oxygen probes.

The overall goal as a group was to investigate the current technology available and develop a method of measuring the true percent moisture (by weight) within an accuracy of 5%. Initially, ReoTemp wanted to have a 1.5 meter long insertion probe that can reach the center of a large compost pile where the bulk of the bacterial decomposition is occurring. ReoTemp also wanted this probe to be able to determine the percent moisture in bio-solid waste, a new and emerging composting market.

Our approach was to investigate, test, and exhaust all known technologies that are used in determining the moisture content of various materials, focusing on soil probes because of the similarity in composition and density. Through our research, we narrowed our efforts on a specific technology called time domain reflectometry, or TDR, which is the industry standard for measuring the moisture content of soil on farms and golf courses. In this sector there are two distinguished companies, Field Scout and Environmental Sensors Inc., that have the same technology packaged in different probes.

After several rounds of testing both probes and verifying their accuracy using the gravimetric method, we decided to move forward with ESI probe because it’s readings were the least affected by the high temperatures (70oC) that the green waste compost process occurs at. The field scout proved to be more accurate when taking measurements at room temperature but because of the inconsistencies in the operating temperature range it would not be a viable option for our purpose.

Further testing with the ESI probe yielded encouraging results with biosolids, all measurements were within the acceptable range of ±5% accuracy. However, tests within the green waste compost did not have the same precision. Accuracy among those tests yielded errors from 1-8% depending on the density and temperature. Results showed no direct correlation when attempting to fit the data for a calibration curve.

Integrating the ESI sensor into the hardware of the insertion probe was determined to be a viable option when used in conjunction with the ramrod. However, after weeks of abusive testing with the ESI sensor, it fell apart after 15 insertions with our final setup. In the future, the ESI sensor would need to be redesigned to withstand the harsh environments of being inserted into compost piles repeatedly.