Best Temperature Sensors for Compacted Soil, Arduino vs Raspberry Pi Data Logging

Hello, I am conducting research on the optimization of geothermal heat pump systems. I am a geotechnical engineer and have little background in electronics. I need to install about 20 temperature sensors in a 1 ft long, 6 in diameter, cylinder of compacted soil. These sensors need to be robust because they will have to bear load, they will be exposed to moisture during and after compaction, and I need them to be as accurate as possible. I am working on a very tight budget so each sensor can be no more than about $15-20, and I know I loose accuracy and precision on that budget, hence "as accurate as possible". I will be collecting the data from the sensors using either an Arduino or Raspberry Pi, which ever would be best. 
I just need to know what kind of sensor I should be looking into, and if you can name specifics that would be incredibly helpful. Also, if you could give any insight into  whether the Arduino or Raspberry Pi would be more beneficial in this project for data collection and why, I would greatly appreciate the advice.
Thank you for your time and attention, CR

Without knowing what maximum temperature, accuracy, span, and resolution you require  it is difficult to be too specific.Thermocouples are very robust, can cope with very high temperatures if you use the appropriate type, and you can build them up yourself to suit the application. However, the total cost of the wire and the electronics to process the signal may put you over budget.Semiconductor sensors operate over a  much smaller temperature span than thermocouples, and the package encapsulation may be an issue.  I would hesitate to suggest  them as an alternative however, since I suspect if you are talking geothermal that their high temperature limit will be well below what you need to measure.There is a lot of information on the internet if you need to look into these options further.Cheers,Richard

Twenty channels is not without its problems. It is common to use a scanner and pipe them into one analogue processor. Your unit is already starting to look quite complex. It is unlikely that you will get a twenty channel temperature sensing system that will work directly with either of the processing systems you have mentioned, if one is available for a PC and works on a USB bus, then it may work on a RPi, but you would need drivers, and if not available, you would need to write them yourself.As regards the processor type, it depends on how comfortable you are working with either platform. Python on the RPi would  probably do it, since I expect that your sampling rate will not be excessive, although you have to be aware that hooking up your sensors to the RPi may not be as easy as it is on something like the Arduino, you will need to look around for a suitable analogue module in all likely hood.If you are comfortable with C, and need to get closer to the hardware depending on what it is you end up with, the Arduino would be a better bet.How do you wish to analyse and present the data is another issue. If you are going to do this on a laptop or desktop PC remotely,  then  how  much data will you be collecting. In both cases  external removable memory will be necessary, an SD card or USB stick of some kind, and you need to consider how that will work with either platform. If it is done nearby, then there are temperature logging systems that plug directly into PCs, and you can avoid the use of both the RPi or Arduino altogether. See my initial comment on the channel capacity.What is the temperature range you are interested in, everything I have said may have been pointless if you are working with temperatures at and below 120 degrees C, the problem immediately becomes simpler, since semiconductor sensors could be used.Cheers,
Richard

Hello,I can't thank you enough for all your insight. I've been at a loss with this for a while. The temperature range would be from about 5-30°C. I would take a measurement from all 20 sensors every minute for 12 to 24 hours.I don't need to observe the data remotely.I do have access to an Agilent 34972A LXI Data Acquisition / Data Logger Switch Unit. I've used it before, but only with one very expensive sensor. If I could use it, are there cheaper sensors that are compatible? Thank you for all your help,CR

OK, that has helped. It  is always a good idea to provide as much info as possible, and you are still missing some re the Agilent system. What modules does it have installed. They have a 20 channel multiplexer for this unit. The cheapest sensors are thermistors. They need to be calibrated/linearized, this unit probably takes care of that for you if it has the module installed. Thermistors come in many different configurations, many are not robust physically, but if you insert and glue them into a small metal block with low thermal mass, they would be fine.Re thermocouples, spools of thermocouple wire are not expensive, and if you can spot weld or braze weld them yourself (some wire is also solderable) then the cost should be within budget. Note that there are a number of thermocouple types, you can check out more about that on the web,  J and K are the most common and the Agilent will definitely handle them.Cheers,Richard

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In the interest of starting simple and adding complexity only as needed. Some ideas to consider:"Processor"  Arduino Mega 2560,  $38           https://www.sparkfun.com/products/11061Has 16  10-bit A/D analog input channels. Assuming you can pare down from 20 to 16 temperatures. Programming is simple. Many tutorials online. Very popular.
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Thermistor sensor, $0.91 ea.
https://www.digikey.com/product-detail/en/mur...FA2B100/490-5630-ND/2533821   
These are small 1% accuracy temperature sensors, less than $1 each. You put them in a simple voltage divider using a single resistor. They will be quite linear over your small temperature range.================================Cheers!

IC temperature sensors like the LM34 cost almost a dollar apiece, and need a power connection. They give a 10 mV per degree C output (some have other scaling). A regular diode is only 2 mV per degree; but you can string 5 in series to give higher output (10 mV). Feed a current from a resistor into the anode of a string, with the cathode end grounded, and sense the anode voltage. You can get 1N4148 or similar diodes for pennies apiece. I've used this in my solar hot water collector for 40 years now. I feed tank vs. collector sensor voltages into a comparator circuit to run a pump control.  I haven't built a data logger yet...