Sensor Specifications

This page continues the sensors page in further detail.

My Personal Rules of Electronics

First Rule of Electronics: Always get the part number or part name, go on the internet, find the device, get the data sheet (usually a pdf file), print the datasheet out, read through it front to back, highlight the important stuff, put it in a binder for future reference, and save the pdf file in a folder on the field computer.

Second Rule of Electronics: No matter how many tools you move to a new bench you will always forget one that you will need. This is an indirect way of suggesting taking considerable care in building a fieldwork toolbox.

Water Turbidity

Courtesy of Shannon (and Gary Clarke and Neil Hall) we have a home-built turbidity sensor or turbidimeter that measures received IR sent by an LED across a gap. The components are embedded in epoxy and the channel between LED and sensor allows water to intervene in the light path. The Clarke Turbidity Sensor page includes xeroxes of the 6 pages of text/diagrams we received and some more notes. Open voltage is measured at about 2 volts, high turbidity drops to about 0.8 (using milk).

Implementation options in the field:

1. Attached to microserver
2. Attached to mote
3. Attached to the Campbell etc of your choice

The turbidimeter requires 12 volts so driving it off a microserver is the simplest way to go since this will have an external soft-switched 12V line. (I don’t know anything about Campbell dataloggers yet.) We can also use the Field Mote external-thru power supply feature which is convenient because of the Tmote signal bus access to an ADC. In more detail: The Field Mote is placed adjacent to a 12V external battery which is connected to the Tmote Power-In bus. This supply is switched internally by the Tmote according to a sampling schedule, and the 12V power is routed to the Power-Out bus where it is used to drive the turbidimeter. The turbidimeter signal is then coupled to a line on the Field Mote signal bus which goes back into the Field Mote enclosure where the analog signal is converted to a digital data number.

A final option would be to get the turbidimeter circuit diagram and redesign it to run off something like 3.3 or 5 volts.

Water/Air Temperature and Relative Humidity

Temperature Sensor
Not active yet: Relative Humidity Sensor

Any small temp sensor can be embedded in epoxy and deployed in air or water or soil etcetera. Related: Tmote Sky devices can be purchased with a Sensirion T/RH sensor pre-attached to the board, a convenient way to begin. However there is a risk of heat contamination from the mote board so here we want to imagine a wire going to a sensor which returns a "nearby" temp reading. The device may be analog or digital and for simplicity we'll start with an analog component, the LM19, and see how it behaves.

This is an analog temp sensor (no relative humidity) from National Semiconductor, part number LM19, aka the 2.4V 10 microAmp "TO-92 package" Temperature Sensor. All of the example applications listed on the data sheet are indoors or in-case but... we don't care. We're going to hook this up to an ADC pin on a Tmote, calibrate it, and go stick it in a creek.

Here are the specs:

NS Package Number Z03A, Device Marking LM19CIZ
Rated -55C to +130C
Available as a 3-pin (TO-92) package for easy extensibility/access.
Predictable curvature error
Suitable for remote applications (ok, there we go)
Accuracy at +30C: +-2.5C max
Accuracy at +130C and -55C: +- 3.5 to +-3.8C max
Power supply voltage range: 2.4--5.5V
Current drain: 10 microAmps
Nonlinearity: +-0.4% typical
Output impedence: 160 ohms (max)
Load regulation, 0 uA < I-sub-Load < +16uA: -2.5mV (max)
Equilibration time: Order of one minute.

Integration with a Tmote Sky

See this page for a quick-and-dirty Tmote modification to the Oscilloscope program that gets us data off this device. The LM19 datasheet tells us how to convert a temperature to an output Voltage and vice-versa, see below, but what we really need is a conversion from a digitized value to temperature.

V_out = (-3.88e-6 x Temp^2) + (-1.15e-2 x Temp) + 1.8639.
Temp = -1481.96 + sqrt(2.1962e6 + RATIO) where
RATIO = (1.8639 - V_out) / (3.88e-6)
Example results:
100C <==> 675 mV
25C <==> 1574 mV
0C <==> 1863.9 mV
-40C <==> 2318 mV

The voltage goes down as the temperature goes up.

Viewing the TO-92 package--it looks like a 3-pin transistor--we have a bottom-view schematic with the flat side up on the page and the pins left to right reading +Vs, V_out, GND. We "jumper" the supply voltage pin Vs to GND using a (say) 0.1 microfarad capacitor in order to smooth out jiggles in the power supply. (The capacitor acts like a little charge-bucket shock absorber.) The +Vs power can be provided by the Tmote (10-pin connector Vcc, pin 1 (square)). The V_out wire can be connected to a Tmote Sky ADC pin.

Light

The Tmote Sky sensor package includes two Hamamatsu light sensors: Photosynthetically active radiation (PAR) and total solar radiation (TSR). These can be taken off the board and mounted external to a case.

Campbell Scientific makes a radiometer package described here.

Hydrochemistry

LISST Series Laser Diffraction Particle Size Analyzers Suspended Sediment Concentration (SSC) for SSC/turbidity comparison, specificaly LISST-25 not sure if this link will work

Water Depth via Pressure Transducer

Pressure Transducer link.

A pressure transducer is an electronic device that converts local pressure to an electronic signal. We consider here two units, the Druck PDCR 1230 and the Global Water WL15. The PDCR is an analog device so we obtain the pressure by converting an analog voltage to a DN value. The Global Water unit does the digitizing internally and transmits pressure information over a serial data connection.