The Sensors

Sensors are the visible part of a weather station. They must be exposed to the weather so that they may take measurements.

Most of these sensors are mounted on a tower about twenty-five feet above ground level on top of the mountain.


Purpose: To measure the air temperature.

Temperature affects whether precipitation is in the form of rain or snow. It affects how low in elevation snow can fall or remain. If you want to go skiing, it will begin to affect you by the driving conditions you experience on the way.

Temperature affects the type of snow which can fall, and it can change the nature of the snow pack already on the ground. It affects the type of wax to use on our skis, and what you need to wear to be comfortable outside.

General details: The sensor is located inside of a "gilled aspirator", which protects it from heating effects of the sun. The gills allow air to flow freely past the sensor.

The sensor includes both the temperature and humidity sensors in a single package. Power, control and output signals are carried via a captive seven conductor cable.

Specific details: Made by Vaisala (Helsinki, Finland): Humidity and Temperature Probe, Model # HMP45AO, S/N T0440041, stamped "Feb 20 1998", handwritten "#10726" & "50'"

Sensors           Cable (tower)     Cable (12-pair)   Datalogger

  +12V power      red               Wht/Org           (+12V) - Power
  ground          black             Org/Wht           "G" - Ground

  Control         orange            Wht/Grey          "C1" - Control
  Ground          clear/bare        Gry/Wht           "G" - Ground

  Humidity        yellow            Wht/Slate         "1H" - Single-ended input
                  (violet)          Slate/White       "AG" - Analog ground

  Temperature     blue              Wht/Brown         "1L" - Single-ended input
  Signal ground   violet            Brown/Wht         "AG" - Analog ground
The analog ground reference for the temperature & humidity signals ("violet" in the tower's cable) is doubled to provide a ground reference in both the temperature's and humidity's wire pairs on their way to the datalogger.

Connections in the junction box at the base of the tower: Left-to-right screw terminals on the top row are, in order, Red, Black, Orange, Clear, Blue, Violet, Yellow.

Maintenance: Brush-off accumulated dust or salt build-up on the sensor surfaces.

Comments: The sensor has a problem with rime build-up pulling the cable out of the sensor head. Perhaps a rigid support of the drip loop would help.

The sensor has a problem becoming encased in rime: Could it be placed inside a large inverted "bucket" which is open to the air, but which would protect it from ballistic impact from windborne droplets?

Wind Speed

Purpose: To measure how quickly the air mass of a weather system is moving.

When weather systems move past a location the movement of the air is described as wind. The higher the wind speed, the quicker a weather system will be replaced with the next.

Wind can affect the environment in obvious ways, stressing rigid structures (like trees and buildings), and moving portable items (like new snow or your picnic lunch). It contributes to other subtle effects, such as windchill, which is a measure of how quickly heat is lost from a warm body.

Winds can have delayed effects as well. New snow, blown over ridge-tops by the wind, can form avalanche-triggering cornices. This hazard may release during a storm, or remain and continue building throughout a winter.

General details: The sensor uses a hub with six paddles on its side which spins on a vertical axis. A magnet is attached to the hub, and a "Hall effect" sensor detects the magnet's movement as the hub revolves. The sensor provides one pulse per revolution.

Protection against icing is provided by an internal 1100 Watt heater and a temperature sensor connected to a thermostat. The sensor is kept at about 35 degrees (F) to avoid freezing. Its construction is heavy gauge aluminum.

Specific details: Made by Hydro-Tech (Seattle, Washington), heated rotor anemometer Model WS-3. The business name was "Taylor Scientific Engineering" in summer of 2001.

The Hall effect sensor is a three terminal device, powered between +12 volts and ground, with an NPN open-collector output. It's output is a nominally 50% duty cycle square wave.

The +12 volt power is jumpered from the temperature/humidity sensor's supply at the base of the tower. Ground and the output run on a single pair of wires to the datalogger.

Hall sensor       Cable (tower)     Cable (12-pair)      Datalogger

  +12V power      .                 x
  Output          .                 Blu/Wht ->(Buffer)-> "P1" - Pulse counter
  Ground          .                 Wht/Blu              "G"  - Ground (digital?)

The datalogger's "P1" pulse input should not be driven with signals which swing over +5.0 volts, so a signal level conditioning circuit is used to reduce the swing:

The Hall-effect sensor's output is buffered using a "NE555" chip (near the datalogger), to allow level shifting to a 0-5 volt swing. The same buffer output provides the line drive to send this signal to the bottom of the hill.

The downhill wire pair is "Wht/Grn" at the terminal block in the radio cabinet. It is connected to the Yellow/Orange wire pair in the large cable going to the lodge. This wire pair is about one mile in length, each leg has about 37 Ohms of resistance (73 Ohm shorted loop), in excess of 20 MegOhm leakage to ground (or other wires). Its capacitance is unknown.

Maintenance: The manufacturer's manual calls for an annual check of the bearings for noise and starting torque. Any noise from the bearing will be associated with increased friction, which will affect the sensor's calibration.

The starting torque (required to get the rotor to start spinning) can be measured with a "torque watch". (This is a very sensitive instrument, the torque is very small.)

Consider lubrication or replacement of the bearings as a result of this check.

Another way of detecting bad bearings: When readings start dropping to "0" in light wind conditions. It seems that the wind is rarely calm for long on top of a mountain.

The deicing heater controller should be turned-on and set to around "65" on its 0..100 scale for winter-time operation (Nov 15 to Apr ?). Set to about "50" for one month prior to and after "Winter". Turn-off the heater during warm weather months.

Calibration: The physical structure of the wind rotor essentially defines how it responds to wind speed. As long as the rotor is not deformed and it spins freely on its axis, there should be no concerns about its calibration.

The rotor has a nominally linear response of revolution rate (RPM) to wind speed (MPH). At 100 MPH it should spin at 850 RPM and generate 850 pulses per minute. Due to starting friction the minimum measureable wind speed is around three (3) MPH.

Comments: None.

Wind Direction

Purpose: To measure the direction from which the wind is coming.

Air masses coming from different directions often have different characteristics, so a change in wind direction often "announces" the arrival of a new weather system.

In case of high winds, the direction from which it comes can dramatically affect what sort of dangers those winds may create, and where those hazards are located.

Mountains tend to block winds, so conditions are calmer off the top of the mountain and on the leeward side (away from the wind).

General details: The sensor uses a simple weather vane (a hub with a tail) mounted on a vertical axis. Directions are sensed with a 5000 Ohm linear potentiometer attached to the vane. The electrical resistance between the potentiometer's wiper and one of its ends varies with its rotation. The zero (0) Ohms direction is usually aligned with "North", and resistance increases proportional to compass bearings (0..360).

Protection against icing is provided by an internal 1100 Watt heater and a temperature sensor connected to a thermostat. The sensor is kept at about 35 degrees (F) to avoid freezing. Its construction is heavy gauge aluminum.

Specific details: Made by Hydro-Tech (Seattle, Washington), heated direction vane Model WD-3. The business name was "Taylor Scientific Engineering" in summer of 2001.

Sensor connections are the three potentiometer terminals.

Jumpers at the base of the tower connect the 3 wires coming from the potentiometer to 3 pairs of wires going to the datalogger. This allows "4-wire" resistance measurements to be made at the tower's junction box.

To make a measurement a 1.0 Volt signal is applied to the ends of the potentiometer using one of the wire pairs. A second wire pair allows a differential voltage measurement from the wiper to common. The third wire pair allows a differential measurement across the whole potentiometer (high-end to common). The ratio of the two measurements is computed and scaled to provide an output with values ranging 0-to-360 (degrees) for the compass direction.

Potentiometer     Cable (tower)     Cable (12-pair)   Datalogger

  High end        .                 Org/Red           "E1" - Excitation
  Common end      .                 Red/Org           "AG" - Analog Gnd

  High end        .                 Grn/Red           "2H" - Differential input
  Common end      .                 Red/Grn           "2L" - Differential input

  Wiper           .                 Blu/Red (ring)    "3H" - Differential input
  Common end      .                 Red/Blu (tip)     "3L" - Differential input

Maintenance: The manufacturer's manual calls for an annual check of the bearings for noise (& starting torque (??)); lubricate or replace the bearings or potentiometer as necessary (??).

Calibration: As long as the potentiometer is not damaged or worn, its linearity should remain adequate for the task. Since the measurement computes a ratio of voltages, the absolute value of the potentiometer's resistance is not critical.

The potentiometer's resistance is expected to be proportional to the compass bearing, so its resistance should go from minimum to maximum in the direction of true North. The physical orientation of the sensor should be verified if there is any reason to suspect that the mechanical set-up has turned.

As mounted on top of Hoodoo Butte, "North" is in the direction of the bottom of the ravine containing Hoodoo Creek, just west of Hogg Rock. (Midway between a left tangent to the highway around Hogg Rock and the rock outcrop at the east end of Potato Hill, about 1000' west of the highway. As a secondary check, the lift cable's axis runs almost exactly NE/SW.

Comments: None.


Purpose: To measure the amount of moisture in the air.

Humidity is a measure of how much water vapor is in the air, as compared to the maximum possible amount. It is expressed as a percentage.

Humidity usually doesn't affect much until it gets close to 100%, at which point the vapor turns to liquid and we get fog, clouds, or precipitation.

If we monitor the humidity we can estimate how close we are to losing visibility. At "saturation", as the temperature drops, fog will form. As the temperature goes below freezing, rime can form as supercooled (still liquid) water droplets freeze on contact with cool objects like towers, trees and skiers. As the temperature continues to drop the droplets freeze into ice crystals, and we get snow.

In the summer, low humidity is very important when considering forest fire danger.

General details: This sensor is combined with the temperature sensor in a single package. It uses a material which changes its electrical resistance in response to how much moisture it has absorbed from air.

Specific details: See the "Temperature" sensor's section, since the sensors are combined.

An internal "drive" circuit applies a stimulus (e.g. current) to the sensor, and measures a response (e.g. voltage).

Maintenance: See the "Temperature" sensor's section.

Calibration: Nothing is actively done. Measurements and visual observations are often compared, to ensure that a reading of 98% or higher is associated with the sensors being in fog or heavy precipitation events.

Comments: None.


Purpose: To measure the air pressure and express it as the equivalent "sea-level pressure".

Since the absolute air pressure varies with both weather and elevation, it is customary for stations to convert their reading to a sea-level equivalent so that readings from different stations can be easily compared. For this station, at 5720', the absolute pressure is 80.2% of "sea-level pressure".

Air pressure and winds are closely related. Dropping pressure can be a leading indicator of winds to come, as air masses are drawn toward lower pressures. High pressure tends to hold-out winds, and is often associated with clear and calm conditions.

General details: The sensor uses a silicon pressure sensor whose output is buffered by operational amplifiers. The sensor also contains its own voltage regulator and internal temperature sensor.

The pressure sensor requires a temperature correction.

Specific details: The sensor is mounted within an aluminum block, with a pigtail of wires.

Sensor function  Attached wire  Datalogger
  +12 Volt         Red            +12 Volt (power)
  Ground           Black          Ground   (power)
  Pressure(+)      .              "4H" Differential input
  Pressure(-)      .              "4L" Differential input
  Temperature(+)   .              "5H" Differential input
  Temperature(-)   .              "5L" Differential input

Maintenance: None.

Calibration: There is no calibration schedule, so this sensor's readings should be considered to be an "indicator" and not a "gauge". While the absolute value reported has an unknown error, it is still useful for detecting trends from the graphical display.

Attempts are occasionally made to develop correction factors by tracking this sensor's pressure & internal temperature readings, as compared to a weighted average of pressure readings for the Corvallis and Redmond airports. Redmond is closer, so its reading is given a 2/3 weighting.

Comments: Design and hardware courtesy of Mike Linse.

Snow depth

To measure the depth of the snow pack.

The snow pack depth affects how fully rocks & trees may be covered, and how much skiable terrain may be available. For skiers there can never be enough snow (well, outside of the roads & parking lot).

By watching short-term variations in snow depth, we can tell roughly how much new snow has fallen during a storm.

If you have ever made a snowball, you know that snow can be compacted. The snow pack is continuously "settling" under to its own weight, even if it isn't melting. Lighter (colder) snows will eventually compress significantly over time, while heavier (warmer) snows will compress less, but do so more quickly.

General details:
The sensor is an ultrasonic range-finder. In our application it is mounted on a tower, looking straight down. It measures how long it takes for an ultrasonic "ping" to reflect from whatever is below the sensor. It also measures the free air temperature.

The distance from the sensor to the ground (or snow pack) is computed based on the echo timing. It is also corrected for temperature, as the speed of sound in air is a function of temperature. By subtracting the measured distance from the distance to the ground, we get the snow depth.

Specific details: Made by Judd Communications, it is a reasonably general purpose depth sensor which is suitable for sensing a snow surface.

Maintenance: Inspect the air temperature sensor housing, to make sure it is free of insect nests.

The manufacturer cites the average life of the ultrasonic transducer at about 3-5 years. After a little over 7 years of operation, as we see degradation in the ability to take consistent measurements, they suggested it may be time to replace the transducer. They offer a transducer replacement kit, or the unit can be sent back for servicing and a full check-out.

When there is no snow pack, the average measured snow depth should be about zero ("0.0").

A tape measure blade attached to the sensor tower allows mid-season verification.

The sensor uses a quartz crystal timebase to measure the echo times. These oscillators tend to be very stable.

Wind and air temperature variations affect this sensor.

The sensor measures and applies a correction for temperature, but the air path is not always uniform. Wind may tend to blow the sound beam around, leading to longer apparent path lengths.

The sensor is mounted on a wooden (4x6) mast, which might tend to warp due to changes in humidity and heating from the sun.

The snow depth sensor may also get "fooled" due to an incorrect air temperature reading: The mid-day sun may heat the sensor more than the air, which results in an incorrect speed-of-sound correction in the echo distance measurement. If the sensor is warmed more than the air, and the distance calculation assumes a faster speed-of-sound than actual, it will computer an echo path length which is greater than what it should be. Since snow depth is computed as the height of the sensor MINUS the echo path length, a longer path results in a lower computed snow depth. An that is what we often see around mid-day and early afternoon on sunny days.

Overall, the readings seem to vary plus or minus an inch or less.

This sensor was installed in the Fall of 2001.


To measure the water content of all precipitation (liquid or solid) which has fallen since the start of the "water year". This includes rain, snow, sleet, and hail, making it an "all season" gauge. The "water year" in this area begins on October 1st.

General details:
Note: The following is speculation, based on inspection of a SNOTEL site and searching the web for information.

The sensor collects precipitation in an antifreeze solution, converting it to a liquid. The solution is stored in a tall cylinder, where the depth of the "water" corresponds to the amount of precipitation received.

The hydrostatic pressure at the bottom of the cylinder corresponds to the height of the water level, so a pressure sensor is used to measure how much water has been collected.

The cylinder is occasionally drained, so it doesn't overflow. If drained before the end of the water year, an offset will need to be added to subsequent measurements, to account for the equivalent amount of precipitation removed from the cylinder.

Temperature variations affecting the cylinder, the water, and the pressure sensor contribute to short-term variations in the meaurement. The cylinder and water may expand or contract in volume at different rates with variations in temperature, similar to a liquid-in-glass thermometer. Due to thermal gradients in the structure, these effects are not easily corrected. The pressure sensor can be temperature compensated for its own part.

Long-term variations can be expected due to evaporation, and of course, the stuff we are trying to measure.

Like most designs, advantages come with disadvantages: Ordinary "tipping bucket" rain gauges can easily resolve 0.01", but their mechanism doesn't work well with frozen precipitation. (Heaters can be used, but they require significant energy which may not be available at battery or solar-powered sites.) An antifreeze-filled bucket doesn't suffer if ice or snow falls into it, but resolving small changes in its depth is difficult.

To reduce wind effects, a "calming device" of hanging vanes surrounds the collection funnel. It helps somewhat to reduce horizontal wind velocity, but is primarily intended to reduce updrafts. Since the collection funnel is many feet above ground level, wind currents not present at the ground might otherwise tend to blow lighter precipitation (e.g. snow) past the sensor.

Specific details:
standard SNOTEL precipitation gauge (which looks like a rocket motor wearing a grass skirt) is essentially a tall 12" diameter metal tube.

A web page by Honeywell Sensotec was found, which briefly describes the gauge. The pressure transducer is a Honeywell Sensotec model TJE.

The antifreeze solution may be a mixture of ethyl alcohol and propylene glycol, which is used in some snow pillow sensors.

It is not known if a floating cover layer (like an oil) is used to inhibit evaporation.

Assuming the annual rainfall does not over-fill the sensor, at the start of the "water year":

If serviced mid-year, after "zeroing" the sensor, an offset amount will need to be noted to account for the amount of fluid removed.

The fundamental equation for hydrostatic pressure is:

Pressure = density * g * depth
Attention to each variable in the equation would assure good correlation of pressure (what we can measure) to precipitation (what we want to measure).

The pressure gauge should provide calibrated measurements of hydrostatic pressure.

The antifreeze solution's density, possibly varying with the amount it has been diluted, is important to track.

The local gravitational constant ("g") probably hasn't changed enough to worry about.

Unless hit by a falling tree or bulldozer, the shape and size of the collection funnel and cylinder probably do not need to be re-verified. Their physical sizes result in a volume of fluid being collected, and made measureable as a depth of water.

This "precipitation" data channel is borrowed from a nearby "SNOTEL" site which is operated by the Natural Resources Conservation Service (NRCS). This service maintains many sites from New Mexico to Alaska.

The Hogg Pass SNOTEL site is about two miles northeast of Hoodoo Butte, near Santiam Pass on highway US20. Historical data, beginning October 1, 1979, is available. The next closest SNOTEL site is at Santiam Junction, about three miles northwest of Hoodoo Butte. It's about 1010 feet lower than the pass, in a slightly warmer and drier location.

This data channel was added to this weather page in the Spring of 2005, after requesting permission from the local NRCS office.

Static Bursts / Lightning

To get an estimate of the amount of local lightning activity.

General details:
An AM (car) radio is tuned to an unused frequency, and listens for the static "bursts" associated with lightning activity.

A counter keeps track of the number of bursts which are heard.

Specific details:

A detection circuit is attached to one of the AM radio's speakers. A mute switch is used to silence the speaker during normal operation.

The audio is run through a half-wave rectifier (using an operational amplifier, two diodes and two resistors), and then simultaneously applied to "slow" and "fast" RC filters:
- "Slow" RC filter = 3 MegOhms & 0.22 uFd -> ~660 millisecond time constant
- "Fast" RC filter = 10K Ohms & 0.22 uFd -> ~2.2 millisecond time constant.
The outputs of the filters are buffered with output-follower op amps, and sent to a voltage comparator.

The "slow" filter output represents the long-term background noise. Its voltage level tracks up and down as the noise level increases and decreases. To trigger the voltage comparator, the "fast" signal will need to rise above the "slow" signal's level.

The "fast" filter output responds to brief noise events, like static bursts, and for a short time can exceed the "slow" filter's output level because it reacts faster.

The time when the voltage comparator's "fast" input is greater than its "slow" input is when a "burst" is detected. To reduce false triggering, the "fast" signal is passed through a voltage divider before it gets to the voltage comparator. The "fast" signal now can only trigger the voltage comparator if it is significantly higher than the "slow" signal. This should be easy for lightning related noise bursts, but harder for music and speech from a distant radio station.

The voltage divider brings a disadvantage of reducing sensitivity to lightning.
- At 2.43::1 no lightning was ever detected. False counts seen every 5 hours 42 minutes.
- At 1.69::1 lightning within a two mile range was detected, but many reasonably strong "bursts" were not counted. False counts still minimal.
- At 1.27::1 there were an excessive number of false counts.
- At 1.39::1 . . . we shall see . . .

The voltage comparator's output triggers a ~0.2 second 1-shot, which is used to activate a white LED as a visual indicator, a beeper as an audible indicator, and an optoisolator on the input of a "network counter".

The weather computer periodically polls the counter using the "cURL" program, parses its XML response, logs the results, and displays the data in a useful manner.


Information for this parameter will not be shown if no events have been counted within the time period of interest (i.e. 6 or 48 hours for the "short" and "long" weather pages). After a count is recorded the pages will keep reporting this parameter until the time period of interest again contains no counts.

The original motivation for this data channel was to use a counter in the datalogger which has been unused for years. As it turns out, the "network counter" was used instead, so the datalogger's P2 counter is still unused. The newer counter can be located anywhere on the network related to the weather station, so testing and debugging can be done in more accessible locations than where the datalogger is located.

One challenge with this sensor is that the radio is located inside a building with metal siding. It's hard to hear local radio stations inside this box. For now, a long-wire antenna is snaked across the ceiling below the (non-metal) roof. An attempt to use a "rod" style car antenna hanging outside worked for a while, but running coax through a doorway can be really hard on the coax if the door seals tightly.

The voltage divider is subject to further improvement, but the fundamental the design has been proven to work. Having "mute" switches on all sound makers (speaker & beeper) is a good feature for the sanity of folks who don't share your excitement at detecting events like this.

2013 08/18

Rain Gauge

Purpose: To measure rain (or rain equivalent) amounts of precipitation.

In contrast to a "precipitation gauge", this rain gauge cannot measure frozen precipitation, unless it melts and runs through the measuring mechanism. That's not going to work in sub-freezing weather, but Summer hail storms should be measureable. Results would be delayed by the amount of time it takes for the precipitation to melt.

The advantage of this sensor is that its measurement resolution is 0.01", which is much better than the precipitation gauge (at 0.1").

Since snow storms often start with a warm-front before a cold-front, the rain gauge might be used to detect the start of a precipitation event. Or maybe just remind us of how much snow "could have been" if it had only been a bit colder.

This sensor will be removed when the weather turns cold, as it is not intended to be used under freezing conditions.

General details: This gauge uses a "tipping bucket", which alternately directs water from the collecting funnel to one of two small cups. When the equivalent of 0.01" of rain has been collected in a cup, it tips and dumps its water, and the other cup begins collecting water. The cups alternate collecting water. An electrical switch is used to detect each tipping event.

The collecting funnel is 8 inches in diameter.

Specific details: This is a Rainew 111 rain gauge. The link given here is representative of many sites which offer this unit. Shop around for price, availability, and delivery times.

Rather than using the wall display counter, the switch in the gauge was wired to a counter input on a "Five Input Network Counter". (This is the same unit used with the Static Crash counter, which means three counter channels are still available).

Maintenance: We will probably have to occasionally pull pine needles out of the strainer at the bottom of the collecting funnel. Almost anything but water will probably jam the tipping bucket mechanism.

Let's hope a bird doesn't adopt it for a nesting site.

Calibration: Not checked.

Information for this parameter will not be shown if no events have been counted within the time period of interest (i.e. 6 or 48 hours for the "short" and "long" weather pages).

2015 03/22

Counter 3 - SPARE

Purpose: To have a data recording channel ready to go into service.

General details: Nothing to see here folks . . . Move along . . . :-)

Calibration: No worries (yet).

Information for this parameter will not be shown if no events have been counted within the time period of interest (i.e. 6 or 48 hours for the "short" and "long" weather pages).

2015 03/23

Counter 4 - SPARE

Purpose: To have a data recording channel ready to go into service.

General details: Nothing to see here folks . . . Move along . . . :-)

Calibration: No worries (yet).

Information for this parameter will not be shown if no events have been counted within the time period of interest (i.e. 6 or 48 hours for the "short" and "long" weather pages).

2015 09/18

Counter 5 -SPARE

Purpose: To have a data recording channel ready to go into service.

General details: Nothing to see here folks . . . Move along . . . :-)

Calibration: No worries (yet).

Information for this parameter will not be shown if no events have been counted within the time period of interest (i.e. 6 or 48 hours for the "short" and "long" weather pages).

2015 09/18

Light Level

To measure the general outdoor light level.

Irregularities in this parameter's daily cycle allows the general nature of the cloud cover to be estimated. On clear days light levels fluctuate slowly, primarily due to the sun as it travels across the sky and illuminates objects in different ways due to its light and shadows. On cloudy days the light levels are usually lower, and change much more quickly due to the shadows of clouds as they pass by.

Outdoor light levels affect what we can see and do outside. "Civil twilight", when the sun is six (6) degrees below the horizon, is about the point where you cannot see well enough to do outdoor activities like hiking. While the web page calculates this time, the graphical display of the measured light level is a reminder of how much (or little) useful daylight there is in a day, how cloud cover can make it darker, and how quickly daylight comes and goes at sunrise and sunset.

General details:
The sensor uses a Cadmium-Sulfide (CdS) sensor and a 1000 Ohm resistor to form a voltage divider whose output voltage rises with increasing light levels.

The sensor is located in one of the building's west windows. The general intent is to sense the overall outside light level. The setting sun, especially in the Summer, may directly illuminate the sensor.

Specific details:
The voltage divider is excited by one of the datalogger's excitation channels, with +1.0 Volt. The voltage divider's output is sensed by a single-ended input. Both excitation & sense channels are relative to the analog ground.

Voltage divider  2-pair line      Datalogger
  "High"           Red              "E2" Excitation channel 
  "Center"         Yellow           "6L" Single-ended input
  "Low"            Green & Black    "AG" Analog ground

Maintenance: None.

Calibration: None required, this sensor does not report measurements in any units of measure.

There is no guarantee that this sensor's response is linear, indeed a non-linear ("logarithmic") sensor would allow measurement of a wider dynamic range of light levels. A diode on the "reference" side of the voltage divider could implement this feature.

This sensor was added just because it could be done, before web cameras were available. It's like having a camera with one black-and-white pixel.

Lessons Learned

Sensors are exposed to the weather, and it's not always easy to keep them running.

Not all water which is below "freezing" is actually frozen. It is possible for water droplets in a fog to remain liquid even as its temperature drops below 32 degrees F. Rime is a form of ice which forms when these supercooled water droplets freeze on contact with things which are below freezing. Everything exposed to this freezing fog can become encased in a thick layer of ice.

The amount of rime which may form depends on the wind speed, temperature, time of exposure, and how much rime has already formed. The wind is the delivery mechanism for the freezing fog; the faster it blows, the faster the fog droplets can accumulate. Temperature affects how much moisture the air can hold; colder air can hold less water. How much rime has already formed affects how big of a target there is to accumulate even more rime.

Rime can be annoying in that moving instruments are literally frozen in place. Cables can become extra large and heavy, and may pull out of their connectors, stretch or break.

To minimize damage, rime should be allowed to go away on its own due to drier and/or warmer weather. Few items encased in rime can be hammered free of the ice without damaging them.

Rime can be prevented by keeping things heated above the freezing point, or by keeping them out of the wind.

Rime can be tolerated by designing things so that they will not be damaged by the additional weight of the ice, or by forces of the wind acting on a now much larger sail area.

Wear & Tear
Moving instruments like anemometers have bearings which allow them to spin. In a place where it's rarely calm these instruments may run continuously for years, from the date they are installed until their bearings wear-out. You might consider keeping a few spare parts handy, since it's only a matter of time before some things wear out.