Instromet Sunshine Hours Sensor

This is a sensor able to record accurate values for hours of bright sunshine. It can be supplied with a counter/display option as a standalone sunshine sensor or with a counter/pulse output option to interface to the rain gauge input of Davis Vantage Pro 2 weather stations (and also to the older Davis WMII and Wizard III models) to allow the sunshine hours figures to be logged by all standard Davis Weatherlink logging systems.

Important: Rainfall and sunshine hours cannot both be monitored simultaneously by the same Davis station, unless it uses an Envoy8X console or Meteobridge Pro unit. But many users may have a secondary or retired older Davis station whose rain gauge input could be used to provide a rain gauge input for the IM sun sensor.

The role of a sunshine hours sensor

Instromet Sun Sensor

This is a sensor which measures sunshine duration directly as sunshine hours. The Davis VP/VP2 range does of course already include a sunshine intensity sensor, which performs well. But sunshine hours and sunshine intensity sensors fulfil two quite distinct roles in weather monitoring.

Sunshine hours (or, more correctly, hours of bright sunshine) is the traditional measure of the amount of sunshine that a location has received. For many years, sunshine hours was the only way in which the amount of sunshine could be monitored and recorded (by the well-known Campbell-Stokes recorder, with its glass-sphere lens and burned-paper trace). So there are extensive long-term records of sunshine hours in the weather archives and there is still considerable interest in sunshine hours as a measure, partly for comparison with historic records, but also because it sums up the amount of daily sunshine in a single, easily-understood daily value, which can then be aggregated into monthly and yearly totals.

In contrast, global solar intensity, as measured by the Davis 6450 sensor, provides a continuous measure of how sunlight strength is varying throughout each day. Intensity is course a very useful measure, especially for any applications that require a value for total solar energy but it is fundamentally different from sunshine hours and less easy to summarise in a single widely-understood value.

While it is possible to fudge a conversion from intensity values to sunshine hours, this is a crude calculation and prone to large errors, especially under certain sun and cloud conditions (and if done too simplistically, as with the algorithm that the present Weatherlink program uses). In contrast, this new sensor adaptation is able to measure sunshine hours directly by means of a tried and tested shadow sensor and to present the resulting values via an interface designed to be compatible with Davis rain gauge inputs. Readings from this sunshine hours sensor can therefore be logged by standard Davis Weatherlink loggers.

IM Sunshine Hours Sensor

The IM sensor consists of two separate components: the sensor itself and a control box.

The sensor component is pictured above and comprises the actual sensing element set within a glass dome and fixed to a mounting bracket, with U-bolt for attaching to the top of a mounting mast. In size, the unit is about 15cm (6″) long overall, with the glass dome about 5cm (2″) in diameter. The sensor contains a vertical shadow pillar around which are set eight light sensitive detectors in a circular pattern. The unit works by detecting the presence of a strong shadow cast by the central pillar causing a differential response across the set of detectors – in conditions of non-directional light (ie a cloudy day) there will be no shadow cast, with consequently a broadly uniform detector response and, conversely, when the sun shines brightly, a strong shadow will be cast and the resulting contrasting signal pattern will be picked up by the detectors and signalled as a period of bright sunlight.

This methodology for sensing and recording bright sunshine has been available for some years now and is therefore a tried and tested technique. It is capable of giving results that compare favourably to other established approaches to recording sunshine hours, but which require either a daily dedication  to manual reading such as the Campbell Stokes recorder or are very costly to buy and to run, such as high-end electronic sensors as made, for example, by Kipp & Zonen. Indeed, the detection of strong shadows is arguably the best methodology for recording bright sunshine because it tallies most closely with normal human experience.

The sensor component is supplied with a 25m (80 feet) sensor cable, which must be wired into the control box supplied with the sensor.

IR enhanced version: An enhanced version of the sensor is also available featuring infra-red filtered photodiodes to more accurately reflect the power spectrum of normal sunlight. This version conforms to strict WMO guidelines and achieves good comparability with high-end reference sensors.

Instromet Sun Sensor Control Box

The control box (left- minus lid) contains the main circuit board for the sensor which processes the raw sensor data and creates an output signal consisting of a voltage pulse every 36 seconds of bright (shadow-forming) sunlight seen by the sensor. Counting these pulses externally therefore gives a reading of bright sunshine hours in units of 0.01 hours.

The control box is sealed but not absolutely waterproof and therefore needs to be further protected from the elements inside a building or shelter of some type. So a typical configuration would place the sensor unit up high where it can get an optimum all-round view of the sky and with the cable running down to the control box up to some 25m distance away and in a more sheltered location (but which doesn’t necessarily need to be a heated indoor space). Note also that, as standard, the control box needs to be supplied with a 12v DC supply, which would typically be provided from the AC mains adapter supplied with the unit, so the box would also need to be sited within a few meters of a mains supply. (The control box may also be fitted with backup batteries, but these are not sufficient for more than a few days’ operation of the unit.)

Configuration options for the Instromet Sun Sensor

2017 model Instromet Sun Display
2017 model Instromet Sun Display

This sunshine sensor can be used in one of three ways (which can, in principle, all be used simultaneously – please consult us for advice on how to build up your preferred configuration from the component parts):

  • As a standalone sensor, in which case an optional display unit – right – is available (in a brand new version as from late 2016), which provides two readings on LCD displays – one for short-term (eg daily) sunshine totals and the second panel for accumulated totals over longer (eg monthly) periods. The daily total can be re-zeroed  independently of the monthly reading. A second, enhanced version of the display is also available with a bright OLED blue-on-black display at extra cost. Finally, a separate standalone Instromet logger can also be specified to accompany the sensor and display, if required;
  • As a sensor for use with the Instromet Climatica weather station;
  • Connecting this IM sensor to a Davis weather station and logging the sunshine totals automatically via the weather station – see the next section below for further details. Note that this option does not require the display above for connection to a rain-gauge input – the pulse output is taken direct from the sensor’s control box.

Using the IM Sunshine Hours Sensor with a Davis weather station

By a happy coincidence, the units of 0.01 for sunshine hours, as measured by the IM sensor are exactly the same as one of the options for the rainfall increment (of 0.01″) on the Davis weather stations that use a separate rain gauge (ie VP/VP2 models and the older WMII and Wizard stations). This 0.01″ increment is available on all these Davis stations, whether US or metric units as the original calibration. The rain gauge input on the Davis stations is also just a simple contact closure.

As a result, it’s possible to connect the pulse output of the IM sensor’s control box to the rainfall input socket of a Davis station set to an increment of 0.01″. The station will therefore display daily sunshine hours in exactly the correct numerical format (except of course that the nominal display units will be inches rather than hours, but this small nicety is of little practical consequence). If the station has a Weatherlink logger attached then a fully detailed record of daily sunshine hours may also be logged, with the accumulation of daily hours updated at the end of every archive interval so that a fairly good record of the sunshine pattern throughout the day can be captured, for example at 5 or 10 minute intervals.

There is however an important limitation to note in connecting the IM sensor to a Davis station: In the standard Davis configuration, only one rain gauge input is permitted per station. Typically this will mean that if the station is already recording rainfall from its rain gauge then there will be no spare input to which the IM sensor could be connected. So connection of the IM sunshine sensor to a Davis station would only be possible in the following circumstances:

  • If the user has a second Davis station at the same site then the rain gauge input to the second station could be dedicated to the IM sensor. This will often be the case where a user has an older station such as a WMII or VP1 station that has been superseded in day to day use by, for example, a newer VP2 station. It will often happen that the display and logging functions on the older station will still be working perfectly well, even if some sensors such as wind or temperature had become troublesome, and so this may well be a way of getting some extra useful life out of an older system.
  • An Envoy8X-based Davis system or Meteobridge Pro unit will be able to able to accept inputs from multiple rain gauges and so it will be perfectly possible to attach the IM sensor interface output to, for example, a 6332 Anemometer Transmitter board and to receive the resulting data stream as a second ISS. 6372 and 6382 Supplementary Temperature and Temperature/Humidity stations would be further alternative transmitters to a primary ISS. This is obviously the most attractive approach for logging the IM sensor’s data;
  • There may well be some users for whom logging of good sunshine hours data is a higher priority than rainfall via their Davis station. They might, for instance, have a high-accuracy rain gauge that they prefer to use for their primary rainfall recording;

NB The Instromet pulse output is suitable for connecting to either Vantage Pro or Pro 2 station or to an older Weather Wizard III or Weather Monitor II station. It cannot be used with a Vantage Vue station (because the Vue has no accessible rain gauge input socket).