4.4.1. Combined T/H Sensors

General Description

*During 2016 and with the introduction of ‘AS’ revision VP2 models, Davis switched to using the Sensirion SHT31 T/H sensor. This marks a step-change from the SHT1x range of digital T/H sensors used for the preceding 10 years (2006-2015). When reading the notes below it is important to be aware of the revision version of your VP2 station. If you’re unsure which version you have then please decode the Mfg Code for your ISS.

!In August 2017 Davis discontinued production of replacement T/H sensors containing the SHT11 and SHT15 parts. There is some stock of these parts still in the supply chain but, from 2018 on, it will become increasingly necessary to use the SHT31 parts as a substitute, which will work perfectly well (and indeed more accurately than the original parts) but will require the appropriate temperature offset to be used if connected to a SIM board of pre-AS revision.

While not officially disclosed, it is widely understood that Davis has always used the Sensirion make of T/H sensors in all Vue and VP2 models from 2006 on. From 2006 to 2015 the Sensirion SHT-1x series range of temperature/humidity (T/H) sensor elements was used in all VP2 weather stations (and also in the Vue model). This is an integrated digital sensor that delivers its data on an interface similar (but not identical) to the I2C protocol and called Sensibus, although this has never been officially confirmed by Davis. From 2016 on, in VP2 ‘AS’ revisions and later, the T/H sensor was changed to the new Sensirion SHT31 sensor . Differences between the SHT1x and SHT31 ranges are detailed below. There has been no announcement from Davis of a change away from SHT11 sensors in the Vue models and we assume that this older sensor is still used in the Vue, probably as another distinguishing feature between Vue and VP2.

For both SHT1x and SHT31 sensors, the Sensirion sensor element is mounted on Davis’s own small PCB assembly using surface mount methodology and the T/H chip is therefore not suitable for user replacement by traditional hand-soldering techniques. Exposed tracks and connections on the PCBA are thoroughly protected from moisture ingress by epoxy-type coating.

Sensirion SHT1x sensor elements

There are three types of SHT-1x sensor element made by Sensirion, SHT-10, SHT-11 and SHT-15 which are identical in general design, but with progressively better accuracy specifications. All three sensors share the same overall characteristics, with an operating temperature range of -40°C to (nominally!) +100°C and relative humidity (RH) covering the full range of 0-100% RH. See accuracy profiles for temperature (right) and RH (left) in the graphs below.

This sensor family is designed with an optimum temperature accuracy at 25°C and accuracy steadily decreases away from this central design temperature. In other words, the accuracy specification typically quoted will only be fully valid at 25°C and accuracy will be lower away from the design temperature. Also, with this profile, it is inevitable that potential inaccuracy will be somewhat greater at low temperatures (eg -20°C), which are relatively commonplace in winter, than at the maximum temperature reached across most of the world (eg 45°C).

Humidity accuracy has its own characteristic profile: Maximum RH accuracy is attained across a central band of RH values spanning roughly 20-80% RH. Accuracy then decreases markedly towards 0% and towards 100% RH. Since fog and heavy rainfall are obviously associated with high RH values, it’s worth noting that humidity in the range 95-100% is a tricky region to measure accurately and potential errors will always be higher here. It is not uncommon to find that 98% or 99% is the highest RH value that a station will display (unless a console offset is deliberately entered).

Sensirion SHT31 sensor element

The SHT31 sensors used in VP2 stations of AS revision and later look very similar to the SHT1x family and are mounted on a small Davis PCB assembly in an identical manner.

Accuracy characteristics for a typical SHT31 sensor are shown to the right. (NB The SHT30 sensor is not used by Davis). The notable further improvements in accuracy over even SHT15 should be clear to see. The main benefit is that optimum accuracy is now maintained for a much wider spread of temperature and humidity, especially so for temperature, and so intrinsic temperature readings should be significantly more accurate, even well away from the nominal calibration temperature of 25C.

Davis T/H sensor PCB assemblies for VP2 stations

There are now three distinct replacement T/H sensors that are available from Davis and compatible with all VP2 stations made since 2006, subject to appropriate temperature offsets being used. (The situation with Vue stations is different: For Vue stations prior to revision M, a single replacement T/H PCB assembly is all that’s available. For revision M and later, a complete harness change is required if the T/H sensor becomes faulty.)

!Note that the comments below about SHT11 and SHT15 T/H sensors are purely for historical information. SHT31 replacement sensors are the only type now available as spares. Stocks of the earlier types are now fully exhausted.

SHT11: The previous standard VP2 T/H sensor is part 7346.166 which has now reached revision J. This part is thought to use the middle-ranking SHT-11 part (SHT-10 being unused by Davis), which is specified as having a typical temperature accuracy of ±0.4°C (at 25°C) and for humidity of ±3% over 20-80% RH, rising to 5% at 0 and 100% RH. The Vantage Vue is also thought to use this same SHT-11 sensor element.

SHT15: In 2015, Davis introduced part 7346.174 which uses the enhanced SHT15 T/H sensor. This option has two main benefits:

• In addition to lowering the typical temperature accuracy at 25°C to ±0.3°C, the SHT-15 has an altogether shallower temperature/accuracy profile, which offers a roughly doubled temperature accuracy at eg -10°C and 40°C;
• The baseline RH range of the SHT-15 is ±2% over 10-90% RH and maximum error at 100% RH is reduced to 4%.

This 7346.174 sensor is a direct drop-in replacement for the older 7346.166 part and may be used with no change in temperature offset being necessary. Note however that 7346.174 has only been made in limited quantities and may become increasingly difficult to source.

SHT31: The more recently-released SHT31 sensor is built into Davis part 7346.070. This is plug-compatible with the 7346.166 sensor and can be used with any VP2 station from 2006 on. However: The temperature calibration of the SHT31 sensor is slightly different from the previous sensor and if the 7346.070 sensor is used with a VP2 station prior to the AS revision then the temperature will read 0.5C high and therefore a temperature offset of -0.5C needs to be entered into the console in this scenario. (And conversely if eg an older 7346.166 sensor is used with a VP2 station of AS revision or later then the temperature will read 0.5C too low and an offset of +0.5C needs to be entered.) NB See your console manual for details of how to apply any temperature calibration or correction.

The design of the VP2 ISS circuit board was updated slightly with the AS revision. Provided the SHT31 sensor is used with an AS revision SIM board (part 7345.952 for the main ISS board) then the calibration difference is allowed for and no offset is needed.In other words, provided that an AS (or later) T/H sensor is paired with an AS SIM board then no calibration change is needed. It is only when mixing older and newer T/H sensors and SIM board that this calibration issue will arise.

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