An introduction to Tilt Meters
Technology
All Geosense Tilt Meters use MEMS or Electrolytic technology.
MEMS
MEMS is a chip-based technology, known as a Micro Electro-Mechanical System, that is composed of a suspended mass between a pair of capacitive plates. When tilt is applied to the sensor, the suspended mass creates a difference in electric potential which is measured as a change in capacitance. This signal is then conditioned to provide either an analogue or digital signal.
One advantage of MEMS sensors is that temperature effect on the sensor is very small compared to Electrolytic although temperature effects of the system and in particular the fixings should always be considered.
Electrolytic
An electrolytic tilt sensor provides an output voltage that is proportional to the tilt angle of the sensor with reference to gravity. A typical tilt sensor has three electrodes: a common electrode and two outer electrodes. As the sensor is tilted, the electrolytic fluid inside the sensor covers more or less of the outer electrodes depending on the direction of tilt. The tilt is calculated by measuring the difference in electrical impedance between the two outer electrodes to the common. This signal is then conditioned to provide either an analogue or digital signal.
One of the disadvantages of electrolytic sensors over MEMS is that they are significantly affected by temperature but are more resistant to vibration over MEMS.
Biaxial or Uniaxial
All Tilt Meters have the option of being supplied with Single or Dual axis Tilt boards. Biaxial is generally slightly more expensive so depending on your application or measurement requirements, you are able to make a decision on which would be better suited and potentially save some cost where you can get by with Single Axis measurements.
Water resistant
Standard Tilt Meters are supplied in an IP66 enclosure meaning they are protected against strong jets of water (i.e. heavy rain). If there is any chance at all of the Tilt Meter becoming submersed, a Submersible Tilt Meter (IP68 to 200m) should be used.
EMC Compliant
Our Tilt Meters have been manufactured to meet the Electromagnetic Compatibility Directive 2004/108/EC, to ensure there is no chance of being affected by electrical noise on any site. When a cable is being routed from each sensor there are specific requirements that need to be met with regards to the type of cable and how it is terminated. Geosense provide a Low Smoke Halogen Free cable with a braid with all Tilt Meters, Tilt Beams and In-Place Inclinometers. When the cable needs to be spliced it is important to contact Geosense to supply an EMC splice kit which will ensure the EMC compliance is throughout the cable route.
SELECTION
Type
There are two output types we can offer for our Tilt Meters:
- 4-20mA
- RS485
4-20mA which is an Analogue signal may be preferable to those that want to use their own Datalogger or simple readout device.
Should there be a situation where you require a number of Tilt Meters in a string to achieve a profile, then RS485 would be the preferable choice. The benefit of RS485 signal is that the Tilt Meters can be daisy chained, minimising the amount of cables running to a datalogger such as the GeoLogger CR Series. They can also be read using a Windows tablet with our G-Tilt or Tilt Checker software. Each Tiltmeter is identified and communicated to using the unique serial number assigned to it and sends 1 or 2 Tilt values depending on whether it is Uniaxial or Biaxial (usually Sine of the angle) and Temperature reading.
Multiple monitoring points in close proximity to each other
Digital Bus Tilt Meters would be the way forward here to allow you to connect each Tilt Meter in series and have only one cable sending the data from all Tilt Meters in that array. Saves a considerable amount of cable needed for the job.
Tilt Meter or Tilt Beam?
It depends on whether you can comfortably assume that the structure you are measuring will move as a uniform entity. If this can be assumed then a Tilt Meter will suffice. However, as in most cases, this cannot be assumed and this is where a Tilt Beam would be superior in providing you with a better understanding of how that structure is moving. For example, if you are measuring the Tilt on a church steeple with a 3m Tilt Beam, you cannot assume that the displacement from the vertical plane you are measuring can be applied to the entire height of the structure. Therefore it would be preferable to place numerous Tilt Beams to span the full height of the structure. This way it can provide a useful Profile of what is happening to the building as a whole.
Orientation of Tiltmeter:
Geosense Tilt Meters have a measuring ranges of +/- 5,10 & 15°. Generally, a Tilt Meter is either mounted Horizontally or Vertically. A suitable mounting bracket would then be supplied depending on this detail. Each sensor is supplied with a label showing the measurement range it has been calibrated for.
Cable options
Geosense Tilt Meters are manufactured to meet the Electromagnetic Compatibility Directive 2014/30/EU . To comply with this directive, special braided cable needs to be used which ensures the EMC protection throughout the cable route. If the cable is to be spliced, a special splice kit is to be supplied to ensure the EMC compliance is continued. Should the cable terminate in a data logger, Geosense supply EMC compliant cable glands which crimp on to the braid of the cable. All RS485 digital output Tilt Meters and uniaxial Analogue Tilt Meters require a 2pair braided cable. Biaxial Analogue Tilt Meters require 3 pair braided cable to be able to send the signal from the extra measuring axis.
INSTALLATION
Multiple Digital Tilt Meters in one String
RS485 signal has a maximum distance of 1200 meters in a continuous line that is properly terminated. Provided that a suitable cable is used and there is not a large distance between the first sensor in the string and the datalogger location, then you should be able to successfully connect up to 250 number sensors in one string. This may also require the need of an RS485 Booster interface.
Of course this would not be advisable as should there be a failure at one sensor, it could potentially affect the readings of the entire string. It would be best to split into as many strings as conveniently possible to cover any possible failures that may occur.
Mounting:
Geosense Tilt Meters are usually supplied with a Mounting Bracket designed to either be mounted Vertically or Horizontally. Sometimes these brackets are already attached to the Tilt Meters ready for installation. The brackets are attached to the surface either by using an adhesive such as Epoxy putty or drilling holes for bolts. The important thing to remember is to ensure the Tilt Meter itself is as level as possible before fixing the bracket in place. The use of a spirit level is essential for this stage. You may also want to consider an area that is less susceptible to being in direct sunlight. Although the Tilt Meters measure temperature and account for any changes this may have on the readings, it must not be overlooked that the surface or building itself may also be affected by the temperature changes that may occur. Any anomalies this may cause can then be ruled out.
Cabling:
Protection of the cable from any sensor is essential. Tilt Meters are rated IP66 however this does rely on the cable gland being tightened as much as possible. Once this is ensured, depending on how the Tilt Meter is to be monitored (readout or datalogger), the cable would need to be protected and out of the way and off the ground as much as possible, cable tying any multiple cables as required. Although the cable is well insulated and braided, this does not stop it being susceptible to damage whether it is from fellow workers on site or even rodents.
OPERATION
Power consumption of the Digital Tiltmeter
| Digital Tilt Bus (Single Node): | ||
| Awake Current: | ~200 uA | |
| Transmit Current: | ~1000 uA | Reading complete, sensor not powered |
| Uniaxial Sensor: | ||
| Reading Current | ~4.5 mA | Sensor powered |
| Biaxial Sensor: | ||
| Reading Current | ~9.5 mA | Sensor powered |
Readout or Datalogger
Tilt Meters can be monitored using a handheld readout or a central datalogger. This would depend on the following:
- Required monitoring interval
- How the data is to be accessed
- How the data is to be presented
- Location of the Tiltmeters
For short-term projects where data is required infrequently, there is no point implementing a fully automated system with alarms and remote communication unless of course there is no one available on site to take manual readings. The use of a manual readout should not be overlooked in these situations and could save on time and money. Wherever possible it is always good practice to take as much baseline data as possible so that you have some know zero movement readings you can compare to at a later date when you may start to see movement. This can almost always rule out any suspected fault that may be accused of the sensor(s).
For analogue – MP12 or Multi Meter readout can be used
For digital – Windows based device with G-Tilt or Tilt Checker software
TROUBLESHOOTING
Do Tiltmeters need Temperature compensation?
Geosense MEMS Tiltmeters have on-board temperature compensation whilst the Electrolytic Tiltmeters require individual temperature compensation. However neither takes into account the mounting bracket to which the Tiltmeter is fixed to and the fixing themselves to the structure. Generally the structure on which the Tiltmeter is placed on will be moving in relation to temperature as well. It is always important to monitor the Temperature as well as the Tilt and if possible get some good baseline data for which you can use as a comparison for when you do see any movement, you can rule out temperature effects.
“NAN” reading from my GeoLogger data file
“NAN” stands for “Not A Number” and is always displayed where a reading should be when the datalogger is not able to receive a reading from the sensor in question. This could be related to a number of various reasons which all fall under the following categories order of likelihood:
- Incorrect logger programming: For RS485 digital output sensors, the datalogger relies on the serial number of the sensor programmed into it correctly and in the right sequence. If this is not achieved then you will always get an NAN reading.
- Cable damage: More often than not, a cable gets cut accidentally on site on a frequent basis.
- Incorrect logger wiring: It is important to double check the order of the wiring for any datalogger as this can prevent successful communication to the sensors. GeoLoggers are provided with detailed wiring instructions however if in doubt, please contact the Geosense sales team. Ensure a good amount of strands are available from each core of the cable you are trying to wire and always fold back on itself (not too far that it touches back on the insulation) to prevent any potential for a “dry” connection. Geosense cabled sensors are always provided with bare wires stripped and tinned ready for wiring with ease however sometimes these become damaged during installation and/or transit. If a significant amount of strands are missing, cut the wire back and re-strip the insulation.
- Battery level: If there is insufficient battery life to be able to power the Tilt sensors connected to the datalogger then it simply will not be able to communicate with them and produce NAN readings as a result.
- Sensor damage: It’s as simple as if the sensor gets damaged then there is no reading to send back to the datalogger.
My sensor is showing unexpected movement
Geosense often receive calls from customers saying that they are getting unexpected movement from their sensor. Before making that call, it is important to have the following information to hand to ensure we are able to assist with the problem as efficiently as possible:
- Details of the sensor arrangement
- Data from other sensors in the same area to the suspected faulty sensor
- Historic data from all sensors
- Raw files of the data (not manipulated)
- Temperature readings if available
- Site conditions before, at time of unexpected movement and after (if available)
- How the sensor is being monitored
- The structure of which it is monitoring
9 times out of 10, the sensor is actually showing movement and when all of the above are taken into account it generally explains its actions. Geosense are always happy to take a look at any data presented to us and with all the information available we can provide our experienced opinion on whether the data is valid or not. This should then assist the customer on making an educated decision on what is happening on site.
MAINTENANCE
Re-calibration
Tilt Meters are usually used in long term static monitoring where it would not be subject to large and sudden movements, unlike an Inclinometer Probe. If the Tilt Meter has been moved around a lot (around 10 moves per Tilt Meter), and/or dropped or damaged, it would be advisable to get the Tilt Meter inspected and the calibration checked. Of course it cannot always be confirmed whether the Tilt Meter has been knocked or not so if you start to see some obscure readings which are not expecting and every other site indicator has been ruled out, it would be advised to get the unit inspected.
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