Operational specifications

Relative dielectric permittivity specifications provides the operational specifications.

Relative dielectric permittivity specifications
	CS650	CS655
Relative dielectric permittivity
Range	1 to 81
Accuracy† 1 to 40:	±(2% of reading + 0.6) for solution electrical conductivity (EC) ≤3 dS/m	±(3% of reading + 0.8) for solution EC ≤8 dS/m
40 to 80:	±1.4 for solution EC ≤3 dS/m	±2 for solution EC ≤2.8 dS/m
Precision‡	<0.02
Volumetric water content
Range	0% to 100%
Accuracy†	±1% (with soil specific calibration), ±3% (typical with factory VWC model) where solution EC < 3 dS/m	±1% (with soil specific calibration), ±3% (typical with factory VWC model) where solution EC < 10 dS/m
Precision‡	<0.05%
Electrical conductivity
Range solution EC	0 to 3 dS/m	0 to 8 dS/m
Range bulk EC	0 to 3 dS/m	0 to 8 dS/m
Accuracy†	±(5% of reading + 0.05 dS/m)
Precision‡	0.5% of bulk electrical conductivity (BEC)
Temperature
Range	–50 to 70 °C
Resolution	0.001 °C
Accuracy†	±0.1 °C (for typical soil temperatures [0 to 40 °C] when sensor body is buried in soil), ±0.5 °C for full temperature range
Precision‡	±0.02 °C
Sensing volume*	7800 cm³	3600 cm³
Sensing volume approximately 7.5 cm radius around each sensor rod and 4.5 cm beyond the end of the rods †Accuracy* specifications are based on laboratory measurements in a series of solutions with dielectric permittivities ranging from 1 to 81 and solution electrical conductivities ranging from 0 to 3 dS/m. ‡Precision describes the repeatability of a measurement. It is determined for the CS650 by taking repeated measurements in the same material. The precision of the CS650 is better than 0.05 % volumetric water content and 0.01 dS/m electrical conductivity.

FAQ(s)

Can the CS650 and the CS655 measure water content between 0 and 0.05?

Probably not. The principle that makes these sensors work is that liquid water has a dielectric permittivity of close to 80, while soil solid particles have a dielectric permittivity of approximately 3 to 6. Because the permittivity of water is over an order of magnitude higher than that of soil solids, water content has a significant impact on the overall bulk dielectric permittivity of the soil. When the soil becomes very dry, that impact is minimized, and it becomes difficult for the sensor to detect small amounts of water. In air dry soil, there is residual water that does not respond to an electric field in the same way as it does when there is enough water to flow among soil pores. Residual water content can range from approximately 0.03 in coarse soils to approximately 0.25 in clay. In the natural environment, water contents below 0.05 indicate that the soil is as dry as it is likely to get. Very small changes in water content will likely cause a change in the sensor period average and permittivity readings, but, to interpret those changes, a very careful calibration using temperature compensation would need to be performed.

Can the CS650 measure permittivity and electrical conductivity of sea water and sea ice?

The electrical conductivity (EC) of sea water is approximately 48 dS/m. The CS650 can measure permittivity in water with EC between 0 and 3 dS/m. EC readings become extremely unstable at conductivities higher than 3 dS/m and are reported as NAN or 9999999. Because EC is part of the permittivity equation, an EC reading of NAN leads to a permittivity reading of NAN as well. Thus, the CS650 cannot provide good readings in sea water.

With regard to sea ice, the electrical conductivity drops significantly when sea water freezes and the permittivity changes from approximately 88 down to approximately 4, as the water changes from a liquid to a solid state. With both EC and permittivity falling to levels that are within the CS650 measurement range, the sensor is expected to give valid readings in sea ice. The sensor is rugged and can withstand the cold temperatures. However, as the ice melts, there will be a point at which the electrical conductivity becomes too high to acquire a valid reading for either permittivity or electrical conductivity.

With the CS650 and the CS655, where is the soil temperature measurement made?

A thermistor is encased in the epoxy head of the sensor next to one of the stainless-steel rods. This provides an accurate point measurement of temperature at the depth where that portion of the sensor head is in contact with the soil. This is why a horizontal placement is the recommended orientation of the CS650 or CS655. The temperature measurement is not averaged over the length of the sensor rods.

Can the CS650 and the CS655 measure water content in greenhouse pots?

Yes, but the pots would have to be large. The CS650 and CS655 can detect water as far away as 10 cm (4 in.) from the rods. If the pot has a diameter smaller than 20 cm (8 in.), the sensor could potentially detect the air around the pot, which would underestimate the water content. In addition, potting soil is typically high in organic matter and clay, causing the probable need for a soil-specific calibration.

Can the CS650 and the CS655 measure gasoline or other hydrocarbons in soil?

No. The principle that makes these sensors work is that liquid water has a dielectric permittivity of close to 80, while soil solid particles have a dielectric permittivity of approximately 3 to 6. Gasoline and other hydrocarbons have dielectric permittivities in the same range as soil particles, which essentially make them invisible to the CS650 and the CS655.

Can the CS650 and the CS655 detect stream bed erosion?

The permittivity of saturated sediments in a stream bed is expected to read somewhere between 25 and 42, while the permittivity of water is close to 80. A CS650 or CS655 installed in saturated sediments could be used to monitor sediment erosion. If the permittivity continuously increases beyond the initial saturated reading, this is an indication that sediment around the sensor rods has eroded and been replaced with water. A calibration could be performed that relates permittivity to the depth of the rods still in the sediment.

Can the CS650 and the CS655 measure water content in compost?

Campbell Scientific does not recommend using the CS650 or the CS655 to measure water content in compost. A compost pile is a very hostile environment for making dielectric measurements with soil water content sensors. All of the following combine to make it very difficult to determine a calibration function: high temperature, high and varying electrical conductivity, high organic matter content, heterogeneity of the material in the pile, changing particle size, and changing bulk density. The temperature and electrical conductivity values reported by the CS650 or CS655 may give some useful information about processes occurring in the compost pile, but these sensors will not be able to give useful readings for water content.

Can the CS650 and the CS655 measure water content in gravel?

Yes. Keeping the sensor rods parallel during installation is especially difficult in gravel, but it can be done. Gravel has large pore spaces that drain quickly, so the water content readings will likely show rapid changes between saturation and very dry. If small changes of water content at the dry end are of interest, a soil-specific calibration may need to be performed to convert period average directly to volumetric water content.

Can the CS650 and the CS655 measure water level?

The CS650 and the CS655 are not ideal sensors for measuring water level. However, these sensors do respond to the abrupt change in permittivity at the air/water interface. A calibration could be performed to relate the period average or permittivity reading to the distance along the sensor rods where the air/water interface is located. From that, the water level can be determined. The permittivity of water is temperature dependent, so a temperature correction would be needed to acquire accurate results.

Can the CS650 or the CS655 be partially inserted into the soil?

Because the reported volumetric water content reading is an average taken along the entire length of the rods, the sensor should be fully inserted into the soil. Otherwise, the reading will be the average of both the air and the soil, which will lead to an underestimation of water content. If the sensor rods are too long to go all the way into the soil, Campbell Scientific recommends inserting the rods at an angle until they are fully covered by soil.

With the CS650 and the CS655, where is the electrical conductivity measurement made?

The bulk electrical conductivity (EC) measurement is made along the sensor rods, and it is an average reading of EC over that distance at whatever depth the rods are placed.

How can the CS650 or the CS655 readings be corrected for temperature?

This manual gives a temperature correction that works in coarse sand, but it should be used cautiously with other soil types. If a temperature correction is required, it is best to determine a soil-specific temperature correction.

When correcting for temperature, the following effects contribute to the sensor output:

The effect of temperature on the measurement electronics inside the sensor head. This is a relatively small effect compared to other temperature effects.
The change in the dielectric permittivity of water with temperature. At 0°C, the permittivity of water is approximately 88, at 20°C it is approximately 80, and at 70°C it is approximately 64. If the sensor is in a soil at any given water content, the changing permittivity of water will cause the period average at 0°C to be higher than it is at 20°C. The same soil will have a lower period average at 70°C than at 20°C. In other words, the sensor will overestimate water content at colder temperatures and underestimate it at warmer temperatures. However, that is only true if electrical conductivity is negligible.
The change in water content as bound water is captured and released. In soils with high clay content, some of the water is partially or fully immobilized by electrical charges on the surface of the clay minerals. The amount of bound water is temperature dependent and may have a small effect on the sensor readings.
The temperature effect of bulk electrical conductivity (EC) on period average. Bulk electrical conductivity increases with temperature; as it increases, it slows down the period average.

The interaction of these effects may be complicated. For example, with increasing temperature, two things happen at the same time: the falling permittivity of water is decreasing the period average, and the increasing EC is increasing the period average. The net result as to whether the period average goes up or down depends on how conductive the soil is and the contributions of the other temperature effects.