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Grounding Sensors

Typically the terms “ground” or “earth” or “earth ground” refer to the voltage potential of, literally, the earth.  Most buildings have a “grounding rod” which is pounded into the earth and all electrical devices that are “grounded” within the building are ultimately electrically connected to this grounding rod.

The distinction between "earth ground" and "data logger ground"

In data acquisition systems all measurements are ultimately based on measurements of the voltage potential between various points in the circuitry and a common reference voltage often referred to as the "data acquisition system (or the data logger) ground." The internal circuitry of the data acquisition system is all referenced to the data logger ground.  It is important to understand that the data logger ground is not the same as the earth ground unless the former is explicitly tied to the latter.  Even in a hand-held volt meter (which is an example of a simple data acquisition system) there is a common reference voltage within the circuitry that is the data logger ground.  In the case of a hand-held volt meter, this reference voltage is not connected electrically to any other reference, and indeed may be at a very large potential above or below the earth ground. 

This difference between the data logger ground and the earth ground is illustrated in Figure 1. Here, a hand-held volt meter is used to measure the voltage across battery #2.  In this case the negative lead of the volt meter, which is internally connected to the circuitry’s reference voltage (the data logger ground), is 12 volts higher than the earth ground.  The volt meter reads the correct value of 13 volts across battery #2 even though the reference voltage of the meter is 12 volts above earth ground.

Figure 1. Measuring Battery #2's voltage using a volt meter. The volt meter ground is 12 V higher than the earth ground.

 

A multi-channel data logger can be thought of as several volt meters combined into one package, with all voltage measurements internally referenced to the data logger ground. 

Grounding to avoid exceeding the input limit and common mode range of your data logger

The data logger has two voltage limits that must be observed:

  • The "input limit," which is the maximum voltage between the two measurement terminals, and
  • the "common mode range," which is the maximum voltage between any measurement terminal and the data logger ground.

The input limit may be software-configurable or may be fixed, whereas the common mode range typically varies as a function of the voltage across the two measurement terminals.  The input limit is always greater than or equal to the common mode range. 

If either the input limit or the common mode range is exceeded by a small amount the measurement is likely compromised. In fact, it is possible that all measurements on the data logger are compromised, although the data logger may not be damaged.

To avoid exceeding the common mode range, each sensor connected to the data logger must be connected to the data logger ground. 

Data loggers may be capable of making single-ended measurements, differential measurements, or both.  In a single-ended measurement, one of the two terminals across which the voltage is to be measured is internally connected to the data logger ground, so that the measured voltage is referenced to the data logger ground.  In this case the input limit will be exceeded before the common mode range because the input limit is always greater than or equal to the common mode range.

In a differential measurement neither terminal is internally electrically connected to the data logger ground.  The data logger measures the voltage difference between the two terminals even if neither terminal is at the same potential as the data logger ground.  In a differential measurement it is possible to exceed the common mode range without exceeding the input limit, and it is important to avoid this situation by explicitly grounding one of the terminals to the data logger ground.  Many self-powered sensors and devices such as thermocouples, pyranometers, and batteries, are not, by default, connected to the data logger ground when the voltage generated by the sensor is measured as a differential measurement.

Consider Figure 2, in which a data logger is used to measure the current generated by a thermopile-type pyranometer.  The data logger has an input limit of 250 mV (that means the maximum voltage difference it can measure between the two terminals is 250 mV), and the pyranometer generates a maximum of 200 mV under full-sun conditions, so the input limit will not be exceeded. Let's look at the common mode range, which is 2.5 V for this data logger.  In Figure 2a, the pyranometer is not connected to the data logger ground.  Even though the voltage generated by the pyranometer will never exceed the input range of 250 mV, the common mode range can be exceeded because the potential between the pyranometer and the data logger ground can “float”: atmospheric conditions and electromagnetic fields can cause the voltage between the sensor and the data logger to be several volts.  To make this problem particularly tricky, this often does not happen immediately after connecting a sensor, but the sensor’s voltage gradually floats farther and farther from the data logger ground until the common mode range is exceeded days or weeks after installation.  The result is that the measurements look fine for many days, then suddenly are clearly wrong.  Sometimes after a while the sensor’s voltage will float back down on its own and the measurements make sense again.  In Figure 2a the sensor has floated to 4.1 volts above the data logger ground potential, which exceeds the common mode range of 2.5 V. 

To keep the sensor’s potential from “floating away,” a jumper must be added (Figure 2b) to connect one side of the sensor to the data logger ground.  The jumper can be a small length of wire or a resistor of relatively low resistance (less than 1000 W).  The sensor is thus now grounded (at the data logger ground) so that neither measurement terminal will ever be more than 200 mV above the data logger ground and the common mode range will never be exceeded.

Figure 2a. Sensor is not grounded, risking common-mode voltage over-ranging.

 

Figure 2b. Sensor is grounded, preventing common-mode over-ranging, but data logger ground is floating.

 

Figure 2c. Recommended wiring. Common-mode voltage over-ranging is prevented and data logger is grounded for safety.

 

Grounding the data logger to earth ground for safety

In Figure 2c the data logger ground is connected to earth ground.  This is generally done for safety reasons, to keep the data logger and all its connected sensors from floating far from earth ground.  Because people and buildings are generally connected to earth ground, if a data logger has floated 100 volts above earth ground (this can happen under some circumstances!) then when someone touches the data logger he/she will receive a dangerous electrical shock.

Avoiding ground loops

Often in electrical circuit diagrams the electrical potential of the “ground” is depicted as being at exactly the same voltage in all places in the diagram.  In fact, because all conducting materials have some electrical resistance, if there is any current flowing from one point to another along a conductor connected to ground there will be a small voltage difference between one point and the next.  This flow of current from one point of the ground to another is what is known as a “ground loop.” 

When making measurements using a data logger it is important to understand that everything electrically connected to the data logger is part of the overall electrical circuit. In Figure 3, for example,  a data logger is used to measure temperatures of water in a pipe using immersed temperature sensors. The data logger is grounded to earth ground by connecting to an electrical conduit, which is ultimately connected to the earth ground via the building's grounding rod.  Two thermocouples are used to measure the temperature of the fluid at two points in a pipe.  The negative leads of the thermocouple wires are connected to the data logger ground to prevent the sensors from exceeding the common mode range.  The thermocouples are sheathed in stainless steel tubes immersed in the fluid in the pipe. The pipe, if it is made of metal, is also connected to the earth because it is electrically connected to the metal cold water mains pipe entering the building.  Even if the pipe is made of a non-conductive material like PEX, the water in the pipe is still electrically connected to earth ground, with some resistance, because non-distilled water is somewhat electrically conductive and the water is touching the inside of the metal cold water mains pipe.  A person looking at Figure 3 might conclude then that the voltage potential at the data logger ground is the same as the potential at each pipe since all are shown connected to earth ground.  This is not necessarily true, however, because the electrical resistance between each pipe and the data logger ground is non-negligible, the path being a circuitous route through the earth, grounding rod, electrical wires, and conduit.  If two points are not connected with near-zero resistance, there is always the possibility that the voltage between the two points is not zero.

In Figure 3 the voltage between the data logger ground and the pipe is shown to be 1.0 mV when no conductor is connected between the pipe and the data logger ground (upper sensor).  This magnitude of potential between one grounding point in a building and another is not uncommon.  If the data logger terminals are at 20 oC and the water temperature is 40 oC, the voltage generated by a Type T thermocouple will be about 0.790 mV.  This voltage is measured by the data logger and used to calculate the temperature of the thermocouple in the pipe.  If the thermocouple wire is not electrically connected to the pipe or water the voltage between terminals H and L will be 0.790 mV and the correct temperature will be calculated (upper thermocouple in Figure 3).  If, however, the thermocouple junction inside the metal sheath is connected to the sheath (this is called a “grounded sensor”), a ground loop will be present as current will flow from the junction, which is now at a higher voltage than the data logger ground, to the data logger ground.  Because the electrical conductivity of the thermocouple wire is fairly high, the voltage between the pipe and the data logger ground may be reduced from the initial 1.0 mV, but it will not be brought to zero because the wire has a non-zero resistance.  In this example the voltage between the lower pipe and the data logger ground is reduced to 0.5 mV because the negative lead of the thermocouple is conducting some current from the pipe to the data logger ground.  Because of this ground loop, the voltage between terminals H and L will be raised from 0.790 mV to (0.790 + 0.5) = 1.290 mV.  From this measured voltage the data logger will calculate a temperature of 52.3 oC instead of the correct temperature value of 40 oC.

Figure 3. Upper (ungrounded) sensor reads correct value. Lower (grounded) sensor reads incorrect value due to ground loop between lower pipe and data logger ground.

 

Conclusions / Rules of Thumb

If the following rules-of-thumb are followed, problems with common mode over-ranging and ground loops can be avoided:

  1. The data logger should have its ground connected to earth ground.
  2. Each sensor should be reliably connected to a ground to avoid common mode over-ranging.
  3. Each sensor should be grounded at ONE POINT ONLY to avoid ground loops.
  4. In order to achieve a single, reliable grounding point for the sensor, it is best to ground the sensor at the data logger and be sure that the sensor is electrically insulated from all electrically-conductive media.  Keep in mind that even a partial connection to earth ground, such as through water or by direct burial of an uninsulated sensor, can cause a ground loop if the sensor is grounded at the data logger.