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How Ohm’s Law Affects Level Sensors & Pressure Transducers

careful planning of your wiring helpsOccasionally, we hear puzzling tales of sensors that just won’t work, even though troubleshooting measures confirm the sensor is operational. If this ever happens to you, it’s worth taking a look at the cable between the sensor and your control system input.

There are two forces at play that need to be managed: resistance and noise. The effects of each are critical to receiving your sensor’s signal. One is just a part of the equation, while the other is a persistent threat. Let’s take a look at resistance, and deal with noise as we go along.

The Law of Ohm

Ohm’s law is really a statement of proportionality and the equation to figure it out. Georg Ohm discovered that current through a conductor is inversely proportional to the resistance of that conductor. The modern equation tells the story well: I = V/R, where I represents current, V is voltage, and R is resistance.

This directly affects the signal output from your level sensor or pressure transducer (or any other sensor, for that matter). In other words, the voltage has to be high enough to overcome the resistance in a circuit. If the voltage drops too much, the result is a loss of signal. Therefore, voltage drop across a conductor is a primary concern.

Voltage Drop

As every conductor has at least a little resistance, voltage drop is going to happen. Resistance is determined by the characteristics of the conductor – in this case, the cable. The two determining factors are the length and the gauge of the cable.

A shorter length of cable will equal a smaller voltage drop, as will a lower gauge (larger diameter wires).

Voltage drop can be a concern for any type of analog output, but primarily for voltage and millivolt outputs. In each case, you’re monitoring the voltage directly. Therefore, if it drops too far, it can alter the signal coming from the sensor and give you a false reading or create a loss of signal.

4-20mA outputs are less susceptible to voltage drop, but it is still a problem if the voltage supply isn’t high enough. Most of our sensors work on 12-24 VDC. Drop below 12 VDC and you likely won’t have enough power to operate a sensor, much less drive a signal through the wire. Conversely, if you have a particularly high load, you may need a solid 24 VDC just to get a reliable signal. Refer to the manufacturer specs to make sure you have the right power supply and resistance load for your sensor.

Resistance is inherent in any cable. In some cases, you may just have too long a cable. It’s worth knowing how far a given output signal can travel. 4-20mA outputs can travel long distances – over 1000 ft. Voltage outputs can travel around 100 ft, while millivolt per volt outputs can only reliably travel about 40 to 50 ft. Typically, 22 gauge wire is recommended.

Digital outputs travel long distances, similar to 4-20mA, and are generally not susceptible to voltage drop as long as the power supply is adequate. Generally speaking, they follow a different set of rules than analog outputs.

Noise

In scenarios where voltage drop is a concern, so is noise. Noise is a problem because it is an inductance of voltage on the wire. Voltage and millivolt per volt outputs are very susceptible to noise because the level or pressure indication is manifest by monitoring voltage in the signal cable. Any inductance of voltage from such things as power supplies, high voltage lines, electric motors, or variable frequency drives can render a voltage output useless.

The millivolt per volt (mV/V) output is especially vulnerable. The scale is so small that even a little electrical noise will wreak havoc on the signal. Light bulbs could even be a problem if they’re too close.

Noise, however, can be managed. Cable routing is extremely important. Run your cables away from sources of noise. Finally, use shielded cable and follow proper grounding practices.

If you’re unable to route your cables away from noise, especially if you need a longer cable run, shy away from millivolt outputs, or voltage outputs altogether.

Calculations and Specifications

Thankfully, we have years of scientific discovery on our side. Resistance and the voltage required to overcome it is easy to measure and calculate. We told you earlier that Ohm’s Law is represented by I=V/R, or current equals voltage divided by resistance.

Said another way, you can determine the voltage needed to power through the resistance in your line by multiplying voltage and output current, or V=IR.

Typically, however, the manufacturer has already done the math for you. Following the manufacturer specs will keep you out of hot water. Look for the supply voltage and the load ratings for that range of supply. If you can’t find it, call the support line and find out at what point the signal will drop off. Make sure you stay well north of that point.

We can provide all that information and more for our level sensors and pressure transducers. Contact us if you have any questions about how Ohm’s Law will affect your level or pressure measurement. We’ll help you understand your sensor and wiring needs.


Our pressure sensors are versatile! They work great in just about any application from general process control to heavy industrial monitoring. Check them out below:

top photo credit: TheZionView View via flickr cc

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