4-20mA Pressure Transmitters for Water Level: How to Choose, Scale and Verify One in Canada

A 4-20mA pressure transmitter turns pressure into a current between 4 mA (the bottom of its range) and 20 mA (the top) on the same two wires that power it, so a PLC, SCADA input or data logger can read it over a long cable. Submerged in a well, tank or sump, a vented model works as a water level transmitter: the current says how many metres of water sit above the sensor. ERE carries the Heron 4-20mA Pressure Transmitter, which comes in four ranges from 10 m to 120 m of water. This guide covers how to choose the range, scale the signal, size the loop supply and check the readings against a tape.

In this guide

• What a 4-20mA pressure transmitter does
• Why use a 4-20 mA signal instead of a voltage output
• Transmitter, data logger or telemetry system
• How to scale 4-20 mA into a water level
• Choosing the pressure range
• Supply voltage and loop resistance
• Vented cables and what can go wrong
• Installing and verifying the transmitter
• What common reading faults mean
• Where these transmitters are used

What Does a 4-20mA Pressure Transmitter Do?

It converts a pressure into a proportional current that another device can read. At the bottom of its range the transmitter passes 4 mA, at the top it passes 20 mA, and everything in between is linear. The Heron transmitter is a two-wire design that needs external power, so one pair of conductors carries both the supply and the measurement.

On a submersible model the pressure is the weight of the water above the sensor, so the current is a direct measure of how deep the sensor sits below the water surface. The Heron unit uses a 316 stainless steel piezoresistive silicon transducer in a 316 SS and Delrin body, 22 mm (0.9 in) in diameter and 14.5 cm (5.7 in) long, on a polyurethane-jacketed vented cable (Heron datasheet A-825, Rev 2). It is rated for -20 °C to 80 °C with a two-year warranty.

Two limits define what it is. The output is analog, so the transmitter holds no memory and has no clock: whatever reads the loop has to do the logging. And it needs a reader in the first place, such as a PLC analog input, a SCADA remote unit, a panel display or a data logger with a 4-20 mA input.

Why Use a 4-20 mA Signal Instead of a Voltage Output?

Because current survives long cables and a broken wire is visible. A current loop passes the same current through every part of the circuit, so cable resistance does not change what the receiver reads as long as the supply has headroom. The USGS manual on submersible pressure transducers in water-resources investigations notes that current-mode (4-20 mA) signals are less susceptible to degradation than the more common DC voltage-mode transmissions, and offers them as one solution for the long leads that deep wells need. The ERE product listing describes the same benefit as high noise immunity on long cable runs.

Two practical reasons follow. The bottom of the range is 4 mA, not 0 mA, so a reading of 0 mA means the loop is broken rather than the water being at zero. And the signal is a process-control convention that PLCs, panel meters and telemetry units all accept; analogue direct-current signals for process control are standardised in IEC 60381-1.

The trade-off is that one loop carries one value. If you want a full time-stamped record or several parameters from one probe, a logger or a digital protocol fits better, which is the next question.

Should You Buy a Transmitter, a Data Logger or a Telemetry System?

Buy a transmitter when a controller or SCADA system needs the level continuously; buy a logger when you want a record without wiring back to a panel or an external supply.

Loggers that are not vented, such as the Solinst Levelogger 5, read absolute pressure and need a Barologger 5 to remove atmospheric pressure afterwards; the barologger explainer covers that method, and the water level data logger guide compares the logger models in depth.

How Do You Scale a 4-20 mA Signal Into a Water Level?

Subtract 4 from the current, divide by 16 and multiply by the range: height of water above the sensor = (mA − 4) ÷ 16 × range. The 16 is the span between 4 mA and 20 mA, so 4 mA is 0 m and 20 mA is full scale.

That height is measured from the sensor, not from the ground. To get depth to water you need the hanging depth, the distance from a fixed measuring point (usually the top of the well casing) down to the sensor: depth to water = hanging depth − height of water above the sensor. For a 10 m transmitter hung 8.00 m below the measuring point:

Most PLCs and loggers do this in an input scaling block: enter 4 mA as 0 and 20 mA as the range, then apply the hanging depth as an offset. To work in pressure instead, one metre of fresh water is about 9.81 kPa. The conversion assumes fresh water: a denser liquid such as saline groundwater or brine produces more pressure per metre, so a level scaled as fresh water reads high. That is one more reason to compare the result with a tape.

Which Pressure Range Should You Choose?

Choose the smallest range whose full scale is above the highest water column you expect over the sensor, with margin. Accuracy is quoted as a percentage of full scale, so a larger range gives a larger absolute error. Heron quotes its ranges in metres and feet, which for a level transmitter means height of water above the sensor. It lists four ranges, an accuracy of ±0.05% of full scale, stability of ±0.20% of full scale per year and an overpressure rating of 2X (datasheet A-825). Applied to each range:

The accuracy and stability columns apply Heron's percentages to each range; ERE calculated them, Heron does not publish them per range. Two things stand out. A transmitter in a well with a 5 m water column loses a factor of twelve in absolute accuracy if it is bought on the 120 m range instead of the 10 m range. And stability is four times the accuracy figure, so over a year of deployment drift, not the headline accuracy, is usually the larger error.

The USGS procedure for measuring water levels with a submersible pressure transducer (GWPD 16) puts the target this way: the measurement error and accuracy standard for most situations is 0.01 foot, 0.1 percent of the range in water-level fluctuation, or 0.01 percent of the depth to water, whichever is least restrictive. It also notes that the unit must be installed so the water level stays within the transducer's measurement range, and that wells with a large difference between maximum and minimum water levels may be monitored, at reduced resolution, with a higher-range transducer. On the 10 m range, ±5 mm is about 0.016 ft before drift, close to but not inside 0.01 ft, so a project working to that limit should plan regular tape checks.

What Supply Voltage and Loop Resistance Does the Loop Allow?

The loop supply must be between 7.5 V and 35 V DC, and after the voltage dropped across the receiving input and any extension cable, the transmitter must still see at least 7.5 V at full-scale current. The standard load rule is: maximum loop resistance = (supply voltage − 7.5 V) ÷ 0.020 A. The 7.5 V figure is the minimum supply in Heron's datasheet; the resistance limits below are ERE calculations from it, so ask us to confirm a loop that sits close to the limit.

The common trap is a 250 Ω sense resistor, which turns 4-20 mA into 1-5 V for a voltage input. At 20 mA it drops 5 V, so a 12 V supply leaves only 7 V at the transmitter, below the minimum, and the reading flattens near the top of the range. A 24 V supply leaves 19 V and works. The transmitter is also reverse-polarity protected, according to the ERE product listing, which forgives a swapped pair during commissioning.

Why Does the Cable Need to Be Vented, and What Can Go Wrong?

Because a vented cable carries a thin tube from the sensor's reference side up to the atmosphere, so the transmitter measures water pressure relative to the air and needs no separate barometer. The USGS states it directly: differential-pressure transducers require a vent tube from the reference port to the atmosphere but do not require a barometer for adjustment to submergence. The Heron cable is vented, and the ERE product listing describes it as providing real-time barometric compensation.

The vent tube is also the weak point. The USGS notes that vent tubes on long cables have a greater chance of clogging, and that changes in temperature, humidity and atmospheric pressure between the water level and the surface can congest the tube and let moisture travel down towards the sensor. The GWPD 16 procedure assumes that at an existing installation the vent tube is unobstructed and the desiccant is in place, includes checking the desiccant and replacing it if necessary in its visit steps, and warns that when the vent tube is inside the cable, the cable must not be pinched so tightly that the tube is obstructed. Keep the vented end of the cable dry and open to the air.

How Do You Install and Verify a 4-20 mA Level Transmitter?

Record the hanging depth, scale the input, then compare the result with a manual measurement before you leave. The GWPD 16 procedure explains why the last step is not optional: pressure transducers are subject to drift, offset and slippage of the suspension system, so readings should be checked against the water level in the well on every visit, and the transducer should be recalibrated periodically and at the completion of monitoring. It also says to make a permanent mark on the cable at the hanging point so slippage can be detected later, and to take the in-place calibration measurements to the nearest 0.01 foot. A practical sequence:

1. Pick the range. Estimate the highest water column you expect above the sensor and choose the smallest range whose full scale exceeds it with margin.
2. Hang the sensor and record the depth. Suspend the transmitter at a known depth below a fixed measuring point, write the hanging depth on the field form and mark the cable at the hanging point so slippage can be detected later.
3. Wire the loop. Connect the supply, the transmitter and the receiving input in series using the wiring diagram supplied with the unit, then confirm at least 7.5 V is present at the transmitter terminals at full-scale current.
4. Scale the input. Set the receiving input so 4 mA equals 0 m of water above the sensor and 20 mA equals the full range, then subtract the result from the hanging depth to get depth to water.
5. Compare with a tape. Measure the depth to water with a water level meter, compare it with the transmitter's calculated level and record any offset before leaving the well.
6. Re-check on every visit. Repeat the tape comparison on each visit, confirm the vent tube is open and dry, and correct or recalibrate if the difference grows.

A dedicated Heron dipper-T2 measures static depth to water and falling-head levels in wells, boreholes and standpipes, which makes it a natural tape for this check; the water level meter guide compares the options. The USGS also cautions that manual tape measurements get harder at great depth because of line stretch from weight and temperature, so use the same tape and method each visit and read the offset as a trend.

What Do Common 4-20 mA Reading Faults Mean?

Most faults trace to the loop, the vent or the scaling rather than the sensor. Use this table to narrow them down:

Where Are 4-20 mA Level Transmitters Used?

Wherever a controller needs a live level: Heron lists tanks, wells, rivers and chambers, with uses such as starting a pump on low or high water level and shutting one off on a low or high level. In environmental work that usually means a monitoring well or a sump feeding a SCADA system or a treatment-plant PLC. If you only need a long-term record with no controller, a vented data logger is simpler; if you need both, the two can share a well.

To compare the water level instruments ERE carries, browse the Water Level collection. Multi-brand supply means one quote can cover the transmitter, the tape you verify it with and the logger for the next well.

Frequently Asked Questions

How do you convert 4-20 mA to pressure?

Subtract 4 from the current in milliamps, divide by 16, and multiply by the transmitter's full-scale range: pressure = (mA - 4) / 16 x range. On a 10 m range, 12 mA is (12 - 4) / 16 x 10 = 5 m of water above the sensor.

What is a 4-20 mA pressure transmitter?

A pressure sensor whose output is a current between 4 mA (bottom of range) and 20 mA (top of range), carried on the same two wires that power it. A PLC, SCADA input or data logger reads the current and converts it back to pressure or, for a submersible model, to water level.

Does a vented 4-20 mA level transmitter need a barometer?

No. The USGS notes that differential-pressure transducers require a vent tube from the reference port to the atmosphere but do not require a barometer for adjustment to submergence. The vent tube must stay open and dry, so check it on every visit.

Can a 4-20 mA transmitter connect straight to a data logger?

Yes, if the logger has a 4-20 mA current input and the loop is powered. Heron describes the transmitter as easy to integrate with an existing SCADA system, a digital display or a third-party data logger. The transmitter stores nothing itself, so the device reading the loop must do the recording.

Related articles

• Water Level Data Loggers: Buyer's Guide for Canadian Groundwater Monitoring
• What Is a Barologger? Barometric Compensation for Water Level Dataloggers Explained
• Solinst Levelogger 5: Groundwater Monitoring Guide
• Water Level Meters: A Buyer's Guide for Canadian Groundwater Monitoring

Lire en français : Capteurs de pression 4-20 mA pour le niveau d'eau : comment choisir, mettre à l'échelle et vérifier un appareil au Canada