Siemens LOGO! 0-10V Pressure Sensor Tank Level Control: Setup and Threshold Configuration

This engineering reference covers the wiring, scaling, alarm messaging, and optional pump control for a Siemens LOGO! 8 reading a 0-10V hydrostatic pressure sensor on a 34.25-inch plastic drum. The target configuration uses an Analog Threshold Trigger (B027) and a gain of 1.38 to display engineering units in 1/10 inch, with high-level and low-level alerts at 34 inches and 3 inches respectively.

1. Application Overview and Operating Scenario

The target installation is a single plastic drum acting as a process reservoir, 34.25 inches tall and 23.25 inches wide, prefilled with fresh water to 34 inches at startup. A 0-5 psi gauge pressure transmitter mounted at the bottom of the drum outputs 0-10 VDC proportional to hydrostatic head. The LOGO! reads this signal, scales it to engineering units, and drives two alert outputs:

• Alert 1 (high level): triggered when the drum is at its fill setpoint, default 34 inches
• Alert 2 (low level): triggered when the drum is at its low-level setpoint, default 3 inches (or 2 inches for empty cutoff)

The original design included an optional pump that refills the drum when the level drops. The user later clarified that the drum arrives prefilled and that the primary deliverable is the alert messaging, with the pump as a peripheral. This reference documents both the alert logic and the optional pump control. A 1-inch deadband between the 3-inch low-level alert and the 2-inch empty cutoff prevents the alerts from chattering on turbulent or splashing conditions.

The minimum hardware is a LOGO! 8.3 base module with onboard 0-10 V analog input (e.g., 6ED1052-1MD08-0BA1) and a 24 VDC regulated supply sized for the LOGO!, the pressure transmitter, and the indicator lamps. If the pump is included, add a digital output module (e.g., DM16 24R, 6ED1055-1NB10-0BA2) to drive the pump contactor coil through an interposing relay.

2. Drum Geometry and Sensor Selection

The drum's usable level span is 34.25 inches with a 23.25-inch internal width. With a 0-5 psi pressure sensor mounted at the bottom, the maximum hydrostatic head at full level is:

P_max = 0.0361 psi/in × 34.25 in = 1.236 psi

This is only 24.7% of the 5 psi full scale, which is intentional over-ranging. The remaining 3.76 psi of headroom corresponds to 104 inches of water column before the sensor saturates, well outside the physical drum. Over-ranging protects against water hammer, foam buildup, and partial submersion of the diaphragm in turbulent conditions.

For a fresh water column at 4°C / 39.2°F, the analog signal voltages at the operating setpoints are:

At 0.5 inches the signal is only 36 mV, which is below the practical noise floor of typical industrial pressure transmitters. Do not attempt to control below 1 inch with this sensor. The LOGO! 8 analog input has a 12-bit resolution over 0-10 V, giving 2.4 mV per LSB, so a 36 mV signal is only 15 LSB - the resolution is too coarse for sub-inch control.

Mount the sensor at the lowest point of the drum, ideally in a 1/2-inch NPT boss welded or threaded into the sidewall 0.5-1 inch above the bottom. For dirty water service, install a chemical seal (diaphragm seal with fill fluid) or a purge ring. For clean water, a direct-mounted sensor is acceptable.

3. Hydrostatic Pressure-to-Level Conversion

The pressure-to-level relationship for a fresh water column at standard conditions (4°C / 39.2°F) is:

P (psi) = 0.0361 × h (inches)

h (inches) = 27.7 × P (psi)

The 0.0361 psi/in factor derives from the specific weight of water (62.4 lbf/ft³) divided by 1728 in³/ft³. For fluids other than fresh water, multiply by the specific gravity (SG):

P (psi) = 0.0361 × SG × h (inches)

If the drum holds a fluid with SG not equal to 1.000, adjust the LOGO! gain value by 1/SG. For ethylene glycol 50% (SG 1.07), use gain = 1.38 / 1.07 = 1.29. Verify the fluid's SG at operating temperature; SG typically falls 1-3% between 20°C and 60°C.

4. LOGO! Hardware Selection

Minimum hardware set:

• 1x LOGO! 8.3 base module 12/24 RCEo (6ED1052-1MD08-0BA1): includes two 0-10 V analog inputs (I7, I8) and four relay outputs (Q1-Q4). Sufficient for a single-sensor, three-output application (high alert, low alert, pump).
• 1x AM2 RTD analog input module (6ED1055-1MA00-0BA2) if more than two analog channels are needed or if the base module is a 230 V variant without onboard 0-10 V input.
• 1x DM16 24R digital output module (6ED1055-1NB10-0BA2) if more than four outputs are needed (e.g., adding a beacon, siren, or remote telemetry).
• 1x LOGO! TDE text display (6ED1055-4MH08-0BA1) optional, for operator-facing text messages and acknowledgment.
• 24 VDC regulated power supply sized at 2.5x the sum of LOGO! current draw (typically 200 mA), pressure transmitter (typically 30 mA), and alarm lamps (typically 100 mA each). A 1.5-2 A supply is typical.

The LOGO! 8 onboard analog input is single-ended, 0-10 V, 12-bit resolution (10 V maps to 1000 internal units, 2.4 mV per LSB). Refer to the LOGO! 8 System Manual for the full catalog and electrical specifications. See Siemens LOGO! 8 System Manual (SIOS 109741041) for the block reference and pinouts.

For a quick reference of the LOGO! 8.3 base module ordering data, see the Siemens product page at LOGO! Programmable Logic Controllers.

5. Sensor Wiring and Termination

A typical 3-wire industrial 0-10 V pressure transmitter has four terminals: +V (24 V supply, brown), -V (common, blue), signal out (0-10 V, black), and shield (bare or green/yellow). Wire the transmitter as follows:

Use a shielded twisted pair for the signal leads if the cable run exceeds 5 m. The shield should be bonded to ground at the panel end only; do not bond both ends (avoids ground loops that inject 50/60 Hz noise into the low-level signal). For VFD or contactor-rich cabinets, route the sensor cable in its own conduit or in a dedicated cable tray, with at least 200 mm separation from switching power and motor output cables.

Verify the transmitter's 24 VDC supply at the field end, not at the PSU. On long cable runs, the voltage drop can be 0.5-2 V, leaving the transmitter starved. The transmitter typically needs 12-30 VDC; if the supply at the field is below 14 V, increase the conductor size or install a local 24 VDC repeater.

6. LOGO! Analog Input Scaling (Gain 1.38 Derivation)

The LOGO! 8 maps 0-10 V at the analog input to 0-1000 internal counts. The Analog Threshold Trigger (B027) and the Analog Amplifier (B001) operate on these 0-1000 counts, with a configurable gain and offset to convert raw counts into engineering units.

To display engineering units of 1/10 inch (so that 3.0 inches = 30 and 2.0 inches = 20), set the gain to:

Gain = (FS_level_in_tenths_of_inches) / 1000

For a 0-5 psi sensor with a 0-10 V output, the full-scale head is:

h_FS = 5 psi / 0.0361 psi/in = 138.5 inches = 1385 (in 1/10 in units)

Therefore:

Gain = 1385 / 1000 = 1.385, rounded to 1.38

Offset is 0 for a properly calibrated sensor. If the sensor has a small zero offset (e.g., 0.05 V at empty drum), set the LOGO! offset to the corresponding negative value:

Offset = -V_zero (V) x Gain x 100 = -V_zero x 138

For V_zero = 0.05 V: Offset = -0.05 x 138 = -6.9, approximately -7. Apply a field-tuned zero by adjusting the offset until the empty-drum reading displays as 0.0 in.

Verify scaling in LOGO! Soft Comfort's online test mode (Tools -> Online Test):

• Empty drum: AI = 0 ± 5 (after zero adjustment)
• 3.0 inches: AI = 30 ± 2
• 34.0 inches: AI = 340 ± 5

If the AI reads 280 at 34 inches (a 17% low error), the sensor is reading low, which usually means a kinked reference tube, a clogged diaphragm, or a fluid with SG different from 1.000. Investigate before commissioning. If the AI reads 360 at 34 inches (6% high), the sensor is over-scaled; check the actual supply voltage (some transmitters are calibrated at 24 V but operated at 28 V, giving a 5-15% scale error).

7. Analog Threshold Trigger Configuration

Drop the Analog Threshold Trigger (B027) onto the LOGO! program canvas in LOGO! Soft Comfort V8.3 or later. The block compares the scaled AI value against the configured On and Off setpoints and drives a Boolean output with hysteresis.

Setpoint semantics: the B027 output is enabled (high) when the scaled AI value is at or above the On setpoint, and disabled (low) when the AI is at or below the Off setpoint. With On=30 and Off=20, the output is high while 20 ≤ AI ≤ 30 (i.e., while the level is in the 2-3 inch range), and low outside that band. To invert the logic - for example, to drive the output high when the level is above 3 inches and low when it drops below 2 inches - add a NOT gate (B002) after the threshold trigger.

The hysteresis between the On and Off setpoints prevents the output from chattering on turbulence or surface foam. A 1-inch hysteresis band (10 counts at gain 1.38) is appropriate for a 34-inch drum; reduce to 0.5 inch (5 counts) if the control window is tight, or expand to 2 inches (20 counts) for very turbulent service. Add an Analog Filter (B006) with τ = 2-5 s before the threshold trigger to suppress surface noise.

For the alert messaging application, configure the B027 outputs as follows:

The high-level alert is non-latching (the operator can manually start the next batch from a full-drum state). The empty-cutoff alert is latching and requires operator acknowledgment via the LOGO! cursor key or TDE button.

Reference: Siemens LOGO! 8 System Manual, Analog Threshold Trigger for the exact B027 parameter set and behavior in your firmware version (8.0, 8.1, 8.2, 8.3).

8. Pump Run/Stop Logic (Optional)

If the application includes a refill pump, wire the pump contactor coil in series with these protective elements:

1. Emergency-stop NC contact (per ISO 13850)
2. Thermal overload auxiliary NC contact (per UL 508A / IEC 61439-1)
3. Pump interlock relay NC contact (e.g., from a high-level float switch as a mechanical backup)
4. LOGO! output Q4 (relay contact) -> contactor coil A1

Use a self-holding Set/Reset (B003) block to latch the pump contactor during a brief power dip or sensor dropout. Add a watchdog timer: if the AI value does not change for more than 10 seconds while the pump is running, force the pump off and trip the "SENSOR FAULT" alarm. This prevents the pump from running indefinitely if the sensor cable is severed or shorted.

For 3-phase pumps, the contactor must be sized for the motor FLA per UL 508A. For single-phase pumps (typical for small drum applications), use a 2-pole contactor rated for the motor's locked-rotor amperage (typically 5-6x FLA for Design B motors). The pump motor's nameplate voltage, FLA, and LRA must be verified before contactor selection.

Reference: TE Connectivity - Externally Mounted Pressure Transducers for Tank Level for the sensor failure modes and how they interact with the control loop.

9. Level Alarm Messaging

Use the LOGO! Message Text block (B035) to render alerts on the LOGO! TDE or the base module's built-in display. Configure the message priority, the bar graph (optional), and the acknowledgment mode per the LOGO! 8 System Manual.

For the high-level alert at 34 inches, the hysteresis (340 on, 335 off) prevents the alert from chattering if the level hovers at 34 inches due to surface waves. For the empty cutoff at 2 inches, the alert is latched so the operator must acknowledge before the next batch can begin. For sensor faults, the alert is latched and the pump is interlocked; the operator must verify the sensor and clear the fault before the system can resume.

Reference: Texas Instruments SNAA127 - Liquid-Level Monitoring Using a Pressure Sensor covers tube-damping, noise floor, and low-level detection limits that inform the alert thresholds.

10. Commissioning Procedure

Use this step-by-step checklist at startup. Each step has a pass/fail criterion; do not proceed if a step fails.

1. Verify the 24 VDC supply at the transmitter terminals is 24 V ± 10%. Cable drop is a common cause of low AI readings. If the voltage is below 21.6 V, increase conductor size or install a local 24 VDC repeater.
2. With the drum empty, read the AI on the LOGO!. Expected value: 0 ± 5 counts. If the reading is 50 (equivalent to 5 inches), the sensor has zero-drifted; check for a clogged reference tube, water in the electronics, or a failed sensor.
3. Fill the drum to 34 inches (use a calibrated reference, such as a sight glass or a dipstick). Read the AI. Expected: 340 ± 5. If the reading is 280 (18% low), the fluid's SG is higher than 1.000, or the sensor is over-ranged. Compute SG = 1.000 x 280/340 = 0.82, which would indicate the fluid is less dense than water (e.g., oil), or the sensor is faulty.
4. Open the drain valve and watch the AI count down. The reading should change smoothly. If the noise envelope is more than ±5 counts, add an Analog Filter (B006) with τ = 2-5 s before the threshold trigger.
5. At 3 inches, the low-level alert should activate (Q2 red lamp, "LOW LEVEL - 3 in" message). Verify by listening for any alert buzzer and checking the LOGO! display.
6. At 2 inches, the empty cutoff should latch (Q3 strobe, "EMPTY - 2 in" message). Verify the operator must press the ACK button on the LOGO! TDE to clear the alert.
7. Fill the drum back to 34 inches. The high-level alert should activate (Q1 amber lamp, "TANK FILLED - 34 in" message). Verify auto-clear when the level drops below 33.5 inches.
8. Trip the E-stop and confirm all pump contactors drop out within 100 ms.
9. Pull the sensor cable at the transmitter end. The "SENSOR FAULT" alert should activate within 3 s. Verify the pump is interlocked off.
10. Document the final gain (1.38 typical, ±0.05), offset (0 typical, ±15), and threshold values (30/20/340 typical) on the panel schematic and in the LOGO! program header comment.

11. Troubleshooting Matrix

12. Safety and Maintenance Notes

Pressure sensors used in liquid level service fail in three principal modes: (1) open circuit, where the signal drops to 0 V and the LOGO! reads AI = 0 (interpretable as "empty drum"); (2) short circuit, where the signal jumps to 0 V or full scale depending on topology; (3) diaphragm rupture, where the signal pegs at full scale. The LOGO! logic should treat all three as a sensor fault, inhibit the pump, and alert the operator.

For plastic drums, use only pressure sensors with chemically compatible wetted materials. EPDM and Viton diaphragms are suitable for fresh water, sea water, and most glycol mixtures. Stainless steel 316L diaphragms are required for aggressive chemicals. PTFE-coated diaphragms are available for ultrapure water and pharmaceutical service.

Establish a proof-test interval (typically 12 months): drain the drum, zero the sensor with the offset adjustment, and verify the full-scale reading against a known reference. A 12-inch column of water should produce 0.43 psi at the transmitter, corresponding to AI = 60 in the LOGO!. If the reading drifts more than 2% between proof tests, replace the sensor.

Periodically inspect the cable gland for stress, the conduit seal for moisture, and the impulse line for sediment. For sensors mounted via a chemical seal, inspect the seal fill fluid for discoloration or air bubbles, and refill or replace as needed. If the level ever rises above the 34-inch high alarm due to a fill-valve failure, the operator should manually close the supply valve and inspect the LOGO! program for any latched faults before resuming.

Frequently Asked Questions