Basic Selection Logic for 1151 Level Sensors

1151 level sensors are typically used for liquid-column hydrostatic pressure measurement. By detecting pressure changes at the bottom or lower side of a vessel, they calculate the liquid level height. For open tanks, storage tanks, clean water tanks, sewage wells, and atmospheric process vessels, selection should first confirm whether the level measurement principle matches site conditions, rather than purchasing directly based only on the product name or historical model.

There is a clear relationship between liquid level height and pressure: provided that the medium density remains stable, every additional approximately 10 meters of water column increases pressure by approximately 0.1 MPa. In actual projects, the appropriate pressure measurement range should be determined based on medium density, upper and lower liquid level limits, installation height, and potential level fluctuations.

Xi'an Shenghongchuang Instrumentation Co., Ltd. and its production base, Shaanxi Qinkong Sensor Technology Co., Ltd., can provide pressure-type, submersible, differential-pressure and other industrial measurement and control products, as well as non-standard customization services, for conventional liquid level, high-temperature liquid level, corrosive liquid level, and sealed vessel liquid level applications. For 1151 series replacements or new projects, technical confirmation should be completed based on actual process parameters.

First Distinguish Between Open and Sealed Vessels

The liquid surface in an open vessel is directly connected to the atmosphere, and bottom pressure is mainly generated by liquid-column hydrostatic pressure. A gauge-pressure level sensor can usually be selected. Once the sensor is installed at the vessel bottom, near the drain outlet, or at the bottom pressure tapping location, it can directly measure the pressure signal corresponding to liquid level changes.

In addition to liquid-column pressure, sealed vessels are also affected by vapor-phase pressure inside the tank. If only one gauge-pressure 1151 level sensor is used, vapor-phase pressure fluctuations may be incorrectly interpreted as liquid level changes, resulting in unstable readings. This is particularly unsuitable for pressurized storage tanks, reactors, and sealed process tanks.

For sealed vessels, a differential-pressure measurement solution should be evaluated as a priority: connect the high-pressure side to the liquid-phase area at the vessel bottom and the low-pressure side to the vapor-phase area at the vessel top, thereby canceling vapor-phase pressure and obtaining the actual liquid-column differential pressure. For high-temperature steam, easily condensable gases, or viscous media, impulse lines, condensate pots, and heat-tracing conditions should also be considered.

Common Application Limits of 1151 Level Sensors

1151-type products are suitable for applications where liquid level changes continuously, medium density is relatively stable, and pressure can be effectively transmitted to the sensing diaphragm. Clean water, circulating water, hydraulic oil, light petroleum products, certain chemical liquids, and general industrial wastewater can all be matched according to material selection and measuring range.

In applications with thick foam, significant crystallization, high medium viscosity, or severe sediment accumulation at the tank bottom, the pressure tapping port may become blocked, causing gradual zero drift. In such applications, attention should not be limited to sensor accuracy; diaphragm structure, flushing connections, installation location, and subsequent maintenance conditions also require focused evaluation.

When medium density varies significantly, such as with acid and alkali liquids of different concentrations, mixed liquids, slurries, or liquids with substantial temperature fluctuations, the hydrostatic pressure conversion relationship will change. In this case, it should be clearly determined whether “liquid level height” is the control target or “liquid weight” is the management basis, to avoid mistaking density errors for instrument faults.

How to Determine Measuring Range, Accuracy, and Output Signal

The measuring range of a 1151 level sensor should not be selected mechanically according to the total vessel height. The actual measurement range should cover the highest liquid level and retain a certain margin to address liquid level surges, high-high alarms, pump start/stop fluctuations, and installation datum deviations. A range that is too small can easily cause overload, while an excessively large range reduces effective resolution.

For example, when the actual liquid level height is 0 to 5 meters of water column, it can be converted into the corresponding pressure range based on medium density and system pressure units. For water, a 5-meter liquid column corresponds to approximately 50 kPa. If a margin of 10% to 20% is considered, a standard range covering more than 60 kPa may be selected as a priority.

The accuracy class should match control requirements. Systems used only for high and low level alarms, pump interlocking, or trend monitoring generally do not require excessively high accuracy. For batching, metering, batch management, or precise liquid level control, sensor accuracy, temperature drift, installation error, and control system acquisition accuracy should all be considered comprehensively.

4-20mA Remains a Common Choice for Industrial Sites

In industrial automation applications, 4-20mA output offers strong interference resistance, long transmission distance, and convenient PLC and DCS acquisition. 4mA normally corresponds to the lower range limit, while 20mA corresponds to the upper range limit. Open circuits or faults can be distinguished from the normal signal range, making it suitable for most liquid level monitoring and control projects.

24VDC-powered level sensors have a broad application range and can be connected to control cabinets, remote I/O, variable-frequency drive auxiliary control systems, and intelligent digital display instruments. During selection, confirm the power supply range, such as the common 12V to 36VDC range, and verify the control system input impedance and allowable loop load.

Where local display is required, an intelligent digital display control instrument can be configured to provide liquid level value display, upper and lower limit alarms, relay outputs, and analog signal transmission. LED displays offer better readability in bright environments, while LCD displays feature lower power consumption. The specific choice should be determined based on control cabinet location, ambient brightness, and frequency of use.

Practical Checklist for Measuring Range Selection

First, confirm the zero reference. When the sensor installation point is below the tank bottom, installed on the side wall, or uses an impulse line for pressure tapping, the system needs to include the installation height difference in the range setting; otherwise, the displayed liquid level will have a persistent fixed deviation.

Second, confirm medium density. Water density can generally be estimated at 1.0g/cm³, but the density of salt water, petroleum products, acid and alkali liquids, and high-temperature liquids may differ significantly. When density changes from 1.0 to 1.2, pressure at the same liquid level height increases by approximately 20%.

Third, confirm overpressure risk. Pump startup, water hammer, tank pressurization, and rapid valve opening or closing may all cause instantaneous pressure surges. The sensor's rated range, allowable overload capacity, and burst pressure should all retain an engineering margin.

Matching Requirements for Medium Characteristics and Installation Structure

Components of the level sensor in contact with the medium must be compatible with the medium's chemical properties. Stainless steel wetted structures can be used for conventional water media; when handling acids, alkalis, salt solutions, chlorine-containing wastewater, or solvents, the resistance capabilities of 316L, Hastelloy, titanium, PTFE, and sealing materials should be further confirmed.

When using level sensors for strong alkalis, particular attention should be paid to concentration, temperature, and long-term immersion conditions. Some materials perform stably in low-concentration alkali solutions at room temperature, but the service life of seals and wetted diaphragms may be significantly reduced in high-temperature, high-concentration, or crystallization-related environments. Selection cannot be based solely on short-term corrosion resistance conclusions.

There is no universally fixed service life for corrosion-resistant level sensors. Medium corrosiveness, temperature, pressure pulsation, cleaning frequency, installation stress, and maintenance standards all affect the service period. With proper material matching, correct installation, and stable operating conditions, equipment can generally maintain stable operation for a long period, but regular inspection and calibration procedures should still be established.

Vibration-Resistant Installation Must Prevent Mechanical Stress From Being Transmitted to the Sensor

In pump rooms, near compressors, on mobile equipment, vibration screens, and in applications supporting large machinery, level sensors should not directly withstand continuous strong vibration. Installation brackets should have sufficient rigidity, and transmitters should not be mounted on thin plates, pipe sections prone to resonance, or locations subject to frequent impact.

For 1151 level sensors with impulse lines, pipeline fixing points should be arranged properly to prevent the weight of the impulse line from being concentrated on the instrument connection. For pipelines with significant vibration, flexible connections, vibration-damping supports, or remote mounting structures may be added, while ensuring that the pressure tapping passage is not crushed, bent, or blocked.

When submersible level sensors are used in deep wells, water tanks, and storage tanks, the probe should not swing for long periods and collide with the pool wall or tank bottom. Guide pipes, counterweights, fixing clips, and protective covers can be used to control position, while sufficient clearance should be maintained to prevent deposits from burying the probe measurement surface.

Additional Requirements for Hygienic and Corrosion-Resistant Applications

For hygienic level sensors used in food, pharmaceutical, bio-fermentation, and high-cleanliness processes, attention should be given to wetted surface roughness, cleaning compatibility, sealing dead zones, and connection methods. Where hygienic certification is involved, the specific product structure must be reviewed to confirm compliance with project requirements for material traceability, hygienic design, and cleaning validation standards.

For systems requiring CIP or SIP cleaning, the sensor should be able to withstand the corresponding cleaning temperature, pressure, and chemical media. If cleaning temperatures reach 80℃, 100℃, or higher, room-temperature selection conclusions cannot be directly applied; process temperature and the ambient temperature of electronic components must be verified.

For media containing particles, fibers, or crystals, process connection types that are less likely to accumulate material are recommended, and space should be reserved for flushing, drainage, and maintenance. After installation, one complete production cycle should be observed to confirm whether zero and full-scale outputs show abnormal changes over time.

Recommendations for Commissioning, Calibration, and Subsequent Maintenance

Calibration of high-accuracy level sensors should be based on correct installation. Sensor mounting direction, pressure tapping condition, medium inside the impulse line, supply voltage, and control system range settings all affect the final displayed value. Before installation-related factors are eliminated, simply adjusting the zero point cannot resolve long-term deviation issues.

For on-site verification, first check the zero point under empty-tank or known liquid level conditions, then conduct multi-point comparisons using a standard pressure source, liquid-column height, or an actual scale. It is recommended to verify at least the three measurement points of 0%, 50%, and 100%, while recording sensor output current, instrument display value, and system acquisition value simultaneously.

The maintenance interval for water conservancy level sensors should be established according to water quality, season, water level fluctuations, and the degree of site contamination. Clean water bodies can be inspected quarterly or every six months; for rivers, pump stations, and wastewater systems with high sediment, algae, or debris levels, inspection intervals should be shortened, focusing on probe deposits, moisture in vent holes, and cable damage.

Common Faults Should Be Troubleshot Item by Item Along the Signal Chain

When the liquid level continuously displays zero or full scale, first check the 24V power supply, power polarity, 4-20mA loop wiring, and PLC range settings. Common site issues are often not sensor damage itself, but loose terminals, water ingress into cables, excessive loop load, or incorrect analog channel parameters.

When liquid level readings drift slowly, check whether the pressure tapping port is blocked, whether the diaphragm is fouled, whether the impulse line has accumulated liquid or is leaking air, and whether medium density has changed. For sealed vessels, also check whether the low-pressure-side impulse connection is unobstructed; blockage on the vapor-phase side directly affects differential-pressure measurement results.

When displayed values fluctuate frequently, distinguish actual liquid surface fluctuations from electrical interference. Check whether the liquid level is affected by agitation, pump startup and shutdown, or liquid impact, while also checking shield grounding, the routing distance between signal and power cables, and whether the control system has an appropriate filtering time setting.

Operating Information to Be Submitted Before Purchase

To improve the accuracy of 1151 level sensor selection, it is recommended to provide vessel type, medium name, density, temperature, pressure, liquid level height, installation location, process connection dimensions, and required output signal. For replacement projects, the original instrument nameplate, wiring method, and on-site installation photos should also be provided.

If high-temperature, high-pressure, highly corrosive, flammable and explosive, or hygienic-grade conditions are involved, explosion-proof rating, wetted material requirements, ambient temperature, cleaning method, and certification requirements should also be provided. The more complete the parameters, the more effectively rework caused by incompatible connections, unsuitable measuring ranges, or incompatible materials after delivery can be avoided.

Before finalizing the solution, on-site operating parameters may be submitted to the technical personnel of Xi'an Shenghongchuang Instrumentation Co., Ltd. Professional personnel can confirm the measuring range, structural form, output method, and installation conditions of the 1151 level sensor, and provide actionable selection and supporting recommendations.