Product type
A hydrostatic level transmitter infers the level of a liquid from the pressure that liquid exerts at the bottom of the vessel. The sensing element is lowered to the bottom, or fixed through the wall close to it, and reads a pressure that rises in proportion to the depth of liquid above it. Because the measurement is a pressure rather than a distance, the instrument is indifferent to everything that troubles a non-contact type: foam on the surface, vapour in the headspace, an agitator turning in the tank and a ladder in the corner make no difference to the reading at all.
A hydrostatic level transmitter infers the level of a liquid from the pressure that liquid exerts at the bottom of the vessel. The sensing element is lowered to the bottom, or fixed through the wall close to it, and reads a pressure that rises in proportion to the depth of liquid above it. Because the measurement is a pressure rather than a distance, the instrument is indifferent to everything that troubles a non-contact type: foam on the surface, vapour in the headspace, an agitator turning in the tank and a ladder in the corner make no difference to the reading at all.
It is the type behind the simplest duties - a water tank, a sump, a fire reserve, a drainage pit, a river gauge - and for that reason it is usually the least expensive way to obtain a continuous level signal. The submersible form, with the sensor sealed on the end of a length of cable, is dropped straight into the liquid and needs no vessel modification, which is why it is the standard answer for an existing sump or reservoir.
This page covers what actually decides the instrument: why the density of the medium belongs in the calculation, what the vented cable is for, how the probe is kept out of the turbulence, and how the type is separated from the non-contact meters quoted against the same liquid.
The measurement is Archimedes in reverse. The pressure at a point below a surface equals the depth of liquid above that point multiplied by the density of the liquid and the acceleration due to gravity, so a sensor at the bottom of a vessel sees a pressure that tracks the level directly. The transmitter converts that pressure into a level once it has been told the density. In the vented arrangement the sensor is referenced to the atmosphere through a tube running inside its cable, so that changes in barometric pressure do not appear as changes in level - which is what the vent tube is for, and why blocking or kinking it produces a reading that drifts with the weather.
The consequence a buyer meets first is that the reading is a pressure, so the level it reports is only as good as the density assumed. Two liquids of different density in one tank give two different levels from one reading, and a liquid whose density changes with temperature - or a tank that sometimes holds water and sometimes a process fluid - needs that stated rather than assumed. The second consequence is that the assembly has to be airtight and correctly vented: a submersible transmitter with a blocked vent reads the headspace pressure as well as the level.
What the type does not suffer from is a line of sight or a reflecting surface. Foam, floating debris, condensation, steam and a moving or uneven surface leave the pressure at the bottom essentially unchanged, so a duty that defeats an ultrasonic or radar instrument is often settled by this one.
Most of the specification is fixed by these decisions rather than by the accuracy class. Each one constrains the next, which is why they are settled in order.
| Configuration | Options to fix | What it actually decides |
|---|---|---|
| Sensing arrangement | Submersible, with the sensor lowered into the liquid, or a transmitter fixed through the vessel wall | Whether the vessel has to be modified, and whether the instrument can be taken out for cleaning without draining it. |
| Output signal | A current output, or a serial output | Whether the reading is transmitted as an analogue level to a controller, or read digitally by a system. A serial form suits a network of tanks. |
| Range | The head of liquid the instrument is ordered for | What depth can be measured, and how much of full scale the normal level uses. A range far larger than the duty wastes resolution. |
| Cable length | Ordered with the instrument, not fitted afterwards | Whether there is enough cable to reach from the bottom of the vessel to the terminal and to be run with the slack and the gland the installation needs. |
| Wetted materials | The diaphragm and body material against the medium | Whether the instrument survives the liquid at all, which is settled before range or accuracy. |
| Compensation | Whether the reading has to correct for temperature, or for a medium whose density varies | Whether the level figure stays true across the working temperature band or drifts with it. |
| Control and indication | Whether the loop is read by a controller, an indicator, or a display with alarm and pump outputs | Whether the installation only measures, or also acts - filling, emptying or alarming on a level. |
| Electrical interface | The cable entry, the gland and the wiring arrangement | How the cable leaves the vessel, and whether the termination stays sealed in a wet or outdoor location. |
| Factor | Why it decides the answer | How to settle it before enquiring |
|---|---|---|
| Medium density, and whether it is constant | The level is calculated from pressure, so the density assumed is as much part of the answer as the pressure measured | State the medium, its density, and whether either changes - including with temperature. |
| The depth to be measured, and from where | Decides the range, and whether the reference is the vessel bottom or the instrument mounting | Give the maximum depth, and the lowest and highest levels in service. |
| Whether the vessel is open to atmosphere | A vented reference is what makes the reading a level rather than a difference between tank and atmosphere | State whether the vessel is open, vented or sealed, and where the instrument is referenced. |
| Turbulence, solids and the inlet position | An inlet discharging onto the sensor, or a stirrer nearby, produces a fluctuating and biased reading | Describe the inlet and any agitation, and whether a stilling arrangement is available. |
| Chemical compatibility | Determines the wetted materials, which rule the instrument in or out before anything else | List the medium, and any cleaning agents the instrument will meet. |
| Temperature of the medium and of the environment | Both act on the instrument, and on the density it is set to assume | Give the working and the extreme values for each. |
If any of these are still open, send what is known. A quotation can be issued against a duty description and refined once the gaps are filled.
These are the types an enquiry for a hydrostatic level transmitter usually turns out to be, and the point at which the choice is genuinely decided.
| Nearest alternative | What separates it from a hydrostatic transmitter |
|---|---|
| Ultrasonic level meters | Measures from above and never touches the medium, so it suits aggressive, hygienic or hot liquids where a wetted instrument is unwelcome. It is the one that struggles with foam, vapour and a narrow vessel, all of which the hydrostatic transmitter ignores. |
| Radar level meters | Also non-contact, and the more capable of the non-contact types in dusty, hot and vapour-laden vessels. Like the ultrasonic meter it is a distance measurement and needs a clear path, whereas the hydrostatic type needs only access to the bottom. |
| Float level sensors | A displacer on the surface, mechanically simple and unaffected by density, but limited to a point or a short span and liable to foul in dirty liquids. |
| Capacitive level sensors | A probe whose capacitance changes with the medium, useful for small vessels and for point detection. Build-up on the probe is what limits it on sticky media. |
| Differential pressure transmitters | Compares the bottom of a closed tank with the top, which cancels the headspace pressure without needing a vented cable. It is the arrangement for a sealed or pressurised vessel, where a single gauge transmitter has no atmosphere to reference against. |
| Magnetic level gauges | A local visual indication with no power, independent of density. It needs a chamber on the vessel and it gives a reading rather than a signal. |
The hydrostatic transmitter is the only one of these that reads pressure rather than distance, which is why it keeps working when the surface itself cannot be seen.
The first reply can be a quotation rather than a set of questions if the enquiry carries these. Where a figure is not known, send the duty description and we will confirm what it implies.
Available products
XK-136P · Hydrostatic Level Transmitters
Submersible hydrostatic level transmitter for water tanks, sumps, reservoirs and pits, from 1 m to 10 m as standard, with a probe rated IP68.
Also in Level Measurement
Every type we quote for under Level Measurement. A type with no link yet is still quoted for - it is described on the Level Measurement overview, and gets a page of its own as soon as a model is published against it.
Frequently asked
Because the instrument measures pressure, and the pressure at a given depth depends on how heavy the liquid is. A column of water and a column of the same height of a denser process liquid exert different pressures, so one pressure reading corresponds to two different levels. The transmitter is told a density and calculates from it, which is exact for a single medium of constant density and progressively less so for a tank that holds different products or one whose contents change with temperature. Where the density is not stable, the reading can still be used - but as a pressure, or with a second measurement alongside it.
It keeps the sensor referenced to the atmosphere. The element inside the probe measures the pressure of the liquid above it, and without a reference it would also be measuring the pressure of the air in the tank headspace and the barometric pressure of the day. The tube runs inside the cable to the terminal, where it is left open to atmosphere, so that the reading is a gauge pressure and therefore a level. If it is kinked, pinched or flooded, the instrument reads changes in the weather as changes in level - one of the few failure modes an installer can cause.
Not as a single gauge transmitter, because a sealed vessel has no atmosphere to reference against and the headspace pressure would be added to the reading. Sealed and pressurised vessels are normally measured with a differential arrangement that compares the bottom of the tank with the top, which cancels the headspace pressure whatever it is. The submersible vented form described here is the instrument for an open or freely vented vessel.
Essentially nothing, and that is the type’s chief advantage. The instrument sits at the bottom and measures the weight of liquid above it, so foam on the surface, steam in the headspace, a stirrer in the tank and an uneven or slopping surface do not change the pressure it sees. What does disturb it is the inlet: a stream discharging directly onto the probe adds velocity pressure to the static head and appears as a reading that moves with the flow. The remedy is a stilling arrangement or a different mounting point, not a different instrument.
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