No one technology is suitable for all level applications, however, pressure based measurement is by far the most commonly used. Market studies variously estimate that it is used on between 70 and 80% of all level duties, industry wide, worldwide.
Pressure measurement offers a unique combination of features which permit simple, cost effective installation and can ensure reliable accurate performance over a wide variety of applications and operating conditions.
Sensors are offered for all tank types for both internal and external mounting. Often existing fittings can be utilised, further simplifying installation. Internal sensors may be clamped in place, suspended via a cable, or pole mounted. External sensors may be mounted via threaded or flanged connections or use any recognised sanitary fitting.
Fundamentally reliability is achieved by ensuring that an instrument is operating within it’s design parameters, by choosing the "right tool for the job". However, the basic design of any instrument has much to do with how suitable it is in the first place.
The basic sensor is an all machine welded construction. Welding technology is either pulsed arc tig or electron beam fusion. All critical welds are subject to 100% mass spectrometry analysis. There are effectively no moving parts and no contacting/wearing mechanisms. The sensing element is protected such that even under extreme overload it is not stressed beyond its design tolerances.
Where access to the sensor after installation could be a problem, all active electronics may be remotely sited at a location convenient for routine calibration checks.
The sensor body is "mass optimised" for maximum tolerance to high vibration and rapid reaction and correction of thermal shock conditions. The design has been refined and proven in many thousands of applications where these conditions routinely occur.
One of the key factors influencing an instruments accuracy is it’s sensitivity. The LVDT sensing mechanism PSM employ provides an infinitely variable output, which theoretically equals infinite resolution. In practice resolution is dependent upon the ability of the sensing element to react to applied pressure.
This is one of the key benefits of our design. On first inspection the measurement diaphragm is a simple device which gives little indication of the complex design and engineering process that has gone into it’s creation.
Chemical compatibility with a wide range of process fluids.
Thermal stability.
Linear, repeatable, and hysteresis free, deflection across it’s intended travel.
Resistance to shock overload.
Sensitivity to resolve minute changes in applied pressure.
Each Transmitter has a measurement diaphragm selected dependent upon the duty. Attached to the centre of the diaphragm is a core rod which carries a Ferrite (magnetic) core.
This ferrite core sits within a torroidal Linear Variable Differential Transformer (LVDT) which has one central Primary winding and two Secondary windings of matched characteristics.
An AC voltage is applied to the primary coil. With no pressure applied the core is in the "null" position and the outputs from both secondaries are matched. As the diaphragm is displaced by applied pressure the core moves within the LVDT and the output voltage from the rear secondary increases, while the output from the front secondary reduces, both in proportion.
Thus the differential in voltage between the two coils represents applied pressure.
This signal is then conditioned, via either integral or remote electronics, to provide a 4-20 mA signal proportional to liquid level.
The transmitter may be either Vented Gauge, Absolute, or differential type.
The differences between these types, and where each would be used is covered under a separate document available from PSM.
The standard diaphragm material is Hastelloy C276, a high nickel alloy with excellent chemical resistance properties. Hastelloy is also particularly suited for the fusion and forming techniques we use in production.
The outer convolution of the diaphragm is carefully designed to act as a "hinge", this allows expansion/contraction of the head assembly due to temperature change to be accommodated without any changes in the diaphragms performance characteristics.
In processes where extreme temperature variation may be encountered, , the DC resistance of one of the LVDT copper windings is measured using the same principle as an RTD temperature probe. The change in its value is used to correct the transmitters output signal.
The diaphragm convolutions are a trapezoidal form, this allows a much finer control over the deflection of each diaphragm section compared to the more conventional sinusoidal form.
The pitch and height of the convolutions reduce as they approach the centre. By including more metal in the outer convolutions, the diaphragm effectively has a variable rate across it’s diameter. In this way we can ensure that on full scale the "base" of all convolutions "touch down " on to the overload stop at the same applied pressure.
This provides very high overload tolerance, irrespective of the actual pressure range of the diaphragm.
The desired pressure rating is achieved by variations in the diaphragm’s size and form, the depth and size of each convolution and the thickness of material.
A complete range of diaphragms has been established which all provide a linear full scale deflection of between 0.01" and 0.02" at their "nominal pressure rating".
To the user this means that whether the measurement span is 100 metres or only a few millimetres a diaphragm of optimum range and sensitivity can be selected.
This is not the case with many "solid state type sensors", where lower range measurements are achieved simply by increasing the gain on what is basically a high range element. Inevitably sensitivity and resolution suffer.
