1. Introduction
In pressure and differential pressure transmitters such as the 1151 series, the measuring diaphragm is made of a flat metal sheet. At first glance, the pressure–displacement relationship of a flat diaphragm should be non-linear. However, under specific design and installation conditions, the diaphragm behaves in a nearly linear manner, which is essential for accurate signal conversion.
2. Conditions for Linear Characteristics
2.1 Small Displacement Relative to Thickness
When the diaphragm displacement is much smaller than its thickness (typically less than 0.1 mm in 1151 transmitters), the deformation follows Hooke’s law within the elastic range, and the pressure–displacement relationship can be considered linear.
2.2 Initial Tension During Installation
If the diaphragm is mounted under initial tension (pre-stretched), the restoring force is dominated by tensile stress rather than bending stress. This condition further improves the linearity of the pressure–displacement curve within the operational range.
3. Practical Design in 1151 Transmitters
High-pressure measurement:
A thicker diaphragm is used. Its displacement remains extremely small, ensuring linear behavior.Low-pressure measurement:
A thinner diaphragm is applied. Since thin diaphragms are easier to stretch during installation, they maintain linearity through initial tension.
Thus, whether used in high- or low-pressure applications, the flat diaphragm in the 1151 transmitter achieves good linearity by combining appropriate diaphragm thickness and installation tension.
4. Engineering Implications
Accuracy Guarantee: The linear response allows the 1151 transmitter to maintain a high accuracy level (typically 0.25% FS).
Manufacturing Advantage: Flat diaphragms are easy to machine compared to corrugated or spherical diaphragms, lowering production cost.
Reliability: The small-displacement design also reduces diaphragm fatigue and prolongs service life.
5. Conclusion
Although the flat metal diaphragm is inherently non-linear, careful control of displacement and installation tension allows it to achieve nearly linear characteristics. This principle explains why the 1151 transmitter, and many similar designs, continue to be widely used in industrial measurement applications.