Diffusion Silicon Pressure Transmitter

A pressure transmitter converts a line pressure into a signal a control system can read. The instrument on this page uses a diffusion silicon sensing element inside a 304 stainless steel body, which means the pressure acts on the element directly and the output is derived from the element rather than from the movement of a mechanism.

The series spans -100 kPa to 100 MPa and covers gauge, absolute and sealed gauge references. Seventeen ranges are held as stocked items from 0-5 kPa to 0-2.5 MPa, all with a 4-20 mA two-wire output, a 24 VDC supply, an M20×1.5 process connection and a 1 m cable. The wider series runs from 0-1 kPa up to 0-60 MPa and includes compound ranges that cross zero into vacuum.

The output is 4-20 mA as standard, with 0-5 V, 0-10 V and RS-485 available to order. Sealing is IP65, the stated accuracy on the stocked ranges is 0.2%FS with a stability of ±0.1%FS per year, and the media temperature range is -20 to +80°C as standard or to +150°C in the high-temperature construction.

How a diffusion silicon pressure transmitter works

The sensing element is a silicon diaphragm with a diffused resistor network on its surface. Pressure on one side of the diaphragm deflects it, the resistors change value with the strain, and a bridge circuit converts that change into a voltage. The element is a solid-state device with no lever, no pivot and no linkage, which is why a transmitter of this kind holds its calibration better than a mechanical gauge under vibration and why it can be made small enough to fit a machine port.

The diaphragm is separated from the process by a thin isolation diaphragm and a fill fluid in most industrial constructions, so the medium touches 304 stainless steel rather than the silicon. That decision is what allows the same element to be used on water, oil and gas. It also sets the limit: a medium that attacks 304 stainless steel attacks the isolation diaphragm just as it would any other part of the wetted assembly.

The output stage then converts the bridge voltage into the standard signal. In a two-wire 4-20 mA arrangement the transmitter draws its own supply from the same loop it modulates, so the current flowing in the loop is the measurement and the receiving instrument reads it directly. That is why a two-wire loop is unaffected by a small voltage drop along the cable, and why the same two conductors serve both purposes.

How to select the right transmitter

Choose the measurement range first, and choose it from the normal working pressure rather than from a peak figure. The stated accuracy applies to the full scale of the range selected, so a range several times higher than the working pressure makes every reading less precise than it needs to be. The 1.5 times overpressure figure covers short-duration spikes rather than a sustained overrange.

Then decide the pressure reference. A gauge transmitter reads relative to atmosphere and is the usual choice for a pump discharge or a compressed air line. An absolute transmitter reads relative to vacuum and is used where barometric changes matter. A sealed gauge transmitter reads relative to a sealed reference and suits a case where a vented reference would be filled with the medium.

Then settle the output and the connection. Two-wire 4-20 mA is the default and the simplest to wire. A voltage output suits a short run to a controller that takes a voltage input, and RS-485 suits a transmitter read digitally alongside others. The process connection is usually fixed by the port already on the machine, and the options here run from M8×1 to DN100 flanges with custom threads available.

Finally confirm the medium and the temperature. Water, oil and general gases suit the standard construction. A medium above 80°C needs the high-temperature version, and a medium that attacks 304 stainless steel needs a different wetted material, which is a case to raise rather than to assume.

Limitations and points to confirm

A transmitter measures pressure at one point. It does not measure flow by itself, and a pressure reading used to infer flow depends on the geometry between the tapping and the reference point, so the same instrument gives different results on a different installation. Where a flow figure has to be calculated from a differential pressure, that is a separate measurement arrangement.

The 304 stainless steel body is not suitable for every medium. Chloride-bearing water, some acids and some process chemicals attack it, and the usual sign is pitting at the isolation diaphragm rather than at the body. Compatibility is a question to settle before the order for any medium outside water, oil and general gas.

Two figures in the supplied material need confirmation and are published as open. The accuracy appears as both 0.2% and 0.5%, with every stocked item marked at 0.2%FS and the 0.5% figure described as a series option. And the reference standard behind the accuracy figure is not named, so if a particular standard has to be cited, that is a case to raise rather than to read from this page.

Several other values a specification often calls for are absent from the material and appear here as information required: the operating temperature range of the electronics as distinct from the media range, the calibration documentation, and the certification list. The pin functions also differ between the output forms, so the wiring is taken from the form ordered rather than from a single diagram.