Pressure transducer, 0.5 to 4.5 V

The stainless pressure sender for water, oil and air lines: 0.5 V at no pressure to 4.5 V at full scale, from 30 psi to 1.6 MPa by model.

A steel body on a 1/8 NPT or G1/4 thread, a sealed connector, and a three-wire lead. Aliases: pressure sensor, pressure sender, pressure transmitter, water pressure sensor, oil pressure sensor, DFRobot SEN0257 (the 1.6 MPa one, with a Gravity plug).

A steel diaphragm behind the thread bends with the pressure, a strain gauge bridge on it turns the bending into a few millivolts, and an amplifier in the body turns that into 0.5 to 4.5 V. It reads gauge pressure: above the air around it, so an open port reads zero.

Pins

Pin	Wire	What it does
VCC	red	Supply, 5 V.
GND	black	Ground.
SIG	yellow (green or blue on some)	The output: 0.5 V at no pressure, 4.5 V at full scale.

Properties

model is the range printed on the body: 30 psi, 100 psi (the default), 150 psi, 200 psi or 300 psi, as the 1/8 NPT ones are sold, or 0.5 MPa, 1.2 MPa or 1.6 MPa, as the G1/4 ones are. The slider on the part is the pressure, in that unit, from zero to full scale.

What the model gets right

Most of these come without a datasheet. The numbers are the listings' and those of the branded part they copy, the Kavlico P4055 (also 0.5 to 4.5 V on a 1/8 NPT or G1/4 port):

The output. A straight line from 0.5 V at no pressure to 4.5 V at full scale: on the 100 psi model, 60 psi is 2.9 V, and on the 30 psi model 15 psi is 2.5 V.

Ratiometric. The P4055 says "0.5...4.5 VDC ratiometric": 10% of the supply at zero and 90% at full scale. On 4.8 V the zero is 0.48 V. An Arduino's converter measures against the same supply, so a sketch that works in fractions of it, (reading / 1023.0 - 0.1) / 0.8 * fullScale, cancels the supply out; one that turns the reading into volts with 5.0 first does not.

The supply. 5 V plus or minus 5%, 4.75 to 5.25 V (P4055). Outside that the model leaves SIG floating. It is not a 3.3 V part.

The resolution. One step of a 10-bit converter at 5 V is 0.122% of full scale: 0.12 psi on the 100 psi model, 1.5 kPa on the 1.2 MPa one.

What it does not model

The error: listings claim 0.5% to 2% of full scale and the P4055 gives a 2% of span error band, so a real one reads a little off, and each one's zero is a few millivolts from 0.5 V (measure it with the port open). Temperature drift, the response time (under 2 ms for the SEN0257, up to 10 ms for the P4055), what the output does past full scale (here the slider stops there), vacuum (negative gauge pressure), the proof and burst pressures, and the few milliamps it draws are not modeled.

Common mistakes

Forgetting the 0.5 V. A sketch that maps 0 to 1023 onto 0 to 100 psi reads 12.5 psi with nothing connected to the port.

Buying the wrong range. A 300 psi sender on a 50 psi house line uses a sixth of the converter's steps; pick a range a little above the highest pressure you expect.

Powering it from 3.3 V. Below 4.75 V the model leaves SIG floating, and a real one is out of its range.

See it in action

Water pressure gauge shows the pressure on a water line in psi and bar on an I2C LCD. Open it at /templates.