Determine how to drive and read an unknown three wire pressure sensor

Question

This three-wire pressure transducer pinout, drive type, and electrical specifications are unknown. It should be a typical industrial transducer. I have tried assuming it is an amplified device with a 0-5, 0-10 V output, or 4-20 mA output. No combination of wiring results in an output signal correlating to applied pressure. Edit Note: I applied 10 bar, which the Agilent 1100 HPLC system easily resolves in normal operation.

The transducer comes out of a late 1990's designed Agilent 1100 HPLC scientific pump, where it measures the output pressure up to 440 bar (6500 PSI). It is part of a pulse dampener, so I wish to keep it intact. I do not have access to the drive PCB any longer.

The markings are: Agilent Technologies Part No. 5065-9907 Revision A Date code 21/03 PA-23R / 81088 440 bar

I suspect this may be a Keller OEM transducer. Keller's currently advertised lines do not include a 23R, but they follow a similar product naming convention - PA-23R could make sense.

The transducer has three wires in a 0.100 pitch ribbon cable, so unshielded, One is black striped. It is pin 1 in the tables below.

I've monitored the drive (edited) current while powering the transducer for repetitive pulses as evidence of a microcontroller. I also monitored for pulses on the output and power fluctuations during boot time. No evidence of active electronics.

The only configuration that shows any response at all is #5 below. The output voltage decreases slightly as pressure increases. However, the output voltage depends on the supply voltage. This seems unusual if it is an amplified transducer.

Note: During each resistance check, I cycled the applied pressure between 10 bar and 1 bar (atmospheric).

The pin-to-pin resistance matrix is:

	pin 1 (black)	pin 2	pin 3
pin 1	x	1.52 MΩ	13.98 MΩ
pin 2	1.52 MΩ	x	11.12 MΩ
pin 3	13.94 MΩ	11.08 MΩ	x

The pin-to-pin voltage drop is:

	pin 1	pin 2	pin 3
pin 1	x	1.64* V	open
pin 2	0.73 V	x	open
pin 3	1.93 V*	1.72 V	x

^*Values decreased when attaching the meter.

Powering the transducer with a 15 VDC supply current limited to 60 mA gives this result set:

Config	pin 1 (black)	pin 2	pin 3	current (mA)
1	+15	0	out	43 mA
2	+15	out	0	16 mA
3	0	+15	out	22 mA
4	0	out	+15	0 mA
5	out	+15	0	0 mA
6	out	0	+15	0 mA

Answer 1

Answer

Keller 23 Connection

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References

(1) Keller 23 Datasheet

(2) Keller 23 Connection

Answer 2

1. If you carried out your measurements with the sensor under a few different pressure levels (say 0, 200, 400 bar) showing all the results could provide insight.

2. The symmetry of table 1 suggests a passive sensor (even though the high ohms values of the very same table suggests the opposite: active electronics!).

• If sensor is a variable resistor (maybe 3rd wire is a ground just for noise reduction), or, a dry potentiometer, these cases should be revealed by the measurements I suggested above.
• Ditto if sensor is capacitive (again only 2 relevant wires). Apply a sinusoidal AC to a pin and connect another pin to a resistive load and measure VAC.
• Try to learn if or which passive sensors were in use in the early 90's, especially by Agilent & Keller.

2. I didn't understand your

I've monitored the drive voltage while powering the transducer for repetitive pulses as evidence of a microcontroller.

• If sensor has active electronics with digital interface (open collector output, I2C, UART (USART) / RS-232, RS-485), ofc it would be tough to reverse engineer the RX/TX protocol but if you connected two pins to a DC supply (like you did before) and scoped the 3rd pin (first without, then with, say a 10k pull-up resistor between it and your Vcc, then a pull-down), and also making & breaking the Vcc connection a bunch of times, you might observe some digital activity.
• Again it would be useful to know which 3 wire interfaces were in use in the early 90's, especially by Agilent & Keller.

3. It's not super likely 15V would fry such a unit, but possible (unit could be a custom design for a very specific application which entailed it's being quite fragile), so it might be worth grabbing a second unit and keeping Vcc to 5V.

Answer 3

Before you do anything else, go to a local dentist office and pay them for an X-ray of the sensor, in three radial orientations, roughly 120° apart - mark it on the sensor. That should give you a good idea of what circuitry is present inside. They may need to use a 2-5x longer exposure, depending on how thick the steel is. Anyway, digital X-ray sensors are popular, so on those the results are immediate and exposure can be adjusted quickly.

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In the configurations that consume power, you’re assuming that the output is voltage. Look at it with oscilloscope: what is it? Does it oscillate? Is it stuck high or low? The output could be current, with a separate supply input. In such case, you’d want to attach a resistance from output to either + or -, and see what voltage you get.

You also should have some idea of the impedance between the pins - complex impedance, at least at a couple of frequencies, and with a couple of DC bias voltages applied. In other words, using a digital RLC meter with DC bias option. The resistances at very low voltages are informative but don’t provide much insight. It’s probably not a passive capacitive sensor, since those should have much more than MOhms between terminals. It’s also not a passive inductive sensor.

A low current curve tracer like the Mr Carlson’s Lab one would be very helpful to see what’s connected to those pins.

For identifying the common/0V pin, it’s worth checking if any pin is connected to the case, perhaps though a “solid” capacitance that would show up when measured. It’s easy to tell apart a real 1nF capacitor from a semiconductor input that happens to provide some capacitance multiplication action and mimics one - as long as you can measure complex impedance and/or can do curve tracing.

There’s nothing unusual about ratiometric outputs, and in fact those would be preferred to a calibration related to a fixed reference voltage. But I doubt very much that the “response” you obtained in config #5 is the real deal. Likely it’s a parasitic path though active electronics. I sometimes see similar behavior with resistive sensors connected to unpowered instrumentation amplifiers.