Differential Pressure Transmitter Questions

Connect a loop-powered differential pressure transmitter to a DC voltage source, a milliammeter, a 250 ohm resistor, and a diode as shown in the below figure.

Differential Pressure Transmitter

Differential Pressure Transmitter Question

Schematic Diagram

The schematic diagram shows the above figure connections in the circuit form.

2 wire Transmitter Schematic

We power-up the transmitter and it is now functioning, answer the following questions:

Question 1:

Trace the direction of current through this DC circuit (using conventional flow notation) and identify the polarity of the voltage across each component in accordance with that component’s function as either an electrical source or an electrical load.

Question 2:

Explain how to measure the transmitter’s output signal three different ways:

Measuring a voltage drop across the 250 resistor (1-5 V signal)
Breaking the circuit to directly measure current with a milliammeter (4-20 mA signal)
Connecting a milliammeter in parallel with the diode (4-20 mA signal)

Question 3:

How does an applied pressure (blowing into the plastic tube) to the “High” pressure port on the transmitter affect the electrical signal? How about an applied pressure to the “Low” pressure port?

Question 4:

While measuring current (with the milliammeter shorting across the diode), temporarily short past the 250 ohm resistor with a jumper wire. How does this affect the circuit current, and why?

Question 5:

How would the pressure transmitter respond if equal pressures were applied to both “H” and “L” ports?

Question 6:

One of the basic rules students learn when first using their multimeters is never connect an ammeter in parallel with anything, only in series. Explain why shorting across the diode is okay to do, and whether or not shorting across the resistor would be just as practical.

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