# how to interface a pressure meter?

Discussion in 'Electronic Basics' started by amorphia, Jun 1, 2007.

1. ### amorphiaGuest

Hi all,

I hoped someone might be able to give me some advice about how to
interface a pressure meter I have borrowed. It is a Hopkins Innovex
600D, I can't find a data sheet, but I don't think that matters as it
must operate according to some simple principles which someone here
will probably be able to tell me. It just has three wires, for output,
supply, and ground. The ratings written on it are: supply 12V, range
0-2 inches WC, and output 4-20 MA.

The bit I don't understand is this - when I have dealt with similar
sensors before, the output has been a voltage range, but I guess that
in this case MA means milliamps, i.e. a current. I connected 12V
across the ground and supply, and put my voltmeter across the ground
and the output. When I change the pressure a lot, the voltage only
changed between 6.13 and 6.15. That seems very insensitive, and given
that the output is given in current not volts, I wonder if I should be
using some other method to measure the output, i.e. one that lets
current flow, seeing as it output is given as a current. But I'm not
sure how.

If anyone can give me some advice I would very much appreciate it!

Cheers,

Ben

2. ### WaterSteveGuest

Ben

There are pressure transmitters and pressure transducers.
Transmitters are voltage devices while transducers are amp devices.
The advantage for the trasnducer is that on longer wiring runs you do
not a voltage drop due to the increased wiring resistance as the
length of the wiring run increases. The 4-20 ma is standard
operating range for these devices. Typically, the input voltage can
be 0-12 or 0-24 volts. I am not familiar with the hopkins but most
of these units are designed for a straight line response of amp vs
pressures - i.e. 4 ma = 0 Pressure and 40 ma = top end of the rated
range. Most of these untis have a safety factor of 2 to 3 to prevent
damge due to overpressure events. Given the model number is it
possible that the top end of the range is 600 PSI?

The other issue is whether it is gauge pressure or absolute pressure
transducer. If it is gauge pressure you should see a plastic vent
tube to the sensor so that the readings are normailzed to atmospheric
pressure. The result is that current atmospheric pressure conditions
would read as 0 psi; or 4 ma.

You should see a change in current output when you set your

Alternately, to get output in voltage you would need to run an
appropriately sized resistor across the output and supply leads. To
size the resisor we typically use a high quality potentiometer and
adjust until we get the proper voltage output. Seems to me the
resistor sizes out to the 7500 Ohm or is it 7500K Ohm range.

I suspect what you are measuring is the input voltage, to verify check
the your power source voltage at the source. It should read the same
as what you are measuring at the sensor. Note it is very important
that your ground is run to a ground common to the power source.

I hope this helps.

Steve

3. ### John PopelishGuest

4-20MA means that the span from 0 to 2 inches of water
column pressure is represented, linearly, by an output
current that ranges between 4 and 20 milliamps. This kind
of output is an industry standard that eliminates any errors
from wiring resistance. Typically, the current is converter
to a voltage at the receiving end by passing it through a
precision resistor, often 250 ohms, to convert the output to
a 1 to 5 volt range.
The output is trying to regulate the current and you are
connecting such a high resistance (the volt meter) that it
is saturating to full output voltage in its attempt to drive
the current through that load. Connect a 250 ohm resistor
from output to ground and measure the voltage across that load.
Do you want to connect this output to some computer system,
or just add a remote output meter to it? If the latter,
these are commercial products that include arbitrary scaling.

E.g.
http://xtronics.com/pd/pd690.htm
http://www.process-controls.com/Access_Control_Sales/eagle_process_panel_meter.htm

4. ### amorphiaGuest

I have often wondered what the difference between a transmitter and a
transducer was, and now I know

I've tested putting a resistor between ground and output, and
measuring the voltage across it, and it is working nicely. Now I will
just connect it to my A to D converter and get the reading in my PC
which is exactly what I need.

Cheers!

Ben

5. ### Bob MastaGuest

Ahhh, I think you have the terminology reversed <g>: A transducer is
a sensor or other device that converts from one variable to another,
typically from "something" (like pressure) to voltage. It's the
raw, native conversion device. It would still be called a transducer
if it converted pressure to current, or watt-seconds to coulombs, or
anything to anything else.

A transmitter, on the other hand, is a circuit that has a transducer
(often internally) as its input and "reformats" the output such that
it is a current in the 4-20 mA range. So zero output from the
transducer will give 4 mA from the transmitter output, and full-scale
from the transducer will give 20 mA from the transmitter output.

You can remember the difference by considering the names themselves:
A transducer is for changing the variable, while a transmitter is for
sending it. The standard 4-20 mA system is used for its high noise
immunity, compared to trying to send millivolt or microvolt signals
directly from a transducer.

Hope this helps!

Bob Masta

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