measure

How to measure a 4 to 20 mA Current Signal (Ultrasonic Level Lab #5B)



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alright guys continuing on with the

ultrasonic level I believe this is part

number three so now we've got the

ultrasonic sensor mounted on the top and

we're at a level of fifty centimeters so

right fifty centimeters in our tank so

that's the highest level and so that

ultrasonic sensor has its cable going

over to this interface box right here

and we've powered that guy up with 24

volts DC we've grabbed that 24 volts DC

from our 24 volt power supply down here

and we have a link an rs-232 link right

here going to our computer and the

highest level in the tank being 50

centimeters should be the highest

current value of a 4 to 20 mili-amp

signal so we should see 20 milliamps on

the screen and in the previous video we

saw that on our Synnex view that we were

getting 20 milliamps on the screen here

excellent but how do we take a meter

reading rather than just relying on the

software here to give us our current

value well let's grab a meter so I've

grabbed a fluke 175 from the shop and

right now I'm going to walk you through

how to hook this guy up all right first

thing I want you to notice is I've got

the red lead pushed into the 400

milliamp terminal and I've got the black

lead pushed into the common terminal

we're gonna be looking at a 4 to 20

milliamp signal so we want to have a

range on the meter that's going to be

appropriate for those values okay so

we're on the 400 milliamp terminal and

the common we're going to be looking at

a current value so we're going to be

connecting up in series with that signal

so first thing we need to do is change

this guy over to look at a DC current so

this guy right here is our milliamps so

I'm going to change this setting right

here to milliamps and I'm doing this

while the circuit is dead okay I do not

have any power to my ultrasonic sensor

right now if you change that meter

setting while the circuit is live and

you are connected into that circuit you

will most likely blow the fuse on this

fluke okay so now I should be able to

see milliamps but you can see here

that I've got milliamps of AC so what I

need you to do now is I need you to

press this button right here and that's

going to change from AC to DC excellent

okay so now we're on to let's just take

a step back we're on to the 400 milli

amp terminal and the common terminal

we're looking at DC current and we've

now press the yellow button and we've

changed from the AC milliamps to DC

milliamps all right let's see how this

is connected into a ultrasonic sensor

okay so we need to look at the current

that's coming from these two terminals

right here the 4 to 20 milliamp output I

provided this with 24 volts with these

two wires right here but as we're

setting everything up I want you to have

the power off so you'll see that I have

my 24 volt power supply off I actually

have a lead that has dropped out so let

me drop that guy back in there but keep

your power supply off while you're

connecting all of your terminals in for

this 4 to 20 milliamp meter reading then

once we turn it back on we'll have to

connect back up to our ultrasonic sensor

to have an output value come in okay so

what we need to do first is we're going

to connect in a resistor so I want to

put a resistor into this terminal right

here for the positive of my 4 to 20

milliamp output so we're going to make

use of this guy in a little bit this is

a let me just Center in here come on

buddy this is a 250 ohm resistor here

and we're going to use this later on to

look at and change the 4 to 20 into a 1

to 5 volt signal but I have this lead

right here and you can see that I've

soldered in that resistor into that lead

and that way we can limit our current so

that we're not shorting out our 4 to 20

milliamp supply that one's going to

connect into our red test lead give me

two seconds we'll show you how to do

that so again just to remind you this

test lead right here has a 250 ohm

resistor in it that way we're not going

to short it our 4 to 20 supply

and the beauty of these guys is that I

can take my red test lead and I can push

it into the back of this terminal like

that and that way I've got continuity in

the circuit now okay so I have from my

positive 4 to 20 mili-amp out I've

placed a 250 ohm resistor and now I've

connected that into the red test lead of

my meter and we'll just place it down

and just make sure that it's not

touching any metal that red test lead

again is going into my 400 milliamps

terminal here and then to complete the

circuit I'm going to take the common

connection or the common terminal and

bring it up to the negative of my 4 to

20 milliamp output okay so to complete

that circuit I'm now just going to

connect the other side of the meter into

the negative terminal of my 4 to 20

milliamp so let's take a step back and

we'll see this circuit here I have the

output terminal of my 4 220 going

through a resistor just so I can not

short out my 4 220 output I have that

connected into my test lead my test lead

is going into the 400 Millie after

middle of my meter and then to complete

the circuit the common is coming up and

connecting into the negative of my 4 to

20 milliamp output beautiful ok let's

turn this guy on we'll see what our

value is on the meter okay so we've done

all of those connections with the power

off now I'm going to turn the power on

so now we've got 24 volts to the

ultrasonic sensor and now we should have

current flowing on our 4 to 20 milliamp

signal and now nice look at that meter

immediate reading we got 20 milliamps

here and that corresponds to the 20

milliamps that we saw our display ok so

let's do the same thing now we're going

to now drop the level down by 10

centimeters and we'll see what our meter

measures in current value okay so I'm

going to drop the level in the tank it's

dropping down from 50 down to 40

I'm going to stop a tray when it gets to

40 there we go nice and there's our 16

milliamps okay let me drop it down

another 10 centimeters and you can see

that the current value is directly

proportional to the level in the tank

here's my 50% range coming in right now

and at 30 centimeters I'm seeing 12

milliamps let's just move this over I'm

at 30 centimeters beautiful so these

meter readings are corresponding to

exactly with what we had on our cynics

view on the Display of our computer okay

let's drop it down another 10

centimeters now so the level is dropping

ball is dropping as well and when we get

to 20 we should see 8 milliamps okay so

there's my

drop down just a touch below but that's

pretty good right seven point nine milli

amps and let me drop it down another 10

or to get it exact I'm holding onto the

camera and trying to drop the love on

the tank at the same time so here's our

10 centimeters this is the lowest end of

our range

there's our 4 million suit okay so our

meter reading is corresponding to

exactly with what we had on the display

of our computer here you can see the

output from the rs-232 to the computer

and I have 4 milliamps here and on my

meter here I have 4 milliamps as well

so everything's giving me the same

signal now so the way that we did this

without blowing the fuse was that we

connected up a connection from the

positive output I put a 250 ohm resistor

into that circuit we're going to lose

this one later on to change that for 220

to a 1 to 5 we just use this so we don't

short out our 4 to 20 we had that going

into our meter terminal right here that

meter lead was connected into the 400

milli ampere Murrell right here the

common connection right here went back

to our supply to complete the circuit

and again we were looking at this range

right here so we're on this setting of

the meter for DC current and we have to

press this button change it from AC over

to DC current we're still in milliamps

that'll change the range one more time

there we go so there's our 4 milliamps

of DC alright guys hopefully that helps

on how to take an actual meter reading

off of this ultrasonic sensor the next

step is to try and send that signal back

into the computer and have the lab bolt

software show is the actual level in the

tank