Danfoss Level transmitter AKS 41 Installation guide

Brand: Danfoss

Model: Level transmitter AKS 41

Type: Installation guide

RZ.5K.A1.02 09-1998

Service Instructions

Liquid level transmitter

Type AKS 41

2 Service instructions RZ.5K.A1.02 © Danfoss 9/1998 AKS 41

Contents

Functional principle ................................................................................................................... 3

Operation ................................................................................................................................... 4

Component positions ...................................................................................................... 4

Light-emitting diodes (LED) ............................................................................................ 4

Pins for measuring test voltage....................................................................................... 4

Pins for calibaration of 4 - 20 mA signal .........................................................................4

Programming button ....................................................................................................... 4

Setting of refrigerant................................................................................................................. 5

Setting of signal damping ......................................................................................................... 6

Calibration ................................................................................................................................. 7

Calibration to NH

prior to installation in plant................................................................ 7

Calibration of plant (all refrigerants) ............................................................................... 8

Trouble-shooting......................................................................................................................10

Version number .......................................................................................................................11

AKS 41 Service instructions RZ.5K.A1.02 © Danfoss 9/1998 3

When the example is transferred to the capacitive

rod, the outer tube constitutes the one plate and

the centre electrode the other plate.

Outer tube of stainless steel

Centre electrode of ordinary steel

The centre electrode is hermetically sealed in a non-

conducting PTFE material (Teflon)

Gap between the outer tube and the insulated centre

electrode where the liquid can rise.

AKS 41

The capacitive rod can measure the level of as well conducting as non-conducting liquids.

If the rod is partly immersed in a conducting liquid, e.g. ammonia:

On the part of the rod that is surrounded by liquid, the outer tube may be regarded as short-

circuited to the surface of the Teflon insulation. This will produce an effect as if the distance

between the outer tube and the centre electrode has become smaller which will cause an

increase of the rod’s capacitance. This change of the capacitance is registered and converted

into a signal current between 4 and 20 mA.

If the rod is to measure the level of a non-conducting liquid, e.g. R22:

When the liquid rises between the outer tube and the inner electrode, it will displace the gas

otherwise present. As the liquid has a higher dielectric constant than the gas, a rising liquid

level will be registered as an increase of the rod’s capacitance and hence result in a higher

output signal.

Functional principle Liquid level transmitter type AKS 41 is built up of a capacitive rod with a belonging electronic

transmitter mounted on top of the rod.

As the name implies the basic function of the rod may be compared to the function of the

electronic component called a capacitor. When the size of such a capacitor has to be

indicated we refer to its capacitance, and this is the size that is measured when a capacitive

rod is used for registration of a liquid level.

Example

The simplest form of electric capacitor consists

of two conducting plates insulated from each

other by some kind of material which in this

illustration is free air.

4 Service instructions RZ.5K.A1.02 © Danfoss 9/1998 AKS 41

Operation Component positions

The following visible components are placed

below the cover:

a green LED

a red LED

two pins for calibration of 4 - 20 mA signal

a programming button (at the bottom of a tube)

two pins for measuring test voltage

Light-emitting diodes (LED)

By observing if the green and the red LED are ON, flashing or OFF, you can obtain information

about the liquid level transmitter’s operating conditions.

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*) As long as the programming button is not activated, the green LED will flash. When the programming

button is activated, the LED will proceed to follow the activation. In other words, the LED will be ON

whenever the programming button is pushed.

Pins for measuring test voltage

By connecting a voltmeter to these pins a volt-age can be measured which is an expression

of the “rough” signal measured directly from the rod. On a sign placed inside the liquid level

transmitter housing two voltages are shown that originate from the manufacture of the liquid

level transmitter:

- Voltage at 0%, i.e. the rod is completely out of the liquid;

- Voltage at 100%, i.e. the rod is completely immersed in the liquid.

In a trouble-shooting situation you can decide, by measuring the voltage between the test

pins, whether the mechanical part of the rod is in order. If a voltage is measured which is

within the range between the two values shown on the sign, the capacitive rod itself (outer

tube, insulation, inner electrode, wire connections, etc.) is in order.

Pins for calibaration of 4 - 20 mA signal

Used for calibration. For further details, read the section “Calibration”.

Programming button

There is a pushbutton at the bottom of the small tube. Use a pointed object if it is to be

activated.

AKS 41 Service instructions RZ.5K.A1.02 © Danfoss 9/1998 5

Setting of refrigerant The liquid level transmitter is factory calibrated for four different refrigerants:

, R22. R404A and R134a (R134a as from version 2.0).

If it is to be used for one of these media, you therefore do not have to carry out a calibration,

but merely set the required refrigerant.

On being delivered it has been set for NH

, so if it has to be used for this media, it is ready for

mounting without any form of setting or calibration.

Setting of refrigerant is carried out, as follows:

1. Remove the connecting plug, so that there is no voltage on the liquid level transmitter

2. Push the programming button and keep it down while the connecting plug is being

mounted. By keeping the button depressed in this way while the supply voltage is being

connected, the transmitter will move into a special mode where you can select the

refrigerant. When the transmitter is in this mode, the red LED is flashing.

3. Let go of the button.

4. You can now select the refrigerant by pushing the programming button a number of times:

- Push it once to select NH

- Push it twice to select R22

- Push it three times to select R404A

- Push it four times to select R134a

(The green LED is lit when the button is pushed).

5. Wait ten seconds until the green LED starts flashing again. The new selection of refrigerant

has now been saved in the memory.

6. Interrupt the supply voltage and connect it again to terminate this special setting mode.

Control and useful hints

If you are not quite sure whether you have set the correct refrigerant, you can control the

setting by measuring the signal current from the transmitter.

1. Connect a measuring instrument (mA measuring range) between terminals 2 and 3 on the

liquid level transmitter’s connecting plug.

2. Put the liquid level transmitter in the special mode where you can set refriger-ants, as

described under pts. 1 to 3 in the previous section (keep the programming button down

while the connecting plug is being mounted).

3. During the first 10 seconds, where the supply voltage is connected, the output signal will

indicate the refrigerant that has been set:

- 5 mA if the liquid level transmitter is set to NH

- 6 mA if the liquid level transmitter is set to R22

- 7 mA if the liquid level transmitter is set to R404A

- 8 mA if the liquid level transmitter is set to R134a

10 seconds after the supply voltage being connected, the output signal will adjust itself to 4

mA.

• It is perfectly all right for you to choose to have the measuring instrument connected while

you set the refrigerant, so that you may in this way better follow what is going on.

• If you have somehow got the setting messed up and you are not quite sure how far you have

come, you may always start all over again by removing the supply voltage (the connecting

plug).

• If the wrong refrigerant has been set, a new setting can obviously be carried out, even if the

transmitter has already been mounted in the system and the rod is surrounded by liquid.

• How to set the refrigerant is also described in the instructions accompanying the liquid level

transmitter.

6 Service instructions RZ.5K.A1.02 © Danfoss 9/1998 AKS 41

Setting of signal damping Boiling liquid has an undefined surface which may produce an unstable level signal. To

reduce disturbances from here to a minimum, an adjustable damping of the output signal has

been built into the liquid level transmitter.

The damping is set as a reaction time and the setting range is from 1 to 120 seconds. The

factory-setting is 15 seconds. To give you an impression of the effect, the reaction time can be

described as the time it takes a level signal to adjust itself to a new value, if there is a sudden

leap of the level.

Setting of signal damping can be performed while the transmitter is operating. The 4-20 mA

output signal is not affected while setting is being done, so you need not interrupt the

connected equipment for level control and monitoring.

Setting is simply carried out by your pushing the programming button as many times as the

number of seconds you want the reaction time to be. If you for instance require a reaction time

of 27 seconds, you push the programming button 27 times.

At the first push of the button the green LED stops flashing, and it will now only light up when

the button is pushed. When the programming button has not been activated for 10 seconds,

the newly set signal damping value will be saved in the liquid level transmitter’s memory, and

the green LED will resume its normal flashing pattern.

Whenever you atart a new setting, start over again with a 1 sec. reaction time.

If you wish to increase the reaction time from 15 seconds to 25 seconds, for example, you

cannot make do with 10 pushes, you will have to push the button 25 times.

The setting of signal damping is also described in the instructions accompanying the liquid

level transmitter.

Level

4-20 mA

output signal

Reaction time

Programming button

Green LED

10 sec.

AKS 41 Service instructions RZ.5K.A1.02 © Danfoss 9/1998 7

Calibration In a calibration of the transmitter you set the two outer limits of the measuring range, i.e. the

point on the rod where the output signal is to be 4 mA, and the point on the rod where the

output signal is to be 20 mA.

The active measuring range starts 35 mm from the end of the rod and ends 45 mm from the

top (gasket’s contact face). The liquid level transmitter has been factory-calibrated to NH

, so

that the entire measuring range is utilised, in other words, the transmitter’s output signal is 4

mA when the rod is out of the liquid, and 20 mA when the liquid is 45 mm from the top of the

rod.

If the transmitter is to be used for R22, R404A or R134a, you need not carry out a recalibration,

just set the required refrigerants as described in section “Setting of refrigerant”.

A calibration of the transmitter is only necessary in the following cases:

- if you wish to narrow down the rod’s measuring range, so that the 4-20 mA range

corresponds to a smaller part of the rod’s length

- if it is to be used for another refrigerant than NH

, R22, R404A or R134a.

Calibration to NH

prior to installation in plant

If the liquid level transmitter has to be calibrated (to narrow down its working range), it would

be a good idea to do it in water prior to mounting it in the system. As water tank can be used a

pipe which is closed at one end. As the case is with NH

, ordinary water has a certain

conductive power for electric current. That is to say, if the transmitter is set to NH

(cf. section

“Setting of refrigerant”) and a calibration in water is carried out, the transmitter will measure

correctly when subsequently mounted in a container with NH

Calibration of lower limit of measuring range (4 mA point)

1. Prepare for the liquid level transmitter to be placed in water.

2. At one end of the calibration pins there is a small shorting strap. Remove this strap, placing

it over both pins simultaneously, so that they are short-circuited. The liquid level transmitter

is now in calibration mode which is indicated in that the green LED is OFF and the red LED

flashing.

3. Bring the liquid to the level corresponding to the lower limit of the measuring range, i.e. the

point on the rod where the transmitter’s output signal is to be 4 mA. You will typically want to

use the full length of the rod, and in that case a calibration must be made of the lower limit

of the measuring range, with the rod completely out of the liquid.

4. Push the programming button once and release it within three seconds. The green LED will

be alight when the button is pushed.

5. Wait 10 seconds.

The new calibration has now been saved in the memory. Remember to remove the shorting

strap from the two pins, if you are not to continue with calibration of the 20 mA point.

Calibration of upper limit of measuring range (20 mA point)

Normally this calibration is done in continuation of the calibration of the 4 mA point. If this is

not the case here, carry out pts. 1 and 2 first.

6. Bring the liquid to the level that is to correspond to the upper limit of the measuring range,

i.e. the point on the rod where the transmitter’s output signal is to be 20 mA. You will

typically want to use the full length of the rod, and in that case a calibration must be made of

the upper limit of the measuring range at a liquid level 45 mm from the top of the rod

(measured from the gasket’s contact face).

Measuring range

Output signal

Level

8 Service instructions RZ.5K.A1.02 © Danfoss 9/1998 AKS 41

7. Push the programming button and keep it down until the green LED goes out which will

happen after about 30 seconds. (It is important that the button is released not later than

three seconds after the green LED goes out).

8. Wait 10 seconds.

The new calibration has now been saved in the memory. Remove the shorting strap from

the two pins and check that the transmitter’s output signal is 20 mA. The shorting strap is

stored by placing it over one of the pins.

N.B.

You may calibrate the lower limit of the measuring range (4 mA point) without at the same

time calibrating the upper limit (20 mA) and vice versa. However, be aware of the following:

When you calibrate the lower limit of the measuring range, it is in actual fact the zero-point of

the built-in signal converter you are adjusting.

When you calibrate the upper limit of the measuring range, it is in actual fact the width of the

measuring range (span) of the built-in signal converter you are adjusting.

Consequently, if the lower limit of the measuring range (4 mA point) is moved, the upper limit

(20 mA point) will automatically be moved a similar distance.

Calibration of plant (all refrigerants)

If the transmitter is to be calibrated in the plant it may however be difficult to get the rod right

out of the liquid, and most often impossible to have the liquid reach the upper end of the rod. It

is furthermore a problem to ascertain the liquid level, when the level signal from the liquid

level transmitter cannot be used. If you are lucky, you may have a short gauge glass to help

you, but they hardly ever cover the lower and uppermost parts of the rod.

In short, it is impossible in practice to carry out a calibration of the

extreme points

of the

measuring range, if calibration is to be made with the transmitter mounted in the system.

But a calibration does not either necessarily have to take place at the extreme points of the

measuring range (corresponding to the 4 mA and the 20 mA points). The relation between

signal current and liquid level can be shown as a straight line - the so called liquid level

transmitter working line. In order for you to draw a straight line you must know at least two

points traversed by the line, but these two points need not be the end points of the line.

The calibration function has been designed to take advantage of this possibility. Instead of

calibrating the lower working point to 4 mA, it may be calibrated to 6, 8 or 10 mA, and instead

of calibrating the upper working point to 20 mA, it may be calibrated to 12, 14, 16 or 18 mA.

In practice it works, as follows. (It is assumed here that the transmitter has been put in

calibration mode by short-circuiting the two pins with the supplied strap, as described earlier,

and that a voltmeter has been connected to output terminals 2 and 3):

When the programming button is kept down, the 4-20 mA signal will be increased in 2 mA

steps, where each step is maintained for four seconds. If the programming button is released

at a certain step, the actual liquid level will be calibrated to the value the current had reached.

If the programming button is released when the current has reached 4, 6, 8 or 10 mA, this will

be understood as a calibration of the lower point of the working line.

If you wait releasing the programming button until the current has reached 12, 14, 16, 18 or 20

mA, this will be understood as a calibration of the upper point of the working line.

Signal current

terminals 2 - 3

AKS 41 Service instructions RZ.5K.A1.02 © Danfoss 9/1998 9

Example:

A 1200 mm long liquid level transmitter is to be calibrated, but it is fitted in a system where the

liquid level cannot be brought out to the extreme ends of the rod.

The lowest liquid level that can be obtained is about 100 mm, measured from the end of the

rod, and the highest liquid level that can be obtained is about 950 mm, measured from the end

of the rod. The transmitter must be calibrated in such a way that it will give 4 mA, if the rod is

completely out of the liquid and 20 mA, if the liquid level is right up at the top of the rod.

In the calculations below it should strictly speaking be taken into account that the rod’s

measuring range is not 1200 mm, but only 1120 mm, because the lowermost 35 mm of the

rod and the uppermost 45 mm do not count, but in this example we have decided to disregard

this small inaccuracy.

A change of the liquid level of 1200 mm will thus give a change in the signal current of

20 -4 = 16 mA.

Consequently, a change of the signal current of 1 mA corresponds to a change of the liquid

level of 1200/16 = 75 mm.

A signal current of 6 mA will thus correspond to a liquid level of 2 x 75 = 150 mm, and a signal

current of 16 mA will correspond to a liquid level of 12 x 75 = 900 mm (the figures 2 and 12

appear, because the signal current starts at 4 mA).

As the liquid level could be set manually from about 100 mm and up to about 950 mm, it is

therefore possible in this example to calibrate the liquid level transmitter at the 6 mA point and

at the 16 mA point.

The calibration is carried out, as follows:

1. Connect a measuring instrument (mA measuring range) between terminals 2 and 3 on the

transmitter’s connecting plug and remove all other connections.

2. Put the transmitter in calibration mode by short-circuiting the two pins by means of the

belonging strap, as described earlier.

3. Have the liquid level adjust itself to 150 mm above the lower end of the rod.

4. Push the programming button and keep it depressed. When the current, as read on the

measuring instrument, has reached 6 mA, let go of the button.

5. Wait 10 seconds until the current has again adjusted itself to 4 mA. The lower calibration

point on the working line has now been saved in the memory.

6. Have the liquid level adjust itself to 900 mm above the lower end of the rod.

7. Push the programming button and keep it depressed. When the current, as read on the

measuring instrument, has reached 16 mA, let go of the button.

8. Wait 10 seconds until the current has again adjusted itself to 4 mA. The upper calibration

point on the working line has now been saved in the memory.

9. Remove the shorting strap from the two pins and check that the signal current adjusts itself

to 16 mA. Remove the measuring instrument and re-establish the connection to the

remaining equipment.

(End of example)

If the upper calibration point is sought calibrated at a lower level, it will be regarded as an

error, and the red LED will flash quickly. At the same time the new value of the upper

calibration point will not be saved in the memory. In other words, even if the calibration is

apparently carried out, it will still be the lower calibration point plus the original span that will

be valid as the upper calibration point.

Output signal

Level

Danfoss A/S, AG-METM

Trouble-shooting What is the error, if the output signal does not match the actual liquid level in a tank or

stand-pipe?

First of all it is important to establish that the problem is really due to the transmitter, and not

to the connected equipment. Any trouble-shooting in connection with the liquid level

transmitter’s function should therefore be initiated with the following general check-up:

1. Dismantle the cap on the connecting plug and pull it back a little on the cable, so that there

will be access to the four screw terminals inside the plug where the four wires are

connected.

2. Now measure the resistance in the outer current loop between terminals 2 and 3 (the plug

must not be mounted on the liquid level transmitter). If the resistance is greater than 500

ohm, check for disconnections or excessive resistance in the connected equipment.

3. Mount the inner part of the plug on the liquid level transmitter again, so that the normal

connection between the transmitter and the connected equipment is re-established.

4. Measure the supply voltage directly on the plug’s screw terminals marked “1” and “earth”.

Check that the voltage is between 20 and 30 V a.c. or between 18 and 48 V d.c. If OK,

proceed with the next point. If this is not OK, look for the cause in the transformer and its

connections.

5. Dismantle the wires in the plug’s terminals 2 and 3, so that any errors in wires or external

equipment will not interfere with the measurement of the signal current. Now measure the

signal current directly between the plug’s terminals 2 and 3 with a measuring instrument. If

no current can be measured, check that a fuse has not blown in the measuring instrument.

This is often the case with a service instrument.

6. If you still cannot measure any current matching the actual liquid level, proceed with the

trouble-shooting procedure below comprising five different descriptions of errors:

A. A signal current between 4 mA and 20 mA is measured, but the current does not match

the actual level.

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B. No signal current is measured (0 mA)

C. The signal current is constant (2 mA) and the red LED is ON

(applies only to versions older than 2.0)

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D. Signal current is constant (4 mA)

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E. The signal current is constant 22 mA and the red LED is on.

(applies only to versions older than 2.0)

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Version number

The transmitter’s version number appears from a production code printed on a sign inside

the transmitter housing.

In the example below the code is shown for a transmitter version 1.4, made in the second

month of 1998 with serial number 15, consecutive number 23.

consecutive number

serial number

production year

production month

product type - code number

version number

AC-RDT

Danfoss can accept no responsibility for possible errors in catalogues, brochures and other printed material.

Danfoss reserves the right to alter its products without notice. This also applies to products already on order provided that such alternations can be

made without subsequential changes being necessary in specifications already agreed.

Danfoss Level transmitter AKS 41 Installation guide

Danfoss Level transmitter AKS 41 Installation guide

Danfoss Level transmitter AKS 41 Installation guide

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