Return Air Sensor probe needed

[back2space]

Can anyone source an return air sensor probefor an LG MV12AH indoor unit.

Ours has gone iffy and is out of calibration by about 2c.

Have been quoted £13 for the one sensor from one place... seems excessive.

[cadillackid]

sounds about right.. surprised that you can buy just the one sensor.. for the Chigo and grunaire units you have to buy the complete sensor pack which is 4 sensors wired to a connector and its $145 USD....

you can measure the resistance of a good sensor but i think LG uses standard 5k thermistors(typically 5k at 25 degrees C) which can be had pretty readily....

-Christopher

[back2space]

sounds about right.. surprised that you can buy just the one sensor.. for the Chigo and grunaire units you have to buy the complete sensor pack which is 4 sensors wired to a connector and its $145 USD....

you can measure the resistance of a good sensor but i think LG uses standard 5k thermistors(typically 5k at 25 degrees C) which can be had pretty readily....

-Christopher

Where can you get these from.

THe part numbers are

6323a20004a or 6323a20004t

Both compatible with my unit as I just need the sensor.

[CELSIUS]

Are all wiring connections at the sensor in good condition?
No bad connections??

Measure the resistance of the sensor at a given temperature, ( say it reads 3kOhm @ 20 C ).
Also move the wires gently ,to confirm tight connections, whilst measuring the resistance with the Ohm-meter.

Keep the Meter on the probe-terminals and place the sensor probe on the skin, between your thumb and finger,check the reading--

The higher skin-temperature will result in a lowering of the resistance in the probe.
A small resistor
( say 100 Ohms --0.5W--) can be placed in series with the sensor,
this will increase the resistance value of the sensor,

it would now be reading 3100 Ohms @ 20 C Temperature.
Of course,this procedure will only increase the value of the sensor.

If the Air con is in the Cooling-mode, the warmer return-Air is passing over the 3.1 kOhm sensor and the unit will now switch off earlier, at a higher temperature than before. The return-air is now less cooler than before.

The Sensor has now been a few degrees adjusted.

To find the correct resistor-value, see the sensor temp-resistance-chart.
If no chart available, measure the change of the resistance-value of the sensor, when the temperature changes by exactly 2 degrees.

That would be the value for the resistor to fit.

[Tesla]

Hi Back2space
Listen to Christopher above and buy a generic 5kohm thermistor sensor as suggested. You could also buy a potentiometer to connect for help in calibration of the new or old sensor. Each component should cost not more than 2 pounds from your local electronics supplier.

[cadillackid]

celcius: I "adjust" thermistors with resistors all the time however it also changes the logarithm of the sensor itslef.. as the resistance you add or parallel to a sensor stays constant and does not change with temperature.. so in effect yes you can "re-calibrate" a thermistor with resistance you can also make a mess too as the resistance will always be a different percentage of the total value than before...

take for instance a thermistor that reads 5000 Ohms at 27 degrees C. when the sensor should read 5300 Ohms at 27 degrees C.. so you solder in a 300 Ohm resistor in series so that the sensor appears as 5300 Ohms and controller sees it at 27 degrees C as it should....

you have effectively adjusted that sensor 6% to be dead on at 27 degrees C.

now the temperature rises to 40 degrees and at 40 degrees the total resistance should be say 2500 Ohms...

at 2500 Ohms you are effectively "adjusting" that sensor by 12%..

why?? because that 300 Ohm resistor did not change with the temperature it remained the same whereas the thermistor value got smaller....

just FYI
-Christopher

[back2space]

Hi Back2space
Listen to Christopher above and buy a generic 5kohm thermistor sensor as suggested. You could also buy a potentiometer to connect for help in calibration of the new or old sensor. Each component should cost not more than 2 pounds from your local electronics supplier.


Thanks cadillackid & tesla & the rest of the guys for the comments.

Think I might buy a generic sensor as suggested. One thing thats confusing me though is I dont know if the LG stuff is 5kohm, I think the coil and pipe sensors are 5kohm but I think the return air sensor is 10kohm?

Can you confirm for me by looking at this link and fault code 1... page 3.

http://www.mylg.co.uk/data%20for%20site/Multi%20F_dx_%20Fault%20Codes%20-%20Nov%2007.pdf

At 25C it looks like it should be 10kohm? Whereas if you look at the next page for the coil thermister it reads 5kohm at 25C?

How would I use a potentiometer? I dont actually have any equipment to check the kohm or the voltage as I am not an engineer so would I need one of these. Surely if I buy a generic one it should be calibrated properly or could I calibrate the existing one with the potentiometer?

Once again thanks for the great advice so far :)

[cadillackid]

indeed looks like the air thermistors on thjese are 10k where as some of the pipes are 5k and one is even 200k... differenmt thermistors are best for different ranges so makes sense... the one you are after is likely 10k..

also remember on these types of units that startup and shutdown do not always occur at the setpoints... for instance a High wall split is likely to stop cooling when its return air temperature is 1 or 2 degrees below setpoint... and stop heating at 3 or 4 degrees above setpoint..

why? because they care calibrated for where the occupants are... your wired remote will display the room temp as the thermistor sees it not as the occupants NECESSARILY do... oversized units are likely to overshoot their setpoints easier... even with inverters.. as the controller usually takes its time to ramp up or down the inverter unless it hits an "out-of-bounds" property like too high coil temp in heat or below freezing in cool mode...

some indoor units are design to be multi-position mounted and will have settings to change to maximize comfort level.

and other units are designed to have remote sensors attached and will have controller settings as such for them to work properly.. all depends on the units...

basically what im saying ois you may not necessarily have a bad sensor but just that you dont like how the unit operates... and the tolerances of thermistors is fairly high... the cheapies used in most splits are not precision so they can keep costs down.. and may have a 5% variance either way from actual temperature....
-Christopher

[back2space]

indeed looks like the air thermistors on thjese are 10k where as some of the pipes are 5k and one is even 200k... differenmt thermistors are best for different ranges so makes sense... the one you are after is likely 10k..

also remember on these types of units that startup and shutdown do not always occur at the setpoints... for instance a High wall split is likely to stop cooling when its return air temperature is 1 or 2 degrees below setpoint... and stop heating at 3 or 4 degrees above setpoint..

why? because they care calibrated for where the occupants are... your wired remote will display the room temp as the thermistor sees it not as the occupants NECESSARILY do... oversized units are likely to overshoot their setpoints easier... even with inverters.. as the controller usually takes its time to ramp up or down the inverter unless it hits an "out-of-bounds" property like too high coil temp in heat or below freezing in cool mode...

some indoor units are design to be multi-position mounted and will have settings to change to maximize comfort level.

and other units are designed to have remote sensors attached and will have controller settings as such for them to work properly.. all depends on the units...

basically what im saying ois you may not necessarily have a bad sensor but just that you dont like how the unit operates... and the tolerances of thermistors is fairly high... the cheapies used in most splits are not precision so they can keep costs down.. and may have a 5% variance either way from actual temperature....
-Christopher

Christopher

I understand what your saying, all my units heat or cool to +/- 1C so they will switch on or off when the temp is within the limits.

The sensor does seem to be duff because we swapped it with one from another unit and the problem followed the sensor

Yes we can adjust the remote control set point to counteract the difference but its annoying to have to explain to family that they have to set it to 24C to get 22C otherwise it will cool to 19C instead of 22C.

We also measured temps in various parts of the room and whilst there are slight variations in temp in parts of the room such as floor level or corners of the room the temperature at the remote sensor mounted 1.3ft up the wall should be that what is set on the unit. We put a digital thermometer next to the sensor and the thermister appears to be nearlly 3C out.

Out of interest what are Mitsubishi Electrics return air sensors are they 5kohm? We are using the remote sensor casings of Mitsi as they are better than the lg ones (more air holes) however we removed the thermister from inside it and replaced with LGs as the mitsi ones didnt appear to be compatible?

How can you tell from those graphs if its a 10 a 5 or a 200k I need? Do you go by the value of the units in the kohm up the side of the graph? The return air sensors on the indoor and outdoor units increase by values of 2.

Im confused now.

[Tesla]

Hi Back2space
I recommend you just measure a good sensor of the same type with a good meter like a fluke at 21 or 25degC - it will be very close to 10 or 5 kohm and replace with a generic. You could use a pot to callibrate but it is differcult and not very accurate as Christopher said (due to shifting of the zero point and range of measurement).
You can also buy a what is called a resistance wheel for about 5 pounds - use this as a tool to prove your measurements and help diagnose. It will have resistances of say 1, 2, 5, 10...3k, 5k, 10k, 20k. Keep in mind the sensor may be located in the wrong place.

[back2space]

Well... an update:

Got a new return air sensor from LG comfort cooling. Paid about £10 for it.

Does pretty much exactly the same, surely 2 sensors in a row are not dodgy.

We have tried moving the sensor to different places in the room and its always about 2C out. So if set to 22 it cools the room to about 20-20.5C and then it begins cooling again at about 21.2.

Thing is when we check the temperature where the sensor is it we know that it is out. And when we swap it with one from another room the problem follows the sensor.