engel promo pack temperture guage

"Soo, what's happening is the one on the end of the long lead is also remeasuring the resistance of the lead as well as the sensor and calling the combined resistance the temperature."

Nup they use negative temperature co-efficient wire that does not include the wire length... They are reasonably accurate but not perfect. The lead inside the fridge measures inside an another in the display measures ambient...


Grrr!!! just a guide is just what you want....

Umm what is "negative temperature co-efficient wire" ?

I've just been to the Pirelli, Olex and Bambach Cables web sites and none of them mention such a substance, what is it?

For the price of these devices and the method of construction they use good old fashioned figure 8 copper wire to connect the remote probe, a couple of NTC or PTC thermistors for sensing and absolutely no compensation for cable resisitance.
Hence the introduced error for the remote probe from its own cable.

"The lead inside the fridge measures inside an another in the display measures ambient..."

Which is exactly how all of them work.

As has been stated they are a guide only.

Geoff

G'day Geff,

I don't disagree with "it's a guide only" but the statement that the wire AND the thermister's total (combined resistance) is the temperature is incorrect. Negative temp co-efficient restance wire is designed to REDUCE the cumulative effect of temperature change on the wire and what the thermister is measuring..in other words the wire is designed to reverse the effect of temperature change on the wire.

This type of wire is commonly used where thermocouples etc are in use as well as thermisters etc.

Temperature Coefficient of Copper
The Temperature Coefficient of Copper (near room temperature) is +0.393 percent per degree C. This means if the temperature increases 1°C the resistance will increase 0.393%.

See also http://www.allaboutcircuits.com/vol_1/chpt_12/6.html

http://www.designinfo.com/cornerstone/ref/negtemp.html

I don't want to go too deep in this Geoff but the cable you should be looking for is a cable rated as NTC cable (neg temp co-ef)

Lance

Hi Lance,
Now I'm with you!

I suspect you and I know as much as each other about this subject but have answered it from two entirely opposite sides.

Your talking thermocouple and thermistor compensation in the cable from an industrial perspective, we could even add 3 and 4 wire RTD's in PT100 and 1,000 if we wanted.

I answered it from the perspective of fixing my $30 engel unit last week. I dropped it and cut the remote cable off flush with the units side!
I've also fixed my mates Dicky Smith unit a few times when his kids pull the cable out of it.

I can say for absolute certainty the Engel, Dicky and probably the built to a similar price Waeco's use good old fashioned copper figure 8 flex of about 0.25sq mm for the remote probe. No compensation whatsoever installed or even attempted!

That's why I'm quite happy to state the difference in the two temperatures is the cable.

Out of curiosity mate, what do you do for a quid? What you know about this subject marks you as a rare breed in this world. Something to do with Industrial Process Control is my guess!

Geoff

Yep ya rite Geoff,

I was (now retired) an electronics tech, elec fitter mechanic, biomed tech, med electronics tech, dialysis tech... and manager of a maintenace department in 2 hospitals - one in Melb the other here in Cairns...

Cut my industrial teeth in process control with a mob called ACI Fibreglass back in the early 70's mainly with glass melting furnaces and all types of equipment associated with that..

Grrr!!!

G'day Kim
What he's saying is that if you place both sensors outside the fridge they give slightly different readings. One would think that they'd be the same, given they are measuring the same bit of air.

agree..."Use them as a guide" only

What's happening is the one on the end of the long lead, the external sensor is also measuring the resistance of the lead as well as the resistance of the sensor.
It then calls the combined resistance of lead and sensor the temperature.

The one inside the unit doesn't have the length of cable attached so its resistance is lower giving a different reading.

When the manufactured made the device they'd have known about this but decided the difference between the two readings really didn't matter.

Of course, for a device that retails for $20 to $25 both readings are most likely a bit out anyway!

Geoff