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Old 12-09-2023, 06:51 PM   #1
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Maretron TMP100 inaccurate temperature readings

Anyone using the Maretron TMP100 temperature sensor with the ambient sensors? I'm trying to monitor my refrigerator/freezer temperatures and am getting very inaccurate temperatures which are off about 9-14f. I've extended the probes with about 10 ft of AWG18 wire which according to the resistive table shouldn't impact the reading that much. The TMP100 is measuring my refrigerator at 48f/9c while the actual temperature is 37f/3C. According to the resistive temperature table 3C should be 8415 ohms versus 9C 6264 ohms (see attached screenshot). It seems the table is backwards as if I put a resistor across the temperature probes is raises the temperature rather than drop it.

The easy answer is if Maretron had a calibration offset configured in the firmware but this would be too easy. Anyone using the Maretron TMP100 and figure out a way to get more accurate readings? I could use large size wire but not sure this will solve my problem.
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Old 12-09-2023, 09:33 PM   #2
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Is it possible you are connected to one of the first two ports on the TMP100? Those are for thermocouples, not J-curve NTC thermistors.
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Old 12-10-2023, 08:10 AM   #3
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Is it possible you are connected to one of the first two ports on the TMP100? Those are for thermocouples, not J-curve NTC thermistors.
No I am connected to one of the last 4 inputs. I did mistakenly use the first input and got a really high reading (like 500f) and figured out the first two inputs are for the EGT sensors only. The resistive/temp table seems backwards to me. I tried a 8k resistor across the input terminals and this raised the temperature while the table seems to show it should lower.
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Old 12-10-2023, 08:48 AM   #4
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Maratron just works better with their sensors. Go into the setup and check the max temp setting and set the max setting. Then re check. if still off lower the max setting until you get the correct reading.
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Old 12-10-2023, 11:56 AM   #5
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Maratron just works better with their sensors. Go into the setup and check the max temp setting and set the max setting. Then re check. if still off lower the max setting until you get the correct reading.
These are the Maretron supplied ambient sensors (https://www.maretron.com/support/man...ons%201.1.html). The sensors appear to be manufactured by US Sensor Corp as that is the resistive/temp table provided by Maretron (https://www.maretron.com/wp-content/...?id=567&oid=30). The IPG100 has no setting for min/max temperatures so I'm assuming you are referring to the displays in N2KView. These settings only effect the gauges shown in N2KView not the actual sensor reading.
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Old 12-10-2023, 12:46 PM   #6
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These are the Maretron supplied ambient sensors (https://www.maretron.com/support/man...ons%201.1.html). The sensors appear to be manufactured by US Sensor Corp as that is the resistive/temp table provided by Maretron (https://www.maretron.com/wp-content/...?id=567&oid=30). The IPG100 has no setting for min/max temperatures so I'm assuming you are referring to the displays in N2KView. These settings only effect the gauges shown in N2KView not the actual sensor reading.



Maybe he means setting in the TMP100 setup screens? I don't recall what settings are available.
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Old 12-10-2023, 12:48 PM   #7
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What about connection polarity? You would think it wouldn't matter, but I seem to recall discovering the opposite about one of the maretron sensor. Just can't recall if it was temp or something else.
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Old 12-10-2023, 01:18 PM   #8
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Maybe he means setting in the TMP100 setup screens? I don't recall what settings are available.
Not much available to configure in the TMP100 other than a label, instance # and PGN frequency.
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Old 12-10-2023, 01:20 PM   #9
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What about connection polarity? You would think it wouldn't matter, but I seem to recall discovering the opposite about one of the maretron sensor. Just can't recall if it was temp or something else.
I confirmed I have the correct polarity but like you don't think it matters. On the SIM100 switch module it clearly states in the manual that polarity matters but is silent in the TMP100 manual.
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Old 12-10-2023, 01:38 PM   #10
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According to the resistive temperature table 3C should be 8415 ohms versus 9C 6264 ohms (see attached screenshot). It seems the table is backwards as if I put a resistor across the temperature probes is raises the temperature rather than drop it.

If you put a resistor across the TMP100 terminals in parallel with the temp probe, it lowers the resistance as seen by the TMP, and that corresponds with an increase in temperate. So that part actually sounds correct.


Have you disconnected the probe and measured its resistance with a VOM? That would help narrow down whether the probe is out of calibration, or if the TMP is off.
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Old 12-10-2023, 01:45 PM   #11
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I see Maretron specifies +/- 2C temp accuracy which is pretty poor. I don't know how much of that is the probe vs the TMP100. That said, you are seeing a greater error than that.
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Old 12-10-2023, 01:53 PM   #12
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What about connection polarity? You would think it wouldn't matter, but I seem to recall discovering the opposite about one of the maretron sensor. Just can't recall if it was temp or something else.
TT, ok the mystery gets deeper. I figured I'd measure the resistance across each probe wire and as expected the polarity didn't matter. See screenshot showing the pertinent resistances measured versus what it should have been per the temperatures being displayed. I'm stumped why the large resistance differences unless the table Maretron posted is wrong.

Opening the refrigerator/freezer for a quick 15-20 seconds caused the probe temperature going up 10-15f. I have both of temperatures now using a 10 minute average which tends to eliminate the spikes from opening the refrig/freezer doors. After the temperatures settle I'm off 5-6f which may be as close as I'm going to get.
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Old 12-10-2023, 02:13 PM   #13
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If you put a resistor across the TMP100 terminals in parallel with the temp probe, it lowers the resistance as seen by the TMP, and that corresponds with an increase in temperate. So that part actually sounds correct.


Have you disconnected the probe and measured its resistance with a VOM? That would help narrow down whether the probe is out of calibration, or if the TMP is off.
Duh, what was I thinking. If I can figure out the table I could get the correct resistor and rather than put across the TMP100 terminals only put on one of the wires. This would raise the resistance and act as a temperature adjustment.
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Old 12-10-2023, 05:09 PM   #14
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Duh, what was I thinking. If I can figure out the table I could get the correct resistor and rather than put across the TMP100 terminals only put on one of the wires. This would raise the resistance and act as a temperature adjustment.
Sometimes Ill use a variable resistor so I can dial it up and down to find the best value. Then solder in a fixed one to finish.
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Old 12-10-2023, 05:36 PM   #15
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Sometimes Ill use a variable resistor so I can dial it up and down to find the best value. Then solder in a fixed one to finish.
Good suggestion, I put a 8k ohm resistor on one of my freezer probes which was incorrectly showing 17f (correct temp 4f) and its now showing 3.6f , happy camper.

I suppose this is a one way fix as theres no way to reduce the resistance (short of using a larger wire) but in my experience the probes tend to be too high.
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Old 12-10-2023, 07:35 PM   #16
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Just to confirm, when you measured the probe resistance, at least one of the wires was disconnected from the TMP100, right? That could impact the measurement otherwise.


The thermistors are supposedly accurate to 0.1C, so if your are reading that far off, they would appear to be junk. Are they Maretron branded sensors, or purchased from some other source?
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Old 12-10-2023, 08:48 PM   #17
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Just to confirm, when you measured the probe resistance, at least one of the wires was disconnected from the TMP100, right? That could impact the measurement otherwise.


The thermistors are supposedly accurate to 0.1C, so if your are reading that far off, they would appear to be junk. Are they Maretron branded sensors, or purchased from some other source?
I measured with the wires connected to the TMP100 (power was off) as it was easier to get a good connection. Would it make a difference? Ill get out my alligator clips and measure from the wires only.

As mentioned in a prior post I was able to get one of the freezer probes 99.9% accurate by adding a 8k ohm resistor. Ill do the same on the others to get an accurate reading. I suppose I could have been happy with the freezer probe showing 20f (when actual was 5f) and set an alarm for 30f but Im a little OCD and like accurate readings.
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Old 12-11-2023, 07:00 AM   #18
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I measured with the wires connected to the TMP100 (power was off) as it was easier to get a good connection. Would it make a difference? Ill get out my alligator clips and measure from the wires only.

As mentioned in a prior post I was able to get one of the freezer probes 99.9% accurate by adding a 8k ohm resistor. Ill do the same on the others to get an accurate reading. I suppose I could have been happy with the freezer probe showing 20f (when actual was 5f) and set an alarm for 30f but Im a little OCD and like accurate readings.
Isolating the sensor is important when measuring. Otherwise you are measuring the impedance of the TMP100 in combination with the sensor, not just the sensor. The difference may not be significant, or even noticeable at all, but you dont know until you have checked.


Also, what are you comparing the sensor reading to, because the other temp sensor will have error too. In
My experience, temp sensors of all kinds can be pretty inaccurate. So just for arguments sake, how do you know its the sensor thats wrong, and not your other thermometer?
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Old 12-11-2023, 10:52 AM   #19
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I would first put the sensor on another channel and see if you get a deferent reading out of the sensor.
You can also move a know sensor to that channel and see if the reading changes.
The channels on the unit are independent . 1 channel can go bad.

I had a tm100 give me wacky readings . Was sent back and replaced by maretron.
I have a sim100 have a bad cannel. ( grate on my smoke alarm)

One last thing. the unit only supports 2 temperature ranges for the ambient sensors. (-20C to 80C or -4F to 176F)

Off the maretron website.
Maretron's TMP100 measures the temperature for up to 6 temperature probes and reports the information over an NMEA 2000 network. The TMP100 supports up to 4 thermistor probes and 2 high temperature thermocouple probes. Optional thermistor probes (-20C to 80C or -4F to 176F) cover a wide range of applications including cabin air temperature, engine room air temperature, refrigerator/freezer temperature, under bolt temperature (inverters, charges, pumps, motors, etc.), tank temperatures (live well bait, hot water, etc.), and air duct temperatures. The optional thermocouple probes (0C to 900C or 32 to 1652F) are used to measure Exhaust Gas Temperature (EGT) as part of a comprehensive fuel management system.
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Old 12-11-2023, 12:06 PM   #20
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Isolating the sensor is important when measuring. Otherwise you are measuring the impedance of the TMP100 in combination with the sensor, not just the sensor. The difference may not be significant, or even noticeable at all, but you dont know until you have checked.


Also, what are you comparing the sensor reading to, because the other temp sensor will have error too. In
My experience, temp sensors of all kinds can be pretty inaccurate. So just for arguments sake, how do you know its the sensor thats wrong, and not your other thermometer?
TT, yes you are 100% correct, the TMP100 was effecting the measured impedance. When I measured only from the wires the temperatures/resistance pretty well lined up with the table.

I'm measuring the actual temperatures with a mercury thermometer and a digital infrared thermometer so am pretty confident on the measurements. I've been monitoring the temperatures over the past day and notice that both the refrigerator (39f-46f) and freezer (-10f-5+f) temperatures are oscillating. (see attached screenshot) The screenshot was over the past 12 hours so the middle of the night when neither door was opened so that cant be the source of the oscillation. In addition I have both set for 10 minute averaging to reduce the swings from opening the doors. The temperatures near the bottom of the oscillation are fairly accurate to the actual temperatures. Any idea what would be causing this oscillation?

Interestingly, the dometic freezer is the one I put the 8k ohm resistor on and it measures a consistent 3-4f which is pretty much the right temperature. The dometic freezer is on a separate TMP100 in the engine room.
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