I’m working on custom car gauges for measuring car coolant temperature and oil temperature and pressure. To save some cash I decided to try to use cheap automotive sensors from China, specifically sensors from Banggood.com. The issue with these sensors is the lack of datasheets and/or characteristics specifications, so I decided to measure the characteristics of the sensors by myself.

Temperature gauge sensor

There was basically zero information about the temperature sensor. I expected that it is NTC thermistor and the question was only what is its resistance at 25°C. Sadly my lab is not equipped with any precise heat source or thermometer so I end-up using a kettle and Fluke 17B+ multimeter with K-type thermocouple sensor. I find out that the resistance of the temperature sensor is 100KOhm @ 25°C and I also confirmed the NTC characteristics with my measurement.

https://www.banggood.com/Auto-Gauge-Universal-Water-Temperature-Sensor-Oil-Temp-Sender-p-1370894.html

Temperature gauge sensor measuring setup and results

My setup is really non-scientific and guarantees only poor precision and temperatures below 100°C.
My main goal was just to find its room temperature resistance and confirm NTC characteristics which I did. Reference temperature was measured with Fluke 17B+ with K-type thermocouple. The thermocouple was glued to the gauge sensor with Kapton tape. Sensor resistance was measured with Rigol DM3085E.

Temperature [°C]	Resistance [Ohm]
13,8	165760
20,0	125435
25,1	98401
28,8	83451
35,0	64030
40,3	51386
44,9	42719
50,5	34460
55,1	28950
60,8	23514
65,6	19746
70,6	16613
75,4	14187
80,5	12020
85,2	10346
90,4	8790
95,2	7490
99,6	6640

Exponential regression shows the increased error below 30°C. This was most likely caused by the imprecision of my homemade calibration setup. The sensor behaves as a generic 100 kOhm NTC.

Pressure gauge sensor

The amount of information for the pressure sensor is a bit better, but I wanted to check sensor precision, linearity, and hysteresis anyway.

https://www.banggood.com/18-NPT-Oil-Pressure-Sensor-Engine-Oil-Pressure-Switch-Sensor-Gauge-Sender-p-1027389.html

Pressure gauge sensor measuring setup and results

For the pressure sensor test, I made a simple jig with an analog gauge and a valve that is connected to my small compressor with a pressure regulator. The compressor in my lab is able to achieve pressures only up to 5.2 bar, so I wasn’t able to test a whole range of the sensor.

Pressure [bar]	Resistance (G) [Ohm]	Low-Pressure Alarm (WK) [Ohm]
0	7,5	0
0,5	24,016	0
0,75	27,311	0
1	30,648	0
1,2	34,001	0
1,4	37,386	0
1,6	40,632	Open
1,8	43,935	Open
2	47,269	Open
2,2	53,818	Open
2,4	57,128	Open
2,6	60,48	Open
2,8	67,038	Open
3	67,36	Open
3,1	70,353	Open
3,2	70,399	Open
3,4	73,944	Open
3,6	77,015	Open
3,8	80,328	Open
4	86,896	Open
4,2	83,672	Open
4,4	93,565	Open
4,6	96,861	Open
4,8	100,193	Open
5	103,545	Open
5,2	106,981	Open

Resistivity values are really close to specifications. Low-pressure alarm threshold was observed at 1.5 bar instead of specified 0.8 bar. The pressure dependence of resistance is linear, but the resistance is changing with ~3 ohm discrete steps. This is most likely due to sensor implementation as a pressure-sensitive element that is connected to a wire-wound potentiometer. I observed an error at 4.2 bar. I’m not sure if it’s a sensor design issue or if the sensor is somewhat damaged. Resistivity also has noticeable hysteresis ~0.5bar.