Measuring capacitance as a result of humidity in oil - Page 1
Forum section: electronics / beginners

author: JacobsMess
author_rank: Newbie
timestamp: January 03, 2020, 10:54:13 pm
content: So long story short, I want to measure capacitance as a result of water particles in oil (used oil from restaurants).I've got a Humirel HS1101 sensorhttps://www.parallax.com/sites/default/files/downloads/27920-Humidity-Sensor-Datasheet.pdfAnd I've acquired a capacitance meter by Global Specialities or CSC (model number 3001) that seems to be considered good and should be accurate enough for my uses.When I try to take a cap reading, the level varies by around 10pF and does not stabilise. I was hoping to get a more reliable figure from my setup, is there anyone here who may be able to help me? If I connect the wires together the measurement either zeros out or gives a stable reading, however with the variable capacitor (sensor), the reading jumps about, even when emersed in oil, making the measurement impossible.Would I be better off getting another meter? if so, can anyone recommend a meter for less than £20 with an accuracy of 0.5% in the pF range?Is it a possibility that the sensor is kaput?

author: unitedatoms
author_rank: Frequent Contributor
timestamp: January 03, 2020, 11:03:52 pm
content: The problem could be that capacitance meter is DC coupled at input. Try another measurement with large normal capacitor in series with sensor. That will block DC and whatever DC noise involved in oil caused by material science things like conductivity, ions etc.1000 pF will do, if the expected sample is in single pF range.

author: JacobsMess
author_rank: Newbie
timestamp: January 03, 2020, 11:05:18 pm
content: yes, the expected sample should be in the singe pF range. The meter I am using is an AC desktop meter.

author: unitedatoms
author_rank: Frequent Contributor
timestamp: January 03, 2020, 11:09:15 pm
content: And, why should sensor work in oil ? It looks like it is designed for air. For oil I'd expect some kind of metal tube with center pin made of smaller tube. Then thing is less affected by AC field. And the capacitor on top with coax going to C meter

author: JacobsMess
author_rank: Newbie
timestamp: January 03, 2020, 11:25:47 pm
content: It is designed for air, but I have been given reassurance that similar designed sensors have worked in oil also. More specifically this....https://uk.farnell.com/honeywell/hch-1000-002/sensor-humidity-capacitive/dp/1566265Though unfortunately they are not made anymore.

author: unitedatoms
author_rank: Frequent Contributor
timestamp: January 03, 2020, 11:35:40 pm
content: Then, well. Perhaps, if I allow myself to go into material science voodoo. The water in air is moleculas of few of them together. The water in oil is may be a droplet with few orders of magnitude of this moleculas. The area of volume of sensing is having a higher shot noise of arriving and departing individual droplets causing sudden changes in momentary readings.So to achieve same performance as in air, the oil sensor must have an area or volume few billions or trillions larger. If you connect in parallel an array of N sensors, you may expect the reduction of noise by Sqrt of N. Or do repeat reading of single sensor and average arithmetically.Edit: Calibration. Consider that if all above is what is happening and you want to prove the validity of method. Take a chamber with dry air and heat up the sample to above boiling point of water so water will separate from oil. Then use trustable air humidity sensor to measure the weight of water. Then match the quantity to oil based sensor. That will prove or disprove that oil sensor is not in error. It will not prove that sensor is correct however, only that sensor is not incorrect for given sample.

author: JacobsMess
author_rank: Newbie
timestamp: January 03, 2020, 11:42:59 pm
content: I am testing for suspended water (very fine particulate in the oil) rather than free water, testing for about 500ppm

author: unitedatoms
author_rank: Frequent Contributor
timestamp: January 03, 2020, 11:54:46 pm
content: What if there is a better method. Water is absorbing in certain exact microwave frequency. That will be kind of RF colorimetry method. Tiny amount of water turns transparent oil very dark at some frequency. Make an RF single port network analyzer (colorimeter for single frequency) with test chamber. The magnitude of S11 scattering parameter of chamber will be between 0 and 1. Where 1 is pure oil (assuming that oil has no absorbtion at this frequency) and lower numbers are for higher water content.

author: JacobsMess
author_rank: Newbie
timestamp: January 04, 2020, 09:53:03 am
content: Unitedatoms, a better method would be appreciated but there are certain factors that need to be considered/excluded including FFA (free fatty acids), foreign matter/particulate and cost.This method was devised to give affordable and accurate tests for water particulate in oil, If you could give me more information on how to do a test with your RF calorimetry method I'd appreciate it.regards

author: den
author_rank: Contributor
timestamp: January 04, 2020, 10:18:07 am
content: Completely agree withunitedatoms. The sensor is designed for relative humidity of air (water vapor), not water droplets in oil (which is more than 500 time more dense than air).Learn from the industry - automobiles are doing that for a long time now (combined oil level and oil quality sensors). I think it's a good idea to have a look have these work.

author: mikerj
author_rank: Super Contributor
timestamp: January 04, 2020, 12:03:17 pm
content: Humidity is the concentration of water vapour in a gas (usualy air), the term simply doesn't apply to liquids suspended in other liquids. Unless it was the actual sensor manufacturer that assured you these devices were suitable for your application then you should take such assurances with a large pinch of salt.

author: graybeard
author_rank: Frequent Contributor
timestamp: January 04, 2020, 01:17:12 pm
content: Capacitive humidity sensors work by having a moisture sensitive dielectric. The absorption/desorption rate of the dielectric is usually slow and nonlinear. They are designed detect moisture in a gas, nominally air. Once you contaminate the sensor with oil there is no reason to expect it to work.

author: Marco
author_rank: Super Contributor
timestamp: January 04, 2020, 11:00:03 pm
content: Looking around polyimide has been used for measuring moisture content of oils, indeed polyimide sensors designed for air were used with oil. I don't see why the structure would have to be different from the Honeywell sensor, but unlike that one this one doesn't necessarily use polyimide.PS. also obviously, even when it works you still need to calibrate it ...

author: JacobsMess
author_rank: Newbie
timestamp: January 05, 2020, 09:41:10 am
content: Yes, I have a methodology for calibration that should be straight forward, using spiked samples and dewatering them. Regarding the polyimide sensor, I cannot find anything suggestion the humirel one is polyimide, any ideas on finding a suitable replacement? Thanks

author: Marco
author_rank: Super Contributor
timestamp: January 05, 2020, 11:07:09 am
content: Looking at Google, SMTHS08A uses polyimide, is relatively cheap and seems closer in construction to the Honeywell sensor.PS. did you use the DC blocking capacitor suggested earlier?

author: JacobsMess
author_rank: Newbie
timestamp: January 05, 2020, 12:25:33 pm
content: The meter I am using is AC, so would it not be the case that ECnoise is unlikely. Unfortunately I am not an electrician and don't have any of these parts about so I'm looking for something that will work without too much trial and error.

author: JacobsMess
author_rank: Newbie
timestamp: January 05, 2020, 12:45:56 pm
content: Also, thanks Marco for the help, the 08A sensor seems much closer to the honeywell and the 07 seems identical in terms of the data sheet so I'll order a couple of these and see how good they are.

author: JIANG
author_rank: Newbie
timestamp: July 17, 2022, 09:05:28 am
content: I want to know ,can you use SMTHS08A for Measuring capacitance as a result of humidity in oil?

author: radiolistener
author_rank: Super Contributor
timestamp: July 18, 2022, 05:29:53 am
content: I think you're needs to measure impedance/frequency dependency graph for a radiator inserted into metal tank with oil. Radiator should be hard fixed in the metal tank. Metal tank is required to shield radiator from external materials. I hear such approach was used by NASA to measure fuel amount in the fuel tank for zero gravity and high acceleration conditions.If you put fixed amount of oil in the metal tank, it can be more easy to calibrate such setup to measure oil contamination with water droplets.

author: Marco
author_rank: Super Contributor
timestamp: July 18, 2022, 01:19:30 pm
content: There have been papers published based on those Honeywell polymide sensors. Polymide is likely hygroscopic, so the water displaces both oil and air and changes the capacitance significantly.Whether the SMTHS08A/SMTHS07 can work as well as the Honeywell sensor, that would require experimentation. Regardless of experimentation, even if they worked actually using them would require calibration against known humidity for a given oil.