Wi-Fi Thermometer

[Ali Baba]

I’ve got a UbiBot which connects wirelessly to my modem. The data can be read on my pc and mobile via an app which is very handy . It gives me a real time reading of temperature light levels and humidity, and can be set to alert me if readings deviate from user defined limits. It also stores and graphs data so I can see what has been happening previously.
Not particularly expensive and the bit of kit in the greenhouse is very small.

[ralphrmartin]

Hi Mel,
I used a BME280 which senses temperature, humidity, and barometric pressure.
Ralph

[DrMikeOxgreen]

My wife bought me an UbiBot a few years ago. It’s great - I’ve got it rigged up to report temperatures inside and outside the greenhouse, as well as humidity and light level. It’s a true wifi system, uploading data to a cloud server every 10-15 minutes, so you can monitor it anywhere provided you’ve got internet. I also added an additional temperature probe so I can monitor the temperature inside my propagator…




I’ve now got the propagator on a time switch to create a night-time cooling-off period, as recommended to me in my “poor seed germination” thread.

One thing has always puzzled me. The greenhouse humidity rises and falls in very close synchronisation with temperature, particularly as the fan heater goes on and off at night. But the humidity changes in the opposite way to how I would expect: if it were relative humidity, I’d expect the RH to fall as the temperature rises (assuming no water vapour can enter or leave the greenhouse). But that’s not what I see: it goes up as temperature rises, and falls as temperature falls. Can anyone explain why?

Or is the UbiBot measuring absolute humidity, and what I’m seeing is evaporation of water (maybe also some transpiration) as the temperature rises, thus increasing the absolute humidity?

[Rayc11]

That's strange I'm using an Elitech RCW-360 WiFi unit to monitor Temperature and Humidity in my Greenhouse, the Graph below shows what I'm seeing, RH falls as the Temperature rises.

[el48tel]

That's strange I'm using an Elitech RCW-360 WiFi unit to monitor Temperature and Humidity in my Greenhouse, the Graph below shows what I'm seeing, RH falls as the Temperature rises.


2023-02-28_11-55-42.jpg

RH should fall as temperature rises. If your GH is not totally open to the elements, the amount of moisture will be the same and unchanged. RH is .... relative .... not absolute ..... amount of humidity .... relative to the amount of humidity which could be held at that temperature. The amount of humidity which can be held increases with temperature at the temperatures we deal with. So temperature goes up .... amount of moisture could be more at the temperature.... related to the amount which could be held the amount you have is lower.

[DrMikeOxgreen]

RH should fall as temperature rises. If your GH is not totally open to the elements, the amount of moisture will be the same and unchanged. RH is .... relative .... not absolute ..... amount of humidity .... relative to the amount of humidity which could be held at that temperature. The amount of humidity which can be held increases with temperature at the temperatures we deal with. So temperature goes up .... amount of moisture could be more at the temperature.... related to the amount which could be held the amount you have is lower.

Yep - I fully understand the concept of relative humidity, dew point, saturation, etc etc, having studied meteorology to ATPL level.

But in the top two graphs I posted above you can clearly see humidity falling as the greenhouse temperature falls, and vice versa. As you’ve correctly explained, that’s not what’s meant to happen!

The sawtooth pattern is due to the fan heater cycling on and off with the thermostat. Interestingly the humidity displays the same sawtooth, suggesting that it is very sensitive to the temperature.

(And I withdraw my suggestion that maybe UbiBot is reporting absolute humidity instead, because it’s reporting as a percentage so it can only be relative humidity.)

[Ali Baba]

RH should fall as temperature rises. If your GH is not totally open to the elements, the amount of moisture will be the same and unchanged. RH is .... relative .... not absolute ..... amount of humidity .... relative to the amount of humidity which could be held at that temperature. The amount of humidity which can be held increases with temperature at the temperatures we deal with. So temperature goes up .... amount of moisture could be more at the temperature.... related to the amount which could be held the amount you have is lower.

Yep - I fully understand the concept of relative humidity, dew point, saturation, etc etc, having studied meteorology to ATPL level.

But in the top two graphs I posted above you can clearly see humidity falling as the greenhouse temperature falls, and vice versa. As you’ve correctly explained, that’s not what’s meant to happen!

The sawtooth pattern is due to the fan heater cycling on and off with the thermostat. Interestingly the humidity displays the same sawtooth, suggesting that it is very sensitive to the temperature.

(And I withdraw my suggestion that maybe UbiBot is reporting absolute humidity instead, because it’s reporting as a percentage so it can only be relative humidity.)

Your post made me look again at my graph. Similarly the RH is falling as the heater is on at night and then rises as the temperature rises in the morning when the heating isn’t on. It then starts to drop as the temperature rises further.
I presume that the humidity is lower at night due to the effect of the fan heating and the lack of transpiration from the plants, then picks up at dawn once the plants start to transpire but while the greenhouse is still relatively cool. Then as it warms up further the inverse relationship between temperature and humidity sets in.
In the summer when the heating is off the humidity mirrors the temperature perfectly

[Ali Baba]

My wife bought me an UbiBot a few years ago. It’s great - I’ve got it rigged up to report temperatures inside and outside the greenhouse, as well as humidity and light level. It’s a true wifi system, uploading data to a cloud server every 10-15 minutes, so you can monitor it anywhere provided you’ve got internet. I also added an additional temperature probe so I can monitor the temperature inside my propagator…




I’ve now got the propagator on a time switch to create a night-time cooling-off period, as recommended to me in my “poor seed germination” thread.

One thing has always puzzled me. The greenhouse humidity rises and falls in very close synchronisation with temperature, particularly as the fan heater goes on and off at night. But the humidity changes in the opposite way to how I would expect: if it were relative humidity, I’d expect the RH to fall as the temperature rises (assuming no water vapour can enter or leave the greenhouse). But that’s not what I see: it goes up as temperature rises, and falls as temperature falls. Can anyone explain why?

Or is the UbiBot measuring absolute humidity, and what I’m seeing is evaporation of water (maybe also some transpiration) as the temperature rises, thus increasing the absolute humidity?

Out of interest how have you rigged it to measure the outside temperature? Mine only has one port for an external temperature probe

[ralphrmartin]

Certainly for my greenhouse, relative humidity readings fall as temperature rises, and vice versa, over say a 24 hour period, unless I water all the plants, for example.

Could your sensor location be a problem, possibly? Could the fan heater by blowing moist air over the sensor in some way? E.g. by blowing hot air across across an open tank / bucket of water, towards the sensor, and the air has cooled down sufficiently by the time it reaches the sensor to have a high relative humidity?

[DrMikeOxgreen]

Out of interest how have you rigged it to measure the outside temperature? Mine only has one port for an external temperature probe

I can’t quite remember how it’s wired up - I think the unit has two USB inputs, and the outside temperature probe goes into one of those - this goes out through one of the roof windows and hangs in a shaded spot behind the greenhouse. The other USB input has a splitter plugged into it (which UbiBot sell as an accessory), into which the humidity probe and the second external temperature probe are plugged (the latter goes into the propagator).

Could your sensor location be a problem, possibly? Could the fan heater by blowing moist air over the sensor in some way? E.g. by blowing hot air across across an open tank / bucket of water, towards the sensor, and the air has cooled down sufficiently by the time it reaches the sensor to have a high relative humidity?

I don’t believe so - I think I put the humidity probe into a corner of the greenhouse, well away from the fan heater. I’ll check tomorrow.