Thermometers

What kind of themometers do you use? mercury or digital? Last year I used a mercury min/max & did not have any probs but I knocked it & it broke so this year I've bought a digital type but it seems to be all over the place :confused:

[ 17. February 2006, 11:45 AM: Message edited by: Gardening Newbie ]

 

I use a max/min

 

Digital or mercury?

 

I have got a mercury max/min thermometer in the greenhouse, and another coiled spring one in the garden. I have found that they record the same temperature when put together. It's true that mercury or alcohol thermometers break, but on the other hand there's not much to go wrong. With a digital one you can't tell!

[ 17. February 2006, 02:29 PM: Message edited by: Liz ]

 

I use the old fashioned mercury ones, Max/min, had them years. As long as you site them in a suitable place, I find, they dont get broken.
Even still got the little magnets supplied to reset them. :D

 

Mercury Max/Min. (Actually they are alchohol bulb - the mercury is used to float the iron indicators on )

Avoid the cheap ones you reset by pushing a button on the front as they tend to stick or the mercuy thread gets broken. The ones you reset with a magnet are better. Always store or fix bulb type thermometers vertically and out of any direct sunlight.

Unless you have a digi coupled to a computer so you can draw charts (maybe with a weatherstation) then they are only good for a spot check on the temperature and won't give any indication of cold nights unless you sleep in the greenhouse :D

 

Helen Have a look around (Ebay maybe) for a radio controlled max-min indoor/outdoor thermometer/hygrometer. You can then leave the remote sensor in your greenhouse and monitor things from the house. If you get one with sufficient bells lights and whistles you can even set a frost alarm to tell you to put the heating on.
We've also got a digi thermometer with an extension probe that we can stick in the soil. It's quite interesting using it to see what difference fleece or polythene makes to soil temperature.
If your digi is "all over the place" it looks like you've got a duff one. We've two in use all the time and one occasionally and they've proved very reliable.

[ 17. February 2006, 09:02 PM: Message edited by: Dave W ]

 

I have used for many years several mercury thermometers with the push button, and i agree the mercury does tend to "split" but I have found in many cases that a good shake tends to get it together again.

 

I too use the min/max mercury thermometer in my greenhouse - and despite dropping it a few times, its still working fine some 7 years later :D

 

I bought a radio indoor/outdoor thermomiter at Christmas, and I do enjoy the conveniance. And it gives the max and min as well. But I found the range was not that great - not the 50 foot they say. I tried it at different distances, and if the distance was too great, or it was not in direct line of sight, it was all over the place - because it was not receiving the signal properly. When the signal was too weak it sometimes only updated once an hour rather than every 70 second as it should. If it gets very cold, the outside batteries lose power and you can lose the signal

Having worked in a research lab for a number of years, my gut feeling is that mechanical meters are often more reliable and more sensitive than electronic meters. I bought some electronic bathroom scales some time ago, and chucked them out after a short while - the readings were all over the place. I am still using my 30 year old mechanical scales.

With a short distance my radio thermomiter seems to be working fine, but I suspect that in ten years time it won't be, whilst my mercury thermometer will still be working OK.

 

I'm getting the impression myself that the mercury ones are a little more reliable, my digital one is a min max type but seems to record odd temps, for instance we have the same heater as last year (parasene electric 3kw) & the setting has not been changed yet the ther reads that the overnight temp dropped down to 2.0c last night but all last spring it did not drop below 7/8.0c, I suppose its possible that the heater thermostat is not working but its only just over a year old so not sure on that one! I'm going out today & buying a mercury type that way I can check them off against each other, hopefully its just the ther & not the heater!

 

Helen
Mercury has to be more reliable as there is really only one moving or 'active' part, but it doesn't provide the convenience of a remote radio operated digital.
A decent digital can be very accurate. As I write the digital in my study which reads via a probe through the window frame reads 3.7 C, our radio contolled digital which is reading a sensor on the other side of the house under the eaves of our shed is reading 3.4 C. Much of this insignificant variation is due to the fact that the first thermometer is only a couple of cm from a window and there is some heat escaping from the room through the hole in the window frame (must get round to filling it!).
The batteries in our radio controlled unit last around 8 months. Cold does affect them but only impairs the signal when the voltage has already fallen to a near marginal level. ( Re-chargeable batteries are very much more prone to the effects of cold than 'throw away batteries )
Peter is bang on with his comments regarding the claimed range of these radio units. The makers will quote the best range under ideal conditions. They operate using a very low power signal on 433 MHz and as Peter said "line of sight", but like a mobile phone Location Location can make a big difference and moving one of the units a just couple of feet can make a difference.
Two good reliable and accurate makes I can recommend from experience are Oregon Scientific and Huger.

(Peter, I could debate analogue V digital measuring instruments having used both for measurements in the RF field for many years. My money on the whole goes to quality digital, though I do recognise that analogue can for some purposes show trends better)

[ 18. February 2006, 09:05 PM: Message edited by: Dave W ]

 

Dave - I would not disagree with you about quality electronic instruments - there is some amazing stuff around these days. But at the cheap end, there is a lot to be said for simplicity.

Are they all on the same frequency? If so you cannot afford to ever have a very long range, as your meter will be showing the temperatures of all your neighbours units in turn.

I recently bought a radio door bell with a 100 yard range. It had 20 different frequencies, and I changed the factory setting. But I was woken up a 2am, when it went off. It was the same frequency as a house behind me in the next street, 60 yards away - I could see them from my window. :D :D :D

 

At 433mhz,a metal framed greenhouse could have a big effect on the signal.
As Dave said location is very important..at that frequency anything metallic around 30cm long can act as antenna and redirect the rf.

Falling snow,rain and even fog could affect the low power signal and a greenhouse full of tall plants would absorb a lot of the rf to begin with
A large sized old fashioned mercury thermometer and some cheap hardwired cctv might be a good alternative plus you could see your plants too

 

I cheat a bit and use a belt and braces approach. I've got a 4 inch diameter unit mounted outside the greenhouse which has a long sensor probe fixed half way along the greenhouse. While we can't read the numbers from the kitchen window we do know that if the pointer is near 2 o'clock its time to think about more ventilation.

Peter your experience with the door bell opens up possibilities for a hi-tech version of "knock door run". Just walk up and down the street with a sender unit in your pocket!