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Our A to Z Guide To Thermometers . . . all you need to know

Our A to Z Guide To Thermometers . . . all you need to know

26 minute read

Here is our A-to-Z guide of Thermometers . . . everything you need to know about this invaluable gardeners tool  

Thermometers are basically instruments which have been designed to measure the temperature – some will measure the temperature of air, some liquids, and some solid objects.
You’ll find thermometers being used in lots of aspects of daily life, from your doctor using one to check your temperature, a chef using one to check meat has been cooked correctly, a scientist using one to check the results of a chemical reaction . . . the list is virtually endless!
Here we will be looking at thermometers being used by gardeners . . . although we might refer to other practices just to give you a fuller picture. 

Alcohol In Thermometers
Traditionally we think of mercury as being used in thermometers to measure the temperature. However, mercury was banned from being used in the manufacture of thermometers in 2007 so other liquids are now used in its place, one of which is alcohol.

One of the advantages of using alcohol is that it can measure temperatures which are extremely low due to its low freezing point – it can measure from minus 112 degrees Celsius. It’s also much safer to use than mercury which is highly toxic to living beings and the environment. Alcohol is also less expensive than mercury. It’s a clear liquid, so to be useful for thermometers it must be dyed – usually you’ll see it dyed bright red. Unfortunately, due to it’s low boiling point, using alcohol to measure really high temperature – above 78 degrees Celsius - is not suitable. 

We’ve included barometers in our A to Z of thermometers just because they are so closely related – and can prove an invaluable tool for the avid gardener!

A barometer is an instrument used to measure atmospheric pressure, also referred to as ‘barometric pressure’ hence the name. The atmosphere is the layers of air which wrap around the Earth. This air has a weight, pressing against everything it touches as gravity pulls it to the earth – barometers measure this air pressure.   

The average atmospheric pressure on the Earth's surface varies between 940 to 1040 hPa (mbar). This pressure changes and affects the weather which is why barometers can be used to predict short-term weather changes. 

Low pressure means there isn't sufficient force to push clouds or storms away, so when there is a drop in the atmospheric pressure then you know this type of weather is due to occur.

Whilst with higher atmospheric pressure cloudy and rainy weather will be pushed away - so an increase in pressure predicts finer, cooler, dryer weather.  

Barometers were originally invented in 1643, with many changes in the materials used in barometers and how they looked and worked. However, the importance of using this type of device becomes apparent if we take a quick look at Fitzroy Barometers. 

fishery barometers

Fitzroy Fishery Barometers 

In 1857 Vice-Admiral Robert Fitzroy designed a simple, robust barometer and devised a way they could be distributed and the weather observations communicated to his London office.

About 100 of these barometers were placed around the coast, with the photograph opposite showing one such barometer located in Stromness, Orkney, shown is it's stone housing.

In 1860 Fitzroy's forecasts / storm predictions began to appear in The Times newspaper and after his death in 1865 his forecasts were found to have been 75% accurate and laid the foundations of modern weather forecasting.    

The barometers available to gardeners today are not as cumbersome as Fitzroy's thank goodness, making them a convenient way to predict the coming weather. 

Gilt Dial Barometer

At just 4½ inches in diameter using this barometer outside in your garden will help you to forecast the weather. 

An increase in pressure predicts fine weather, a decrease bad weather. So you'll be able to plan your day accordingly.  

Click here to see full details. 


Celsius is a scale based around the freezing and boiling points of water. 0 degrees Celsius is the freezing point and 100 degrees Celsius is boiling point.

The Celsius scale is in general use wherever the metric system of units is used, the only countries using Fahrenheit only are the United States, Liberia and the Cayman Islands.

Anders Celsius

It was invented in 1742 by the Swedish astronomer Anders Celsius.

Originally called the Centigrade temperature scale, from the Latin 'centum' which means 100 and 'gradus' which means steps.

Celsius devised it to be used with mercury thermometers that fixed the boiling point of water at zero and the freezing point of water at the 100 degree mark.

He described it in a paper read before the Swedish Academy of Sciences 1742. 

In 1745 the scale was inverted to how we know it now by Carl Linnaeus, a Swedish botanist, zoologist, taxonomist and physician.

Since 1948 the Centigrade scale has become commonly referred to as the Celsius scale in the honour of its inventor. 

Soil Thermometer 

You can see the Celsius scale clearly printed onto the glass tube within our Soil Thermometer. This thermometer uses kerosene as the liquid to move up and down the tube as the temperature changes.

Designed for testing the temperature of soil the scale is from minus 15 to plus 110 degrees Celsius.

Click here to see full details. 

Soil Thermometer

It also features the Fahrenheit scale at the same time - you tend to find that most modern thermometers show both scales.

Digital Thermometers

A more modern alternative to the traditional liquid thermometers, digital thermometers use the principle that the electrical resistance of metal changes depending on the temperature. So, within the thermometer will be a sensor that produces a resistance, current or voltage in reaction to a change in temperature. A microchip then converts this reaction into a temperature reading which is then shown on the easy-to-read LCD display.

Digital thermometers should provide the most accurate readings of the temperature as they react to temperature changes faster than liquid thermometers but this accuracy is often reflected in their price as they tend to be more expensive than liquid thermometers.

Some digital thermometers will simply show the current temperature whilst others have the capability to store temperature readings, allowing you to compare maximum and minimum temperatures for example.

Unlike liquid thermometers, digital thermometers do require a power source for them to operate – in most cases they are simply battery powered.

Digital Thermometer

Max-Min Digital Bar Thermometer 

The designers of this digital thermometer have tried to combine the style of a traditional liquid thermometer with an LCD display by displaying a bar which shows the Centigrade and Fahrenheit readings whilst a red section in the centre represents the liquid and will be higher or lower according to the temperature.

Click here to see full details. 


Proposed in 1724 by physicist Daniel Gabriel Fahrenheit, the Fahrenheit scale is an alternative way to measure temperature and predates Celsius (1742).   

He established 0 degrees Fahrenheit as the freezing temperature of an ice-salt mixture (water, ice and ammonium chloride), 30 degrees as the freezing point of water and 90 degrees as the normal body temperature.

After his death in 1736, the Fahrenheit scale was recalibrated to make it slightly more accurate. In this new version the exact freezing point and boiling point of water (with no salt included) were marked as:

  • 32 degrees Fahrenheit = freezing point
  • 212 degrees Fahrenheit = boiling point.

There are only three countries which use Fahrenheit as the standard way to measure temperature - the Cayman Islands, Liberia and the United States. All other countries use both, with Celsius as the standard. 

fahrenheit v celsius

Galileo Thermometers

Galileo thermometers (sometimes referred to as Galilean thermometers) are made from a sealed glass cylinder which contains a clear liquid and glass floats of varying density, each with a temperature marking.

As you can see in the photograph, these floats rise or fall in proportion to their density and the surrounding density of the clear liquid they are in. Some of the floats sink whilst others rise. The temperature indicated on the lowest float is the current temperature.  


The reason they are called ‘Galileo’ thermometers is that they are named after Galileo Galilei who discovered the principle on which this thermometer is based – that the density of liquid changes depending on the temperature.  


Whilst we are talking about thermometers, have mentioned barometers, now is a good time to also give a few minutes to hygrometers as these are also a valuable measuring instrument which a gardener can use.

A hygrometer is an instrument for measuring the humidity of the air or a gas or in soil or in confined spaces – the word is derived from the Greek word ‘hygros’ meaning wet or moist. The history of the hygrometer goes a long way back to around the 1400s when Leonardo da Vinci built the first crude hygrometer to measure the humidity in the air or in a gas. Developments since then have turned them from the crude instrument first designed to the more simple to use instruments which we use today.

Here we are talking about hygrometers used to measure the humidity of the air, an essential part of weather forecasting and a great help for gardeners with greenhouses.

The hygrometers used normally by gardeners are those which measure the relative humidity of the air – the amount of water in the air at a given temperature compared to the maximum amount of water possible and is shown as a percentage. It’s easy to understand if we see that on a very wet day that the relative humidity would be 90-100 percent. A comfortable, healthy humidity for us is 40-50 percent, anything under 30% would be considered as too dry. However this is what we would find comfortable, but not necessarily what your plants would like.

The type of plants you grow in your greenhouse will determine the level of relative humidity you should aim for, however, the optimal humidity level for most plants is 80% - anything higher or lower and your plants will grow but at a slower pace and they would not be as healthy as those grown at 80%.

In most UK greenhouses the amateur gardener should be aiming for humidity of between 40-80% but you are always best to check the level which will suit the plants you are growing at the time. You should also note, that if your greenhouse is showing humidity levels over 80% then you need to be on the look out for such diseases as botrytis or powdery mildew.

You can find hygrometers for sale as a standalone instrument or you’ll also find thermometers or clocks that feature an extra dial on the face for measuring relative humidity.   


The photograph opposite shows a close up of the face of our Gilt Dial Thermometer and Hygrometer.

With this item the hygrometer dial is the smaller face on the main face of the thermometer.

You can see the easy to read scale around the dial showing the humidity as a percentage as we’ve discussed. 

Click here to see full details. 

Indoor Thermometers 

When using a thermometer indoors to measure the temperature of a room, to get an accurate reading the thermometer needs to be positioned correctly:

  • Position in the centre of the room
  • Place at least 2ft off the ground 
  • Do not position in direct sunlight
  • Place away from any direct drafts or heat sources.

Traditional glass / liquid thermometers will take longer to adjust to the room temperature than a digital thermometer, but in either case we would recommend leaving the thermometer in situ for at least 5 minutes before taking a reading.

When taking the reading you shouldn’t pick the thermometer up or be too close to it in case the reading is affected by your body temperature. 

Room and Conservatory Thermometer 

This handy wooden thermometer is covered with handy tips to encourage responsible habits to help reduce energy usage.

It can be used to easily monitor room temperature and displays temperature in degrees Celcius. 

Click here to see full details. 


Jam Thermometers

We’ve included Jam Thermometers in our A to Z based on the fact than you might be growing fruit . . . so could be making jam or marmalade . . . or even pickles from left over harvests . . . plus it’s the only ‘J’ type of thermometer we could find!

An essential addition for your kitchen drawer, a jam thermometer or sugar thermometer is the easy way to know if you are getting to the right temperature when making jam, marmalade, caramel, fudge or chocolate sweets.


Kerosene In Thermometers

Liquid Thermometers

Liquid expands at a regular, measurable rate when it is heated. As a result, many thermometers have a narrow glass tube containing a liquid which is used to measure the temperature. The liquid sits at the base of the tube in a bowl or bulb and as heat rises, the liquid expands, climbing up the tube. When it becomes cooler the liquid contracts and goes back down.

Although the first thermometers produced used water as the liquid, because water freezes at 32 degrees Fahrenheit it was decided to switch the liquid to be mercury. Due to the toxic properties of mercury modern manufacturers have looked for other liquids they could use and this has led to thermometers now being produced using alcohol or kerosene.  

Liquid Thermometer

In the picture opposite we have removed the glass tube from our Wall Thermometer to show you the bulb at the base.

This thermometer features a glass tube inside of which kerosene, dyed red, is used as the liquid to indicate the temperature. 

As this picture was taken indoors the temperature is warm enough to cause the liquid to have expanded out of the bulb and travelled up the tube. 

Wall Thermometer

We replaced the glass, liquid filled tube back into the high impact, styrene case so you could see how, when correctly fixed, the height of the liquid shows the temperature. 

Even though this thermometer is relatively small at just under 6 inches tall, the temperature is still easy to read on the black scales, one Celsius, one Fahrenheit. 

Click here to see full details.  

wall thermometer

Max Min Thermometers
Our Digital Max-Min Thermometer has been our bestseller with gardeners over the past few years, and we can understand why a max-min thermometer would be popular if we look at how they work and how helpful they are to gardeners.

Have you heard of a six thermometer? This was the original name for the maximum-minimum thermometer which was invented in 1780 by British scientist James Six.

Although prior to this date other designs for maximum minimum thermometers had been presented they all proved problematical and it was James Six who was able to design a single instrument that was both robust and simple to use, recording both maximum and minimum temperatures reached.

This invention was considered so important that Six was elected a Fellow of the Royal Society in 1792. Since it's invention this self-registering thermometer has been used in meteorology and oceanology and it's basic design has changed little in over 200 years. 

A max-min thermometer is designed to record the highest and lowest temperatures reached throughout the day and night and allow you to read them until it is re-set. This makes these thermometers ideal for gardeners as it allows you to measure the temperature overnight.

So, you could place this type of thermometer in a greenhouse in order to monitor the temperature to see if it falls below freezing – then you would know if you needed to add a heater. You could also use a max-min thermometer to monitor the outside temperature – enabling you to see when temperatures get high enough to start planting outside for example.

There are lots of designs of maximum minimum thermometers on the market, from analogue to digital, ones for use indoors and ones which are weather resistant. Here are just two examples, which show how diverse the designs are: 

Gilt Dial Max Min Thermometer

Gilt Dial Max Min Thermometer 

The main dial of this round thermometer shows degrees Celsius and Fahrenheit. The three different pointers are used to mark the following:

  • black - actual / current temperature
  • red - highest temperature reached
  • blue - lowest temperature reached.

Click here to see full details. 

To reset the maximum and minimum pointers simply involves using the larger nut and turning the red pointer to touch the black pointer. Then use the smaller nut to turn the blue pointer to touch the black pointer. 

This manual resetting is replaced with the simple press of a button with the digital thermometers. 

Digital Max-Min Thermometer

With this best selling digital thermometer you don't have to read a scale, the temperatures are clearly displayed on the large screen:

  • top - maximum temperature reached
  • middle - actual / current temperature
  • bottom - minimum temperature reached.

Click here to see full details.  

max min thermometer

You can select between seeing the temperatures in degrees Celsius or Fahrenheit simply by pressing a button on the side of the thermometer.  

Nano Thermometers

As the name suggests, nanothermometers have been developed to work at nanoscale, having very high spatial resolution of below one micrometre, where conventional thermometers would be ineffective.

As a gardener, we will never need to use a nanothermometer, but scientists do use them now. For example, Peter Maurer, a physicist at Harvard University uses a nanothermometer in his research to be able to detect temperature changes down to a few thousands of a degree “We now have a tool to control temperature on a cellular level, and we can study how biological systems react to temperature change”. This allows them to map temperature changes within a single living cell as they heat parts of the cell using a laser.

Outdoor Thermometers

Just as you would expect from the name, these thermometers have been designed to be used outdoors. There are lots of outdoor thermometers on the market to choose from, so let’s take a look at the features you should be checking to help you select the best one:

Durability – think about the conditions that the thermometer will need to withstand when it is left outdoors, all year round. You need one which is made from a material which won’t rust or degrade quickly when subjected to the Great British Weather! It needs to be weather-resistant, working correctly whether it’s raining, windy or freezing. Being outdoors all the time also means it’s going to get dirty so ideally you want to choose one which is going to be easy to clean – one which you can simply wipe down and it will look as good as new!  

Outdoor Thermometer

Waterproof Digital Thermometer

This thermometer is suitable for use in all weathers as it is waterproof!

It has an IP rating of 66 which means it is waterproof against hose-directed water and falling drops of rain or snow. 

Perfect for the Great British Weather, measuring outdoor temperatures all year round. 

Click here to see full details.

Easy To Install – so depending on where it’s going to be used you might want one which is free standing – so could be placed on a table – or one which is easy to fix to a wall, fence or post. Does it have a built-in hanging hole? Does it come with fixings or a bracket to hold it to a wall? Does it need a separate stand or place to put a remote sensor? 

You also need to consider how it is being powered – is it a liquid thermometer? does it need batteries fitting? Or does it need access to mains electric? Depending on the size of your garden, or whether it’s being used in your kitchen garden or at your allotment, could affect which you select.

Accurate, Easy To Read, Temperature Range – when choosing a thermometer for use outdoors, consider where it will be positioned and how far away it might be when you read it. Decorative, outdoor thermometers which are fixed to a house or garden wall need to be fairly large so they can be read from a distance.

Greenwich Clock & Thermometer

This traditionally styled, double sided station clock has an inset temperature dial which is approx. 8.5cm wide so still readable when you are stood on the ground. 

Its' ornate scroll bracket makes it easy to secure to a wall or fence.  

Click here to see full details.  

greenwich clock and thermometer

When it comes to accuracy, if you are looking out of your window to decide what you should wear based on the temperature then a large, easy to read dial and accuracy of a couple of degrees will suffice. However, if you are wanting to check the temperature in your greenhouse to ensure it’s adequate for propagation then a more accurate reading will be required.

Thermometer Accuracy

If you look at our Waterproof Digital Thermometer you can see that the temperature is shown in the lower panel to parts of a degree.

The manufacturers advise this thermometer is accurate to plus / minus 1 degree Celsius. For even greater accuracy you then need to be looking at more scientific thermometers – with their associated costs.

Propagation Thermometers 

When propagating from seeds or cuttings there are three main things to check to help with germination success:

  • Light
  • Moisture
  • Temperature.

Getting these three correct to suit the plants you are growing is important as all plants have preferred conditions for germination. To get the temperature correct for germination it is important to measure the soil temperature, not air temperature. This is why, if you grow direct from seeds or cuttings, it's recommended that you have a soil thermometer.

Although a soil thermometer might look like any regular, medical thermometer, soil thermometers are designed for use outdoors, so are more robust, and the temperature range will be calibrated for accurate readings to suit gardeners sowing seeds.  

Propagation & Soil Thermometer

This compact thermometer is the perfect size to keep in your pocket when you are at the allotment or in your greenhouse.

It comes with a sturdy plastic case which the thermometer clips into for safe storage and carrying. 

Click here to see full details.  

propagation thermometer

If you are sowing seeds in a propagator or rooting cuttings in a propagator, then a compact thermometer like the one above is likely to fit within the propagator and allow you to take an accurate reading with the propagator lid / cover in place. Just measure the height of your propagator before choosing your propagation thermometer. 

However, when sowing directly into the ground a larger, robust soil thermometer is a better choice.

Soil Thermometer in use

Soil Thermometer 

This larger soil thermometer is ideal for checking the temperature of your soil outside before sowing.

Its 7½ inch probe makes it easy for you to push into the ground the required 1 to 2 inches so you can check the temperature where your seeds will be sown. 

Click here to see full details. 

A soil thermometer is perfect for testing the ground temperature prior to sowing directly into the ground. But you shouldn’t just go out, check the temperature for a couple of minutes, then carry on.

To get a good, reliable temperature check, follow this basic method:  

  • Prepare the ground for sowing - that includes adding any mulch or fertiliser - basically you want the ground to be the same as if you were about to sow the seeds - so dug over, racked etc etc. Ideally this should be done a minimum of one day before planning to check the temperature. 
  • You'll need to take several temperature checks - across the area where you will be sowing. This is so you get a good average reading, which will take into consideration varying degrees of shade and sunlight. 
  • Prepare the testing holes - before using your thermometer you need to check it's not going to crash into any stones or anything else which could damage the thermometer. So mark each point where you are going to test and use a stick or garden cane or screwdriver and push into the ground for about 4 to 5 inches to make sure the ground is free of obstructions. 
  • Take the temperature - plan to take two measurements in each test position throughout the day - one in the morning, one late afternoon / early evening. For each test position and time you'll need something to write the temperature onto as the aim is to get an average temperature for the whole area. So ideally test over several days rather than all on one day prior to planting. 
  • How to test - ideally you want to insert the thermometer into the ground to the depth you will be sowing the seeds. Once pushed into the ground leave the thermometer in place for 3-4 minutes to allow the temperature to settle. Then remove and note down the temperature. 

Although checking the soil temperature in this way may sound time consuming and tedious, this preparation prior to planting is likely to result in better propagation rates. When you are planting directly into the ground you might be waiting weeks between sowing and the plants appearing on the surface of the soil. It's a long time to wait . . . and wait . . . to then find out that the soil temperature was not high enough and the seeds haven't germinated!


"Cold! If the thermometer had been an inch longer we'd have all have frozen to death!"
Mark Twain

"Public opinion is the thermometer a monarch should constantly consult."
Napoleon Bonaparte

"The best thermometer to the progress of a nation is its treatment of its women."
Swami Vivekananda

Room Temperature

"You think you’re cool but really you’re just room temperature.”

There is a lot on information online about the ideal room temperature so you should turn to the experts to get some reliable figures. A couple of organisations which you can check out for healthy room temperatures are The Energy Saving Trust and The World Health Organisation.

  • The Energy Saving Trust advises you should set your room thermostat to the lowest comfortable temperature, typically between 18 - 21 degrees Celsius.
  • The World Health Organisation stresses the basic level of warmth required for a healthy, well-dressed person is 18 degrees Celsius.

Whilst a study from housing expert Richard Moore outlines the potential dangers to health by having the incorrect room temperature: 

  • 24 degrees Celsius and above - cardiovascular risk of strokes and heart attacks
  • 21-24 degrees Celsius - increasing discomfort
  • 18-21 degrees Celsius - comfortable temperatures
  • 16-18 degrees Celsius - discomfort, small health risks
  • 12-16 degrees Celsius - risk of respiratory diseases
  • 9-12 degrees Celsius - risk of strokes and heart attacks
  • below 9 degrees Celsius - risk of hyperthermia. 

Room Thermometer

This thermometer has been designed for use around your home, to help you maintain a comfortable temperature but also an economical temperature.

Different coloured zones make it easy to see if you should be turning your heating on – but equally as important if you should be turning your heating down when temperatures are hotter than you need!

Click here to see full details.  

Room thermometer - hypothermia design

Santorio Santorio
So you know about Mr Celsius and Mr Fahrenheit, but do you know who invented the thermometer?

Well, the person thought to have been the inventor of the first temperature measuring device was Italian physician Santorio Santorio (1561-1636).

Santorio the inventor of the thermometer

In 1612 Santorio was the first person to apply a numerical scale to a thermoscope, which later evolved into the thermometer.

The thermoscope had no scale and was used to indicate the differences in temperature only – for example, it could show if the temperature were higher, lower or the same, but you couldn’t measure the difference or record the temperature.  

By adding a scale Santorio created an air thermometer. Unfortunately it was fairly inaccurate due to the effects of differing air pressure on the thermometer, which was not understood at the start of the 17th century. 


“A thermometer reflects the temperature of the environment. It simply reacts to what’s happening around it. If the temperature is hot, it tells you so. If its’s cold, the thermometer reflects that reality as well . . . it has one purpose and one purpose only.”

Although the quote above is just part of one describing leadership and being a ‘thermometer’ or ‘thermostat’, it also clearly shows the purpose of a thermometer – to measure temperature. But what is temperature ?

Temperature is basically the degree of hotness or coldness of an object – when we talk about something being hot or cold we are talking about its temperature.

As we have seen earlier, temperature can be measured in degrees Celsius and degrees Fahrenheit

Decorative Thermometer

This attractive wooden thermometer shows both Celsius and Fahrenheit being shown on the same thermometer. 

Its' function is to show you the temperature of the air in the area you have placed it. It doesn't control it, it doesn't record it, it simply displays the current temperature. Perfectly designed to carry out its purpose. 

Click here to see full details.  

decorative thermometer

Unusual Garden Thermometers

Vintage Garden Thermometers

Wooden Thermometers




Zero reading on thermometer

In the photograph opposite we placed a Thermastick Thermometer into a glass filled with ice so we could get a 0 degrees Celsius reading.

As you can see, the Thermastick takes extremely accurate readings so shows it at 0.9 degrees Celsius.  

Click here to see full details. 

Thermastick Thermometer 

The Thermastick uses a thermistor to measure the temperature as this ensures a very accurate reading.

Thermistors are temperature-dependent resistors, changing resistance when the temperature changes. They are very sensitive and react to very small changes in temperature.

As the Thermastick is used for many applications and has an extremely wide temperature range of -49.9 to + 299.9 degrees Celsius, using a thermistor is the obvious choice.  

Click here to see full details.

thermastick thermometer

This a to z guide to thermometers has been created from our personal knowledge, information gathered by speaking to other gardeners or manufacturers in the gardening industry, by reading gardening magazines and devouring information from books and the internet. We aim to be as accurate as we can, so if you find a mistake, please remember, we’re only human.

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