Thermometers

[Martin Aislabie]

Guys, a note of caution.

I bought yet another Thermometer today – saw it in a shop and heard it whisper “buy me” – so I did :rolleyes:

Having a bit of time on my hands I then decided to check its displayed accuracy against the other 5 thermometers I have.

I have a Mercury in Glass Thermometer which I know to be within +/-0.2C of 20C – I had it calibrated at work and that was the best they could calibrate to.

I then used this as the reference against which all the others were judged.

Filled a Pyrex jug with warmish water and placed them all in, arranging them so they were within close proximity but not actually touching each other.

Gave the thermometers 30min to stabilise before the first reading was taken and then further readings were taken at odd intervals as the water in the jug slowly cooled down from 25 to 18C over the space of a couple of hours.

4 out of the remaining 5 were within the range +/-0.2C – consistently either cooler/the same/warmer than the reference – which is fine with me.

Of the 4 thermometers which were OK, 3 were Alcohol in Glass and the 4th a Digital device.

However, the fifth thermometer, another digital unit was consistently 1.8C different – which is far more than I am comfortable with and from now on it will be consigned to the task of a stirring rod.

Has anyone else tried comparing the accuracy of their Thermometers?

Martin

[Barry]

I have a few thermometers around the darkroom. I have not compared them for consistent and accurate temperature readings. The only time I concern myself about accuracy is when I process colour prints, which is very rarely.

I always use the same thermometer for mixing my mono chemistry, if it is inaccurate, I don't worry because I believe consistency in the process is more important. If it were in error by 1 or 2 degree's, it just means that I always process my film at 18 or 19 degree's instead of 20.

However I guess it would be prudent to check just in case my regular thermometer stopped working :eek:

[Trevor Crone]

Thanks Martin; I've several mercury thermometers in the 0-50C/0.2C. range but I haven't compared them against each other for a while.

I check the thermometer I use regularly against my digital thermometer and the digi usually indicates a temperature up to 0.5C higher.

[Dave miller]

I only have one, which I believe to be constantly inaccurate.

[Martin Aislabie]

I have a few thermometers around the darkroom. I have not compared them for consistent and accurate temperature readings. The only time I concern myself about accuracy is when I process colour prints, which is very rarely.

I always use the same thermometer for mixing my mono chemistry, if it is inaccurate, I don't worry because I believe consistency in the process is more important. If it were in error by 1 or 2 degree's, it just means that I always process my film at 18 or 19 degree's instead of 20.

However I guess it would be prudent to check just in case my regular thermometer stopped working :eek:

Barry, I do the same, use only one thermometer to check the developer temperature - which is my Mercury in Glass unit.

I too have wondered what I would do if I accidentally broke it, hence the want to compare it with the others I have, for when the inevitable happens.

Now I can substitute one of the other thermometers.

Glass thermometers are vulnerable to braking from either bending or dropping and electronic devices don't survive anything longer than the briefest of immersions.

Martin

[Bob]

I have half a dozen. I use two Paterson precision thermometers as my standard (they agree as close as I can read them) and calibrate the others against them at around 40C and mark them - a bit of tape with the difference marked on it. I keep the Patersons out of harm's way and use the others.

In practice, only one is out by more than a fraction of a degree (having thrown away the dial thermometer that was over 4 degrees out)...

[Mike O'Pray]

I have two Jobo thermometers that fit either end of a Jobo CPE2. Assuming the water bath element heats the water consistently at both ends I get the same temp on both but both could be out by exactly the same amount although this seems unlikely.

So I have concluded that both are accurate. It raises the question of whether Jobo thermometers as used in their processors were calibrated and guaranteed accurate to within a certain scope.

Anyone know?

Mike

[Niall Bell]

Hi,
This is an interesting thread to which i'd like to chuck in few thoughts, although I'm at risk of being shot down as a pedant- which I fully acknowledge.

Glass column thermometers (the commonly used type being discussed here I think) can be grouped as being either total/complete or partial immersion. As the names suggests a total immersion thermometer needs to be fully immersed in the liquid (or at least pretty much all of it) whilst a partial immersion thermometer should be immersed only as far as is indicated. Whether total or partial immersion (and extent of partial immersion) should be marked on the reverse side of the thermometer.

The thermometers are calibrated only for the immersion indicated, and use of a thermometer at a different immersion gives errors in the reading. Further errors can be introduced through e.g. taking a reading when the eye is not level with the top of the mercury or spirit in the thermometer, or from where on the meniscus the reading is taken.

The interesting thing for me is that none of my photographic thermometers have any indication of the degree of immersion required, and these are made by Jessops (!) and Paterson; although I do try to read the things level with my eyes.

I suppose what I'm saying is that there's errors involved with making all measurements, including temperature and so few of us are measuring this precisely.

Yet it still all seems to work just fine.

[Trevor Crone]

Niall, you are quite right, and indeed I don't think for B&W processing we really need this kind of accuracy. I have a range of mercury filled thermometers, some need total immersion, one requires 80mm immersion another 76mm and one even in old money, 4" immersion. A couple are even nitrogen filled:shock:

Needless to say I completely ignore this data, perhaps that's why my film is always either under or over developed:slap:

[dicko]

Agree with Barry - I just stick with one thermometer, as consistency is the goal rather than specific temperatures. Of course, if I drop it and break it, then I may notice different results with a replacement, but as I do B&W only, it's unlikely to cause any real-world problems for me.

[Trevor Crone]

In case anyone should think I'm crazy in having such a range of thermometers?

I have about a dozen mercury filled which I acquired when a laboratory was closing.

I always stick to just one for consistency. But its nice to have a reserve in case of breakage's.

[Dave miller]

Trevor, you are not crazy, but you are wasting your time, for no amount of protestations will now convince us that you aren’t a closet thermometer hoarder with an uncontrollable mercury fetish. :D

[Trevor Crone]

Trevor, you are not crazy, but you are wasting your time, for no amount of protestations will now convince us that you aren’t a closet thermometer hoarder with an uncontrollable mercury fetish. :D

As they say Dave, "I'm as mad as a hatter":D

[RH Designs]

I forget who, but one customer did tell me that he keeps his mercury thermometer safe as a reference only, using it to check the calibration of his digital one once a week. That way he minimises the risk of damaging the reference, but can be sure the more robust digital one is sufficiently accurate. If the digital breaks, buy a new one and calibrate that, and so on. So you never need more than two at any one time :).

[Martin Aislabie]

The immersion depth of thermometers is really important for stainless steel probes - particularly if there is a large difference between what temperature it was at before being immersed.
Steel is a good thermal conductor, a so need to be deeply immersed in the fluid to make sure it is the fluid temperature you are measuring not a combination of air & liquid temperature.
However, this probably isn't a big deal in the darkroom where the air temp and black and white chemistry are probably at similar temperatures but it will improve the thermometers response time.
Martin

[Martin Aislabie]

As they say Dave, "I'm as mad as a hatter":D

The expresion "Mad as a Hatter" comes from a time when they used Mercury in the manufacture of Top Hats and the unfortunate consequences it had on the people who made them :eek:

I too will confess to being a closet thermometer hoarded with a mercury fettish ;)

Martin

[Trevor Crone]

The expresion "Mad as a Hatter" comes from a time when they used Mercury in the manufacture of Top Hats and the unfortunate consequences it had on the people who made them :eek:

I too will confess to being a closet thermometer hoarded with a mercury fettish ;)

Martin

Martin, "Mad as a Hatter" probably does hold some truth for me:o For having worked in laboratories for 40 years we had a fair amount of mercury filled thermometers getting broken. Some of the mercury ended up down lab. sinks.

I remember early in my career we had a visit from health & safety with their 'mercury sniffer,' needless to say some of the areas had levels of mercury beyond the recommended levels. Thereafter we had to carry mercury spillage kits. Eventually we gave way to the regular use of Hg thermometers and used spirit based ones instead.

[Mike Meal]

Can anyone recommend a good digital thermometer?
I'm about to start processing E6 so I need something which is accurate.

[Dave miller]

Can anyone recommend a good digital thermometer?
I'm about to start processing E6 so I need something which is accurate.

Talk to Richard of RH Designs (http://www.rhdesigns.co.uk/darkroom/html/thermometer.html), he does one.

[Mike Meal]

Great, thanks Dave.

[RH Designs]

I do indeed, but the spec is +/- 1 degree which is not really good enough for colour work on its own. What I'd recommend is getting a high quality conventional thermometer to use as a reference against which to check the digital one from time to time. Digital thermometers are very consistent but not necessarily very accurate unless you pay a lot of money. Colour processing needs to be accurate to 0.25 degree I think.

The advantage to our thermometer is it's waterproof and you can use it to stir your chemicals with while checking the termperature. Many digitals at the cheaper end of the market are not waterproof.

[Martin Aislabie]

I can vouch for many digital thermometers not being waterproof.

The idea that someone can market a photographic un-waterproof thermometer is crazy (to me)

One dip into a chemical tray and they die - not straight away but by the next day they won’t play anymore.

Some of the more expensive hand held digitals are not waterproof either

Martin

[Trevor Crone]

I can vouch for many digital thermometers not being waterproof.

The idea that someone can market a photographic un-waterproof thermometer is crazy (to me)

One dip into a chemical tray and they die - not straight away but by the next day they won’t play anymore.

Some of the more expensive hand held digitals are not waterproof either

Martin

Martin, are you talking about the all in one unit type of digital thermometer? Only I have one with a separate stainless steel probe and the probe itself is completely waterproof.

[Mike Meal]

Thanks Richard, I see Paterson make thermometers which measure in .3 steps which they claim to be accurate, I was thinking I would have to remortgage but its only £12?!:rolleyes:
I'II take your advice and order a digital thermometer aswell.

[Bob]

I have two of those Paterson "precision" thermometers. Bought a few years apart, they agree with each other as close as I can read them. I use them to calibrate my everyday working thermometers.

[Martin Aislabie]

Martin, are you talking about the all in one unit type of digital thermometer? Only I have one with a separate stainless steel probe and the probe itself is completely waterproof.

Yes Trevor - I was talking about the all in one units.

The stainless steel remote probes are fully sealed units and thus completely waterproof.

Although, most of the remote head units I have seen are no more than splash proof

Martin