Archive for the ‘Thermometers’ Category

It is surprising what you can say about such a simple thing as the thermometer.
There are a number of different types that you may come across in a hospital, probably the main one is the mercury or alcohol in glass type. In these the mercury is the silver coloured one, the alcohol is most often dyed blue or red so that it can be seen. In many countries mercury thermometers are no longer used due to the toxicity of mercury.

The trouble with the glass ones is that they can be broken very easily, the other trouble is that the thread of mercury or alcohol can get broken, that is a gap can be seen in the thread.
I will explain how you can have a go at getting around this fault later.
The Clinical thermometer is different from other glass ones in one special way.
With most thermometers when you take it away from the thing you are measuring the reading will start to change, this doesn’t matter if you can read it in place, but if you have to remove it then it makes it difficult to get an accurate reading.
The way they get around this is to put a kink in the tube which the mercury goes up and down in, this means that the mercury can rise easily up the tube as it has the pressure of the expanding mercury behind it, but when you remove it the kink is enough to keep the mercury at the highest point it reached.
After reading it the way to get the mercury back to the start again is to take hold of the thermometer at the end away from the bulb and give it a few short sharp flicks.
There are two ranges of clinical thermometer in normal use, the first is the one used for people with a temperature in the normal range, so this one is marked from 35 to 42 degrees centigrade.
The other is the low reading one for people whose temperature has fallen well below normal, this one will read from about 28 to 40 degrees centigrade, this is called a HYPOTHERMIA thermometer.
The cure for a thermometer with the mercury or alcohol column which is broken is handy to know now and again, mind you it doesn’t often work but is worth trying.
Grasp the thermometer by the end away from the bulb and flick it hard, this will sometimes force the two together again.
The second method is to put the thermometer bulb into a very cold fridge or into liquid CO2 or some such liquefied gas, this will shrink the mercury back into the bulb and when warmed again the mercury should have re-combined.
The third method is only to be tried if all else fails and the thermometer is only fit for the bin because that is where it will probably be after you have tried this!
Get some hot water, the temperature of which is just above the highest reading on the thermometer, and very carefully put the thermometer in, watch it very closely, when the threads are about to rejoin, you have to take it out of the hot water and put it into cold water to stop it rising any further.
As I said before this works once in a while and is very nice when it does, more often than not you will end up throwing it in the bin because the bulb has burst.
The other method, which is a combination of both, is to put it into the hot water until the thread is very nearly joined, take it out and then give it a good flick.
Alcohol is now more often used than mercury in some situations, for example, incubators, this is because mercury is toxic and if a thermometer was to break and spill mercury into the incubator, the chances are that a little will remain behind and might cause problems to the babies.
Mind you I think that the chances of serious problems are very slight, still it is for that reason that alcohol is now used in incubator thermometers.
There are other types of thermometers available for different tasks and I will explain a few of them, the section is not one of maintenance but one that gives you a little information on what is available.

1. Bi metallic thermometers.
These rely on the unequal expansion of two dissimilar metals that have been bonded together into a narrow strip and formed into a coil.
This coil is joined to a pointer which moves around as the coil winds and unwinds with temperature change.
These are rather sensitive to damage and should be treated very carefully otherwise they go out of calibration.

2. Gas thermometers.
These are more accurate than the liquid in glass thermometer, they measure the variation in pressure of a gas kept at a constant volume.
These are rather specialised and you probably won’t come across one, still its nice to know what is around.

3. There are also thermometers that have a very long column of mercury in a metal tube, that may be rigid or flexible, and the expansion and contraction of this moves a pointer around.

4. Electronic thermometers:
These use probes to sense the temperature.
There are four different types:
a. Thermocouples.
b. Thermistors.
c. Platinum resistance thermometers (PRT’s).
d. Integrated circuit sensors.

A. Thermocouples:
These are designated by a letter, each is made up of different compositions of metals.
They are E.J.K.N.R.S and T.
For example,
Type J thermocouples-
They are of Iron and constantan, this gives them good accuracy but across a fairly narrow temperature range, +/-1% between 300 and 850o centigrade.
Type K thermocouples-
They use base metal nickel + aluminium and nickel + chromium alloys.
They should have an accuracy of +/-3% between 0 and 4000 C, and +/-0.75 between 400 and 11000C. they can be used as low as -2000 C and above 11000 C. However above 11000 C they drift with an over-reading when the temperature is falling and under-reading when the temperature is rising.
Type N thermocouples-
These use more modern nickel alloys, nickel + chromium + silicone and nickel + magnesium + silicone.
The silicone, amongst other things, lengthens the life of the sensor wires.
They do much the same things as the Type K but do not suffer the top end drift problems so may be used up to 13000 C.
Type T thermocouples-
These are used where great accuracy is required over a limited operating range, only up to 4000 C.
They can be as accurate as +/-0.250c over the whole range.
Type R thermocouples-
These use platinum + rhodium in an aluminous porcelain sheathing giving an accuracy of +/-1 C up to 1000 C and +/-20C up to 13500 C, they are capable of going up to 17000 C.
All these need some sort of instrument to connect them to, in most cases an instrument can only use one type of probe, though there are some multi probe instruments.
The next two may be better for permanent immersion applications where temperature changes move slowly, they are bulkier, which is no real disadvantage and are more accurate than thermocouples.

B. Thermistors.
The name is derived from the words thermally sensitive resistors, these are semiconductors made from a mixture of metal oxides to behave as a thermal resistor – i.e. experience a change of electrical resistance as temperature varies.
They can be as accurate as +/- 0.10C (over a limited range).

C. Platinum resistance thermometers.
These also measure the electrical resistance of a conductor metal subjected to temperature change.
The accuracy of these can be +/- 0.20 C

D. Integrated circuit sensors.
These are used in electronics to monitor the temperature of circuit boards, though the whole system is enclosed in a compact chip it means the probe cannot be separated and for applications other than electronics they are actually bulky, they have a rather limited temperature range, – 55 to + 1500 C, for electronics this is fine especially when their accuracy of up to +/- 0.050C is considered.

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