EVERYTHING YOU WANTED TO KNOW ABOUT PYROMETERS.

Transcription

1 EVERYTHING YOU WANTED TO KNOW ABOUT PYROMETERS. David Coggins - dcoggins47@gmail.com INDEX 1.0 Introduction 2.0 General Principles 3.0 Thermocouples 3.1 Type R Thermocouples 3.2 Type K Thermocouples 3.3 Type N Thermocouples 3.4 Summary 4.0 Pyrometers 4.1 Analogue Meters 4.2 Digital Meters 4.3 General 5.0 Compensating Lead 5.1 Checking your pyrometer connections 5.2 Use of pyrometers 6.0 Conclusion 1.0 INTRODUCTION Most potters own a pyrometer, but in my experience it is one of the least understood devices in a pottery workshop. This article is written to (hopefully) reveal a little of the mysteries of the pyrometer. A pyrometer is simply a device for measuring temperature using thermoelectric effects. It consists of three parts - the thermocouple, the indicating meter, and the compensating lead connecting the thermocouple and meter. Firstly, I will discuss the thermocouple GENERAL PRINCIPLES The thermocouple works on a principle discovered over 150 years ago - if two dissimilar metals are joined together, a tiny voltage exists between them. People with fillings in their teeth will have experienced this effect when they accidently chew on a bit of aluminium foil!

2 Later it was discovered that a voltage difference existed along a length of wire when heated at one end - and the thermocouple, as we know it today, became a reality. Many different metals are used for thermocouples, all with different temperature ranges and characteristics, but potters are concerned with only three basic types, and I will now describe these. 3.0 THERMOCOUPLES 3.1 TYPE R THERMOCOUPLE Undoubtedly, the best thermocouple for potters is the type R, or platinum/rhodium. Type R thermocouples withstand all temperatures in the normal range of pottery kilns, being rated up to 1700 deg C. The thermocouple is virtually indestructible by heat, and needs only very thin wires (0.5mm) to last for many high temperature firings. One major drawback of the type R is the price currently platinum is obscenely expensive. The type R probe assembly uses a thin wall sheath (which requires extra care in handling) to protect the mechanically fragile thermocouple wires. 3.2 TYPE K THERMOCOUPLE In the past, the most common thermocouple used by potters was the type K or chromel/alumel type. The major advantage of type K was its low price. However, the type K is only suitable for use to 1200 deg C absolute maximum, and does not retain accuracy over many firings. Thick wires are used (3mm) to extend the useful life of the thermocouple, and it is essential to use a sheath to protect it during reduction firings. The type K is adequate for bisque and earthenware glaze firings, and low temperature glass furnaces. 3.3 TYPE N THERMOCOUPLE A thermocouple developed in Australia in recent years is the type N, or nicrosil/nisil. The type N has a similar price to the type K with the advantage of much longer useful life then type K (up to 10 times) plus a temperature range up to 1300 deg C. The type N is generally made with 3mm wires for long life, and must be protected from reducing atmospheres with a sheath. The type N can be used for regular stoneware firings with minimum degradation. 3.4 SUMMARY The following table summarises the characteristics of thermocouples commonly used in pottery, ceramics and glass. TYPE METAL ALLOYS MAX. TEMP COMMENTS K Chromel/Alumel 1200 deg C Low price, short life N Nicrosil/Nisil 1300 deg C Low price, medium life

3 R Platinum/Rhodium 1700 deg C High price, long life The best buy overall is type N, as it combines high temperature rating with reasonably low price. Note that thermocouples are not interchangeable as each type has a different characteristic - the indicating meter requires recalibration if the thermocouple is changed. 4.0 PYROMETERS This section discusses the indicating meter. There are two types of meter currently in use - the traditional "swing arm" or analogue meter, and the more "high-tech" digital meter. In both cases, the meter is simply a volt meter, or more correctly a "millivolt meter", as the tiny voltages generated by a thermocouple are approx. 15mV (type R) and approx. 50mV (type K & N) at maximum temperatures. (One millivolt is one thousandth of a volt.) 4.1 ANALOGUE METERS The analogue meter comes in many forms - as a free standing unit, or built into an electric kiln controller - but in all cases the mechanical construction is similar. This consists of a indicating needle attached to a very fine coil of wire mounted on jewelled pivots. The tiny current from the thermocouple is passed through the coil and moves the needle against a permanent magnet. The whole assembly is very fragile and sensitive to vibration and dust. I have given this basic description of the internals of an analogue meter to show why it will not work very well after it has been dropped onto the workshop floor! This common occurence generally spells the end of the meter, and is one of the major disadvantages of the analogue meter. Another disadvantage is that the temperature scale is usually divided into 20 degree segments, which makes accurate reading very difficult.more importantly, it is impossible to see a change of less than 5 degrees, making it a matter of guesswork whether the temperature is rising, steady or falling during a reduction firing. On the other hand, the analogue meter consumes no power, so the batteries won't go flat in the middle of a firing! A SPECIAL NOTE:- If the thermocouple is disconnected from an analogue meter, the terminals must be connected together with a short length of bare wire. This will stop the needle swinging wildly about during handling, which may cause serious internal damage to the movement. 4.2 DIGITAL METERS The digital meter has many advantages over the analogue type.

4 A change of one degree is readable, which means that we can see if the kiln temperature is rising or falling almost instantly. The rise (or fall) per hour can be calculated in 5 minutes or less. It is much easier to read, particularly for those of us with "tired" eyes. It will also withstand excursions to the workshop floor. However, it needs batteries (or a 240V "power pack") and tends to be a bit more sensitive to heat - the Liquid Crystal Display (LCD) will darken if exposed to temperatures above 50 deg C, until it's impossible to read. (The same thing happens to your digital watch if you leave it in the sun). If the unit is located in a cooler area, the LCD will last indefinitely. The major advantage of the digital meter is that the readings are repeatable - that is, you can obtain the same reading for every firing and know that the temperature is exactly the same. 4.3 GENERAL Because the voltage output of each type of thermocouple is different, it is not possible to interchange meters - each meter is calibrated for one type only, and will need to be recalibrated if a change in thermocouple type is considered. It is relatively easy to change between type N and type K, but type R is very different, and requires special calibration. The accuracy of pyrometers is variable. The accuracy of analogue meters can be as low as plus or minus 10%, which is plus or minus 130 degrees at 1300 degrees C. The accuracy of digital units is generally plus or minus 2% over the working range, means within 26 degrees at 1300 deg C. This is usually sufficient for most requirements. Absolute accuracy is unnecessary, as the pyrometer is only a guide to the temperature in the kiln. It is an aid to kiln management only, and MUST be used in conjunction with pyrometric cones, which measure HEAT WORK, a far more important indicator of glaze or body maturity than temperature alone. There will be more about this later. 5.0 COMPENSATING LEAD The third part of a pyrometer is the compensating lead, which connects the thermocouple and the meter. This cable is not very well understood, as it has far more significance than just a way of connecting the thermocouple and meter Earlier in the article I mentioned that voltage is generated between two ends of a wire by heating one end, in other words, the difference in temperature between each end produces a voltage. The thermocouple uses this principle, and produces a voltage which is proportional to the difference between the tip, or hot junction, and the terminal end, or cold junction. This is a very important point to consider, because the thermocouple millivolt tables which are used for meter calibration are produced with the assumption that the cold junction is at ambient (air) temperature.

5 I bet that if you touch the terminal block (cold junction) of your thermocouple when the kiln is at maximum temperature, you will find it's temperature considerably higher than air temperature, probably 60 to 100 degrees C or higher. Therefore, the temperature indicated on your meter would have a large error incorporated into it, if it weren't for the "compensating" lead. Ideally, the terminal block should be mounted on a cooler surface to "heat-sink" the temperature down, but in practice this is virtually impossible. However, the temperature of the cold junction should be kept to a minimum by preferably not attaching the terminal block to the kiln body. The compensating lead is composed of material with the same temperature response as the thermocouple. One end of the compensating lead will be at the same temperature as the cold junction, and the other will be close to ambient temperature. The compensating lead generates a voltage to compensate for the difference between the cold junction temperature and the ambient. In other words, the compensating lead acts as a thermocouple, attached in tandem with the main thermocouple, and together they give the correct voltage output to the meter. In effect, the cold junction of the thermocouple has been extended from the terminal block to the terminals of the meter, so the cable is often known as the "extension cable". The analogue pyrometer is calibrated for the length of compensating lead supplied with the unit, and the length should ideally not be changed. In practice, small changes of length have only a minor effect on readings. The length of compensating lead has no effect on most digital pyrometers. To obtain correct compensation, the lead which matches the thermocouple type must be used. BPQ can supply information on compensating leads if required. 5.1 CHECKING YOUR PYROMETER CONNECTIONS. Here is a suggested method to check that your pyrometer is correctly connected. 1. Disconnect the compensating lead from the thermocouple, and temporarily connect the thermocouple direct to the meter via short lengths of plain wire. It doesn't matter which way it is connected at this stage. 2. Heat the tip of the thermocouple with a flame, and see if the meter shows an increase. If not, reverse the connections and check again. 3. Mark the thermocouple wire connected to the negative terminal (-) of the meter with red paint (or nail polish). This is the negative lead of the thermocouple. 4. Reconnect the thermocouple to the compensating lead, ensuring that the red wire of the compensating lead is connected to the red painted wire of the thermocouple. 5. Check that the other end of the compensating lead is connected to the meter correctly, ie. with the red lead to negative (-) and the other to positive (+).

6 5.2 USE OF PYROMETERS. When firing a pottery kiln, a pyrometer is essential to indicate the rate of rise or fall of temperature, particularly during reduction. The most accurate pyrometer cannot indicate when a firing is completed, it will only indicates the temperature difference between the tip of the probe and ambient temperature. Pyrometric cones indicate HEAT WORK, which is a combination of TIME and temperature, giving an accurate indication of glaze maturity. Factors such as firing time, degree of reduction, different kiln loadings and probe position can affect the relationship between temperature readings on a pyrometer and cone indications. As an example, a cone 9 may tip at an indicated 1210 degrees C if fired over a 3 day firing cycle, whilst in a fast firing cycle, say 3 hours, the same cone number may tip at 1300 degrees C. The cone MUST be considered as the only reliable indicator of glaze maturity. It is for this reason that glazes are usually designed to cone numbers, not temperature. Absolute accuracy is not available from pyrometers available to the average potter, nor is it required. The pyrometer should be used as an aid to efficient kiln management, it's most important feature being repeatability of readings to produce successful firings. 6.0 CONCLUSION I hope that these articles have shed some light on the mysteries of the pyrometer - it is logical and scientific, not really a "black art"! I will attempt to answer enquiries regarding your pyrometer if you wish to me at dcoggins47@gmail.com