GB2256056A

GB2256056A - Temperature measuring apparatus.

Info

Publication number: GB2256056A
Application number: GB9210425A
Authority: GB
Inventors: David John Fairhurst
Original assignee: KANE MAY Ltd
Current assignee: KANE MAY Ltd
Priority date: 1991-05-16
Filing date: 1992-05-15
Publication date: 1992-11-25
Also published as: GB9110631D0; GB9210425D0

Abstract

A processing unit (102) can be connected to a series of different probes (112;130), each having different characteristics, the selection of a probe being dependent on the temperature measuring task to be performed. A first probe (112) has a thermocouple (108) and a second temperature sensor (122) to provide cold junction compensation. The hot junction (108) of the thermocouple is placed in the medium, the temperature of which is to be measured, and the unit (102) derives the temperature from the voltage generated and the cold junction temperature measured by the second sensor (122). Other probes (130) containing a thermistor or platinum resistance thermometer (132) can be attached to the unit (102) to provide non-thermocouple temperature measurements. Thus, the speed or accuracy of a temperature measurement can be optimised to account for different criteria by the use of different probes with the appropriate type of sensor. <IMAGE>

Description

TEMPERATURE MEASURING APPARATUS The present invention relates to temperature measuring apparatus and in particular, but not exclusively, to such apparatus for measuring the temperature of products such as food.

Modern digital hand held thermometers tend to be based on one of the following types of sensor: thermocouple; platinum resistance; or thermistor (semiconductor). These different types of thermometers have various strengths and weaknesses and are thus each suited to particular tasks and functions.

Thermocouples have a low thermal mass and hence have a fast response to temperature change.

Furthermore, thermocouples have a large number of sensor configurations, can measure over a wide range of temperatures and are relatively cheap to manufacture.

However, thermocouples have two disadvantages. Firstly, they have a poor accuracy at low temperatures and secondly, cold junction compensation is required for changes in ambient temperatures.

Thermistor devices are highly accurate and are cheap to manufacture. However, they have a narrow temperature range and high thermal mass resulting in a slow response to changes in temperature.

Platinum resistance sensors whilst being highly accurate and having good long term stability, are expensive, have relatively few sensor configurations and have a high thermal mass resulting in a slow response to changes in temperature.

In order to achieve optimum results, the user often has to use two or more different types of thermometer when measuring different temperature situations. For example in the food industry, there is the need to measure food product temperatures very accurately and also to be able to measure air temperatures very quickly. To do this accurately, a user may have to use a thermistor device or platinum resistance thermometer to measure the food product temperature very accurately and a thermocouple to measure air temperatures very quickly. This causes inconvenience to the user who is forced to keep at least two different instruments, and increases the expense involved.

It is therefore an aim of certain embodiments of the invention to provide a single instrument which can be used in at least two different modes so as to account for different criteria, for example so as to optimise speed or accuracy as desired.

There are known thermocouple thermometers in which there are two temperature sensing elements. The first element is a thermocouple and the second is usually a platinum resistance sensor, a thermistor, a diode or other type of suitable device.

Thermocouple sensors are active transducers which exploit voltage developed between two dissimilar metals when the "hot" and "cold" junctions are held at different temperatures. Under International standards, the voltage so created is with reference to a cold junction assumed to be at O"C. In a laboratory situation, this can be achieved by holding the cold junction at the ice point in a mixture of ice and water.

However, under normal conditions the cold junction is at the ambient temperature, and it is necessary to correct for the difference between O"C and ambient temperature to determine the actual temperature to which the hot function is exposed. It should be noted that since the temperature of the "cold" junction acts as a reference value, the temperature of the "hot" junction may on occasions be actually lower than that of the "cold" junction.

Thus, the second temperature sensor element is used to measure the ambient temperature so as to provide the compensation for the cold junction. Since the range of ambient temperature is often relatively narrow, for example in the range of 0-40OC, platinum resistance sensors, thermistors and other such sensors can be used for accurate measurements.

In order to measure the cold junction temperature accurately one of the connections from the hot junction (acting as the main temperature sensor) is positioned against a block of material having high thermal conductivity and reasonable mass, which consequently acts as an isothermal block or region. It is here that the cold junction is formed. A second temperature sensing element is positioned so as to measure the temperature of the block and consequently the ambient temperature i.e. the temperature of the cold junction.

Both the second temperature sensing device and the block are located within the housing of the instrument. The block may, for example, be in the form of a block of metal covered with a suitable electrically insulating material.

According to an invention disclosed herein, there is provided a temperature measuring instrument comprising a processing unit for connection to a temperature probe, the unit having a thermocouple mode for determining a thermocouple based temperature measurement in which the unit is arranged to process information received from both a thermocouple incorporated in a probe and a non-thermocouple temperature sensitive device, the information from the non-thermocouple device being used by the processing unit to provide cold junction compensation for the thermocouple, wherein the processing unit has first connection means for connection to the thermocouple and second connection means for connection to the nonthermocouple device, and wherein the unit has a nonthermocouple mode for determining a non-thermocouple based temperature measurement, in which it is arranged to process only information received from a nonthermocouple temperature sensitive device connected to the second connection means.

Thus, the second temperature sensitive device for use in thermocouple measurements is connected to the processing unit via the second connection means. The processing unit does not have to incorporate an internal temperature measuring device itself. If a nonthermocouple measurement is to be made, then the second temperature device in a thermocouple probe could be used to provide a non-thermocouple temperature measurement, the processing unit ignoring the information from the thermocouple and treating the information from the nonthermocouple device so as to provide a temperature measurement rather than compensation.

Alternatively, a specific non-thermocouple probe could be used. The probe would thus have means to connect to the second connecting means and would not be connected to the first. In this case, the lack of thermocouple information could be used by the processing unit to detect that a non-thermocouple measurement is to be made, and to switch to the non-thermocouple mode automatically. Of course, the probe could be provided with an additional input to signal to the processing unit that a particular type of sensor is being used.

For example the input could be arranged to signal that the problem is a non-thermocouple probe. Preferably the signal includes data as to whether the sensor is a thermistor etc.

The use of purely external temperature measuring devices means that there is greater versatility in terms of designing probes. For a thermocouple probe, the cold junction will generally also be separate from the processing unit.

Preferably, the non-thermocouple temperature measuring device and the cold junction of the thermocouple are coupled together via an isothermal point or region in the probe or probe connector ie. the part of the probe which is attached to the connection means of the processing unit.

The non-thermocouple device may, for example, be a thermistor, semi-conductor, platinum resistance sensor or any other suitable device.

The thermocouple cold junction, the nonthermocouple device, and/or an isothermal point or region may be arranged in a handle of the probe or in a housing of a connector for connecting the probe to a main instrument body.

For thermocouple measurements, it is desirable that only one connection operation be carried out even though two devices have to be connected to the processing unit.

Thus, in general the second temperature sensitive device will be incorporated in a single probe with the thermocouple. It may be incorporated in the main body of a probe or in a connector to be physically connected to the processing unit. The probe connector will in any event have sufficient pins, sockets or the like to connect to the first and second connection means of the processing unit.

According to a further invention disclosed herein there is provided temperature measuring apparatus comprising a processing unit for connection to a temperature probe, the unit having a thermocouple mode for determining a thermocouple based temperature measurement in which the unit is arranged to process information received from both a thermocouple incorporated in a probe and a non-thermocouple temperature sensitive device, the information from the non-thermocouple device being used by the processing unit to provide cold junction compensation for the thermocouple, wherein the processing unit has first connection means for connection to the thermocouple and second connection means for connection to an external non-thermocouple device, and wherein the unit has a non thermocouple mode for determining a non-thermocouple based temperature measurement, in which it is arranged to process only information received from a nonthermocouple temperature sensitive device connected to the second connection means and the apparatus further comprising a plurality of temperature sensing probes for selective attachment to said instrument, at least one probe incorporating a thermocouple and cold junction compensating temperature sensing device so as to enable a thermocouple based temperature value to be obtained and at least one probe comprising a non-thermocouple temperature sensing device so as to enable a nonthermocouple based temperature value to be obtained.

Preferably the instrument is arranged so as to determine whether the probe has a thermocouple or not and to accordingly self configure. This may be achieved by the instrument being arranged to record an input from the probe which indicates what type of sensor is being used.

With such an instrument, the user can choose an appropriate temperature sensing device for a particular measurement tasks. Furthermore there is no need to change the configuration of the sensing means depending on which sensing means is being used.

Preferably a number of probes are provided which are each adapted to achieve optimum functionality for different temperature task. Thus a compact device can be provided which uses the same instrument but with different probes to measure the temperature in different situations.

For example one probe may comprise a thermistor whilst another may comprise a platinum resistance sensor. Obviously the temperature sensors in different probes can be of the same type but have different characteristics. Probes with identical temperature sensors can be provided with distinguishing features, such as differently coloured handles, to avoid contamination between different products especially in the food industry. For example one probe can be provided to measure the temperature of meat whilst another may be provided to measure the temperature of fruit.

Embodiments of the invention will now be described with reference to the accompanying drawings in which: Fig. 1 shows a prior art thermocouple thermometer; Fig. 2 shows a thermometer embodying the present invention; and Fig. 3 shows the thermometer of Figure 2 to which a different probe has been attached.

Fig. 1 shows a known standard thermocouple thermometer 2 which has an instrument housing 4 to accommodate circuitry 6 required to provide the temperature from the voltage generated between the hot and cold junctions 8 and 10. Attachable to the instrument housing 4 is a probe 12 having two pins 14 and 16 which are received in respective sockets 18 and 20 on the housing 4. The hot junction 8 is arranged at the end of the probe 12 whilst the cold junction 10 is arranged in the instrument housing 4. The cold junction 10 is formed when the pins 14,16 of the probe 12 are inserted into sockets 16 and 18. Two dissimilar metals are brought into contact in socket 18 to thereby form the cold junction 10. The circuit is completed through socket 20 which makes electrical connection between the hot junction 8 and the circuitry 6.A cold junction sensor 22 and isothermal point/region 24 are arranged adjacent to the cold junction 10.

In use, the hot junction 8 is placed in the medium of which the temperature is to be measured for example air. In this case the temperature of the air needs to be measured rapidly. A voltage is generated between the hot and cold junctions 8 and 10 which is received by the circuitry 6. The circuitry 6 is arranged to derive a temperature value from the generated voltage. In order to compensate for the cold junction 10 not being at 0 C, the cold junction sensor 22 measures the temperature of the isothermal point/region 24 and consequently the temperature of the cold junction and sends a signal indicative of that temperature to the circuitry 6. The circuitry 6 derives from the two signals the actual temperature of the medium measured.

Referring now to Fig. 2 which shows a thermocouple thermometer embodying the present invention. In a manner corresponding to the prior art thermometer, the device 102 has a probe 112 and a instrument housing 104 containing the relevant electronic circuitry 106 which is well known to those skilled in the art. A hot junction 108 is provided at the end of the probe 112 whilst a cold junction sensor 122 is received in a handle 101 of the probe 112.

The probe 112 has four pins 114,115,116,117 whilst the housing has four sockets 118,119,120 and 121 to receive these pins.

When the pins are received in their respective sockets, connection is made in socket 118 between two wires of dissimilar metals to thereby form the cold junction. The circuit from the hot junction 108 to the circuitry 106 is completed via socket 120. In addition, the cold junction sensor is coupled independently to the circuitry 106 via sockets 119 and 121. The connector of the probe 112 contains an isothermal point/region 124, the temperature of which is measured by the cold junction sensor 122. The sensor 122 is in close thermal contact with the pins. This temperature is assumed to be the same as that of the cold junction.

When the device is being used as a thermocouple thermometer, the hot junction 108 of the thermocouple is placed in the medium, the temperature of which is to be measured. A voltage is generated between the hot and cold junctions 108, 110. The cold junction sensor 122 measures the temperature of the isothermal point/region 124 and consequently the cold junction to provide compensation as in the prior art.

As shown in Fig. 3, a second probe 130 containing a thermistor or platinum resistance thermometer 132 can be connected into sockets 119 and 121 as used to connect the cold junction sensor to the circuitry. Thus sockets 118 and 120 are not used. The sensor 132 measures temperature and the signal is fed to the circuitry 106 which is able to detect that there is no thermocouple present as there are no signals from sockets 118 and 120 and according display the measured temperature.

Other different types of probe containing various temperature sensing devices can therefore be attached into the same instrument housing via the sockets so as to measure temperatures. Thus, when the user wishes to perform a particular temperature measuring task, the appropriate probe can be used. If the situation alters the probe can be removed and replaced with a different, more appropriate, type of probe.

In one variation, the connector part of a probe may contain a non-electrically conducting isothermal block.

This is particular suitable for applications where the cold junction sensor is not in direct thermal contact with the input pins. The isothermal block can thus provide the necessary thermal contact between the cold junction and the cold junction sensor.

In a further variation, the probe is provided with a further input pin which is received in a corresponding further socket in the housing. Information as to the nature of the sensor is sent to the processing unit via the further input and socket. The information provided may merely identify the sensor as being either a thermocouple or not a thermocouple. The information provided may alternatively specify what type of sensor is being used e.g. platinum resistance or thermistor.

Claims

1. A temperature measuring instrument comprising a processing unit for connection to at least one temperature probe, the unit having a thermocouple mode for determining a thermocouple based temperature measurement in which the unit is arranged to process information received from both a thermocouple incorporated in a probe and a non-thermocouple temperature sensitive device, the information from the non-thermocouple device being used by the processing unit to provide cold junction compensation for the thermocouple, wherein the processing unit has first connection means for connection to the thermocouple and second connection means for connection to the nonthermocouple device, and wherein the unit has a nonthermocouple mode for determining a non-thermocouple based temperature measurement, in which it is arranged to process only information received from a nonthermocouple temperature sensitive device connected to the second connection means.

2. A temperature measuring instrument as claimed in claim 1, in combination with a temperature probe incorporating a thermocouple device and a nonthermocouple temperature sensitive device.

3. A temperature measuring instrument as claimed in claim 2, in which the processing unit incorporates means for selecting between the thermocouple and nonthermocouple modes, wherein in the thermocouple mode the processing unit processes information from both said devices in said probe, and in the non-thermocouple mode the processing unit processes information only from said non-thermocouple temperature sensitive device in said probe.

4. A temperature measuring instrument as claimed in claim 2 or 3, wherein the temperature probe comprises a probe body incorporating the hot junction of said thermocouple device, and a connector incorporating said non-thermocouple temperature sensitive device.

5. A temperature measuring instrument as claimed in claim 2, 3 or 4, wherein said probe incorporates an isothermal region for thermal coupling of said nonthermocouple temperature device and the cold junction of the thermocouple.

6. A temperature measuring instrument as claimed in claim 4 and 5, wherein said isothermal region is arranged in said connector.

7. A temperature measuring instrument as claimed in any preceding claim, in combination with a temperature probe incorporating at least one temperature sensitive device, the or each of which is in the form of a nonthermocouple temperature sensitive device.

8. A temperature measuring instrument as claimed in claim 7, wherein the processing unit is responsive to the absence of a thermocouple device so as to select the non-thermocouple mode.

9. A temperature measuring instrument as claimed in any preceding claim, wherein the unit comprises means for detecting that a non-thermocouple measurement is to be made and means to switch to the non-thermocouple mode automatically.

10. A temperature measuring instrument as claimed in any preceding claim, wherein said unit is arranged to detect the type of sensor connected to said second connection means.

11. A temperature measuring instrument as claimed in any preceding claim, wherein said non-thermocouple device to be connected to said unit is one of the following types of sensor: thermistor; semi-conductor; or platinum resistance sensor.

12. A temperature measuring instrument as claimed in any preceding claim, in combination with a plurality of temperature probes to be selectively connected to the processing unit, at least one of said probes incorporating both a thermocouple device and a nonthermocouple temperature sensitive device, and at least one other of said probe incorporating at least one temperature sensitive device, the or each of which is in the form of a non-thermocouple temperature sensitive device.

13. A temperature probe for use with a temperature measuring instrument, said probe comprising: a thermocouple device; a non-thermocouple temperature sensitive device for providing cold junction compensation for the thermocouple device; first connection means for connecting the thermocouple device to the measuring instrument; and second connection means for simultaneously connecting the non-thermocouple device to the measuring instrument.

14. A temperature probe as claimed in claim 13, comprising a probe body incorporating the hot junction of said thermocouple device, and a connector incorporating said non-thermocouple temperature sensitive device.

15. A temperature probe as claimed in claim 13 or 14, incorporating an isothermal region for thermal coupling of said non-thermocouple temperature sensitive device and the cold junction of the thermocouple.

16. A temperature probe as claimed in claim 14 and 15, wherein said isothermal region is in said connector.

17. A temperature measuring apparatus comprising a processing unit for connection to a temperature probe, the unit having a thermocouple mode for determining a thermocouple based temperature measurement in which the unit is arranged to process information received from both a thermocouple incorporated in a probe and a nonthermocouple temperature sensitive device, the information from the non-thermocouple device being used by the processing unit to provide cold junction compensatidn for the thermocouple, wherein the processing unit has first connection means for connection to the thermocouple and second connection means for connection to an external non-thermocouple device, and wherein the unit has a non-thermocouple mode for determining a non-thermocouple based temperature measurement, in which it is arranged to process only information received from a non-thermocouple temperature sensitive device connected to the second connection means and the apparatus further comprising a plurality of temperature sensing probes for selective attachment to said unit, at least one probe incorporating a thermocouple and cold junction compensating temperature sensing device so as to enable a thermocouple based temperature value to be obtained and at least one probe comprising at least one temperature sensitive device, the or each of which is a non-thermocouple temperature sensing device so as to enable a non-thermocouple based temperature value to be obtained.

18. Apparatus as claimed in claim 17, wherein a number of probes are provided which are each adapted to achieve optimum functionality for a different temperature task.

19. Apparatus as claimed in claim 17 or 18, wherein a plurality of probes are provided with distinguishing features for use with different products to avoid contamination between said different products.

20. Temperature measuring apparatus substantially as hereinbefore described with reference to Figure 2 and 3.