Querx PT ships with a test sensor. This lets you use the device right out of the box by connecting it to an Ethernet network and a power supply.
However, this sensor is really just a simple test sensor with a 100 mm PVC cable but no protective sleeve. While it returns reliable measurements, it does not allow for much leeway in terms of installation on site. But this is exactly where the advantages of Querx PT lie: Temperature sensors that are perfectly suited to virtually any installation site are available and the sensor cable’s length can be increased up to several hundred meters.
The following article is intended to help you decide which sensor is best suited to your situation.
Types of Sensors
Querx PT is currently available in four variants. The models Querx PT100 and Querx WLAN PT100 are configured for use with Pt100 sensors, while Querx PT1000 and Querx WLAN PT1000 are prepared for Pt1000 sensors.
Such sensors consist of an electric resistor, a case that protects the resistor and makes it easier to install the sensor and usually a cable that connects the sensor to monitoring devices such as Querx PT.
The difference between Pt100 and Pt1000 sensors is easily explained: At a temperature of 32 °F (0 °C), Pt100 sensors have an electric resistance of 100 Ω, whereas Pt1000 sensors have a resistance of 1000 Ω (or 1 kΩ).
The letters Pt refer to the chemical element from which the electric resistor is mainly built - platinum. A change in resistance caused by a changing ambient temperature is not desirable in regular resistors, as used in electronic devices, and generally this characteristic is prevented as far as possible. When measuring temperatures with a resistor, however, this precise change is required. Among many other advantages, the resistance of platinum changes relatively consistently across the relevant temperature range of -328 °F to 1112 °F (-200 °C to +600 °C). But more on this later.
egnite offers a range of temperature sensors which is continually extended. However, an incalculable variety of resistance sensors, which can never be offered by a single retailer, is available. Should a different retailer offer a Pt100 or Pt1000 sensor that is better suited to your needs, it is no problem to connect it to Querx PT.
The construction types of Pt-sensors are highly varied. Apart from the available standard sensors, the manufacturers of different monitoring devices offer their own construction types that are more easily integrated into their products.
The most common construction type is the so-called cable sensor. This usually consists of a metal sleeve with a diameter of 3 or 6 mm, which contains the resistor element and is squeezed onto a connection cable. Cable sensors deliver good results when measuring the temperature of surrounding air and specially labelled, waterproof devices can be used to measure the temperature of liquids. Breathing air or water are mostly not problematic, whereas, when measuring other, especially aggressive media, you should first consult the sensor’s data sheet or ask the manufacturer directly.
Liquids often need to be measured inside piping - a situation to which cable sensors are not very well suited, due to the small contact area. In this case it is advisable to use contact sensors, or, if a higher accuracy is required of the measurements, so-called immersion pockets. The latter consist of a metal sleeve that is screwed into the piping and into which the sensor is inserted. Before installation, the immersion pocket is filled with heat conducting paste, in order to ensure that the heat or cold is transmitted to the temperature sensor.
While simple cable sensors are well suited to measuring the temperature in living areas, they do not necessarily meet our aesthetic expectations. Pt100 sensors are also available as specially designed indoor temperature sensors. They usually ship without cables, making it possible to connect them to cables that are already laid behind plaster.
Monitoring refrigeration appliances or temperature chambers can often be a challenge. In the easiest case, the manufacturer themselves offers a Pt-sensor or has even fitted the appliance with one. If this is not the case, you will need to place a sensor inside the device without obstructing its isolation features. It might not be the optimal solution, but a thin sensor cable could, for instance, be passed through a door’s rubber sealing. A better solution would be a threaded cable sensor, either with or without an immersion pocket, as previously described for measurements in piping. Since this option requires a hole to be drilled into the device, this solution is obviously reserved for specialists who can evaluate the effect of such a modification on the entire system. You will certainly lose the warranty for the refrigerator or oven in your kitchen, should you opt for this method.
Most temperature sensors ship with a cable of some length. Once you have found a sensor with a cable length that fulfils your demands, you should ensure that the wire ends fit Querx PT’s relatively delicate connection terminals. The cross section of the individual wires should be between 0.2 and 0.75 mm2 (24 … 18 AWG). If the wires include ferrules, they should be between 0.25 and 0.34 mm2 large.
Many temperature sensors are sold with cables of up to ten meters length. If this is not long enough, the cable can easily be extended, but the fact that any cable has a certain electric resistance that is added to the sensor’s resistance needs to be taken into consideration. The value Querx PT returns will therefore be higher than the actual temperature.
This deviation is not to be taken too lightly. A meter of copper wire with a diameter of 0.5 mm (cross section 0.2 mm2, AWG24) has a resistance of 0.086 Ω. Every meter that is added to the sensor cable increases the temperature that is displayed by approximately 0.396 °F (0.22 °C). At a length of ten meters, this adds up to a deviation of more than 3.96 °F (2 °C). When using Pt1000 sensors, this error is reduced by a factor of 10.
However, there is a solution to this problem: 3- and 4-wire sensor cables. 4-wire sensors use one pair of wires to transfer power to the sensor and the second pair to measure the sensor’s voltage. 4-wire cables offer the most accurate measurements. Wire can, however, be saved by using 3-wire cables. These let Querx PT measure the wire’s resistance independently and then automatically deduct it from the sensor’s measured resistance.
Querx PT needs to be configured correctly via the web interface, in order for Querx PT to make these additional measurements and calculations. The fact that all the wires of a 3-wire cable are the same, which is not the case with 4-wire cables, needs to be considered. When configuring the device, you will notice that the interface does not distinguish between 2- and 4-wire cables. This is due to the fact that Querx PT actually uses the 4-wire method with 2-wire connections. The two little switches on the terminal each connect two wires, once they are set to ON. Thus, only two wires are required on the sensor cable itself.
So what is the maximum length of a 3- or 4-wire cable? Theoretically, they could be hundreds of meters long. However, in practice one might run into the issue that magnetic of electric fields close to the cable might influence the measurements. This is a problematic topic and can often only be managed by trial and error. Avoid laying the wire in parallel to power lines and maintain a maximal distance between the wire and electromagnetic devices such as motors, relays or the like. Querx PT features an integrated 50 Hz filter which dims these frequencies and their harmonic waves, in order to reduce measurement deviations caused by AC cables. The filter can be set to 60 Hz for use in countries that use 60 Hz AC voltage.
2-wire temperature sensors should only be used with short cables. It is advisable to use 3- or 4-wire temperature sensors for even just a few meters. These wires can then be extended with additional cables. In any case, the switches on the Querx PT sensor terminal and the sensor configuration in the web interface need to be set correspondingly.
Querx PT’s measurement electronics cover the temperature range from 328 °F to 1382 °F (-200 °C to +750 °C). Of course, not every sensor is capable of tolerating such extreme temperatures. One common weak point is the cable, which can break in very low temperatures and melt in very high temperatures. Cable sensors usually feature a metal sleeve that appears to be resistant to high temperatures. Since the resistor inside the sensor is, as we have learned, made of platinum - a heat resistant precious metal - this should not be a problem. But beware, the sleeve contains a casting compound that can react to high temperatures in a variety of ways. Furthermore, the heat conducting metal sleeve might transmit the temperature to the connection cable, damaging it.
You should therefore make sure that the sensors can resist the temperatures you need to measure and are specified for the particular temperature range, before using any particular Pt100- or Pt1000-sensor.
Querx PT itself should certainly never be exposed to such extreme temperatures. The device can only function correctly when operating in ambient conditions that remain within the range of -40 °F to 185 °F (-40 °C to +85 °C), as specified in the data sheet. Please be aware that copper cables conduct heat well and may transfer the high temperatures that the sensor is exposed to into Querx PT’s interior.
As long as you are using Querx PT to measure normal temperatures of indoor- or outdoor-ambient-air and take the notices concerning cable-length and location into account, the device will deliver readings that conform to the specified accuracy when used together with regular Pt100- or Pt1000-sensors. However, the further the measurements move away from the freezing point, the higher the risk of measurement errors will be.
Querx PT measures temperatures at an accuracy of ±0.8 °F (±0.5 °C) across the entire range from 328 °F to 1382 °F (-200 °C to +750 °C). This value, however, only refers to the measurement electronics Querx PT itself uses, while the connected sensor will also exhibit some deviations that will be added to those of the electronics. The temperature sensor’s data sheet will provide more detailed information on its accuracy. The standardized accuracy classes that the sensors are graded as serve to give a general idea of their accuracy. Class B, which allows for a maximal deviation of ±0.54 °F at 32 °F (±0,3 °C at 0 °C) is very common. However, when measuring much higher or lower temperatures, the permitted deviation is many times higher. In the range of 1112 °F (600 °C) it is as high as 5.4 °F (3 °C) for class B sensors.
This deviation occurs because the platinum resistor does not increase absolutely linearly with increasing temperatures. Querx PT employs a special formula - the so-called Callendar-Van Dusen equation - to account for these errors. Nonetheless, a few things need to be considered. The platinum resistors are not made entirely of platinum, making them react slightly differently to what is expected of the pure material. A value a that indicates a rise in the characteristic curve between 0 °C and 100 °C results in three values that are applied to the formula. Querx uses the values α = 3,85 * 10-3 °C-1, which were set in the standard IEC 751 / DIN 60751 in 1995.
The sensor needs to be manufactured in compliance with the standard, in order to ensure accurate measurements in the upper and lower ranges. All sensors offered by egnite comply with the standard.