Especificaciones técnicas
DAMPER CONTROL UNIT 1 CHANNEL (DCU 1CH)
Two different temperature sensors are commonly used for sensing small temperature variations, although both are based on the same principle: the change of a resistive element under tempera- ture changes. The two available types are resistance temperature detectors (RTDs) and negative temperature coefficient (NTC) thermistors. Their characteristics are summarised in Table 2.2.
RTD (Pt) Thermistor
Temperature span -250 to 900 -100 to 450
Sensitivity 0.00385 K−1 '0.04 K−1
Accuracy ±0.01 K ±0.1 K
R − T curve Linear Exponential
Excitation Current or voltage source Current or voltage source
Typical size 6 mm×6 mm 3 mm×3 mm
Table 2.2: Typical properties of platinum RTD sensors and NTC thermistors.
Usually in low-noise temperature measurements thermistors are the preferred option because their high sensitivity. In the following we consider thermistors and platinum resistance temperature detectors.
2.2.1
Resistance temperature detectors (RTDs)
RTDs are resistive elements manufactured with different metals such as platinum, nickel or cop- per. These metals exhibit a known change in their resistance with changes in their temperature. The change of the resistance in these materials is large enough to detect small temperature varia- tions [101]. Commercial RTDs are, however, manufactured mainly with platinum, hence, we only describe these ones here. The relationship between resistance and temperature can be expressed, in general, as
R(T ) = Ro[1 + α1(T − To) + α2(T − To)2+ . . . + αn(T − To)n] (2.2)
where T is the temperature, α1, α2,. . . ,αn are constant coefficients and Ro is the resistance at a reference temperature, To, usually, 273 K. In the linear region (which extends from −250 to 850) Eq. (2.2) reduces to [28]
R(T ) = Ro[1 + αPt(T − To)] (2.3)
where αPtis the sensitivity of the sensor defined as
αPt= 1 Ro
dR(T )
dT (2.4)
which for platinum RTD is 0.00385 K−1. The value of Ro varies from 25 Ω to 10 kΩ, however, the use of large values of Ro does not produce better results in the noise performance of the system as one could expect —see §2.3.2.1—, but does reduce the errors due to the 2-wire measurement set up —see appendix §A.4. Two important features of the platinum sensor are [72, 28]: (i) its long- term stability (manufacturers specify values around 0.05 K yr−1) and (ii) its excellent repeatability. However, as it will be shown in§2.3.2.1 the sensitivity of the platinum RTD is not enough to reach the demanding requirements of the measurement under certain power limitations.
2.2.2
NTC thermistors
NTC thermistors are also resistive elements that change their resistance with temperature. NTC thermistors are manufactured mixing and synthesising oxides doped with metals. The usual oxides used are of manganese, nickel, cobalt, iron, copper or aluminium. The oxide proportion determines the resistance and the temperature coefficient of the sensor [123, 101]. Thermistors are characterised by (i) exhibiting a large change in their resistance under small temperature variations, (ii) a negative temperature coefficient and (iii) an exponential relationship between temperature and resistance, which implies a non-constant, but high sensitivity —see Figure 2.1.
The resistance-temperature relationship of a thermistor can be expressed by means of the Steinhart-Hart equation [133],
T−1= A + B ln R(T ) + C ln3R(T ) (2.5)
where T is the temperature in Kelvin units and A, B and C are constant coefficients that depend on the thermistor type and are given by the manufacturer. Nevertheless, Eq. (2.5) can be simplified, if the working temperature range is small (∆T ' 50 K) to [123, 28, 101]
R(T ) = Roeβ(T
−1−T−1
o ) (2.6)
where β is the temperature characteristic of the material and is constant for small temperature ranges and Ro is the resistance of the thermistor at a given reference temperature, To, usually 298 K. The values of Ro for space qualified thermistors are in the kΩ range (2 kΩ to 20 kΩ [16]), thus, appropriate for the 2-wire measurement configuration [75, 101, 28] —see appendix§A.4. The value of β depends on the thermistor and the typical values are around 3500 K [16].
The sensitivity of the thermistor can be calculated for small temperature intervals by making use of the definition given in Eq. (2.4). The resultant sensitivity is
αNTC= − β
T2 (2.7)
which is ∼0.04 K−1 at 298 K. The sensitivity is the most important parameter of the sensor for our purposes since its value appears in the expression that sets the noise performance of the system: the higher the sensitivity the lower the noise in the measurement —see §2.3.2. For this reason the thermistor appears, in principle, as the baseline for the temperature measurement subsystem (TMS). Figure 2.1 shows the change in the resistance as a function of the temperature for platinum RTDs and for NTC thermistors: the higher sensitivity of the thermistor with respect to the platinum RTD is clearly seen. 0 10 20 30 40 50 0 0.5 1 1.5 2 2.5 3 3.5 temperature [oC] R(T)/R o Pt RTD thermistor
Figure 2.1: Normalised resistance vs. temperature for a NTC thermistor and a Pt RTD.
Thermistors, however, present different concerns that must be taken into account:
thermistors are, in fact, semiconductors and, thus, some 1/f excess noise2 might appear in the measurement [90, 56],
thermistors are manufactured with tiny amounts of ferromagnetic materials (nickel, cobalt or iron) [123, 160, 101]. The severe magnetic cleanliness requirement in the test masses location might be incompatible with the magnetic properties of the NTCs,
the long-term stability might not be as good as in the platinum sensors. Different studies on this subject, however, appear reassuring [79, 80, 41],
the repeatability of the thermistors might not be good enough, i.e., sensors from the same batch actually exhibit different performance. This implies the need of a screening process to select those sensors which work properly.
These concerns are analysed in this thesis in order to validate the use of NTC thermistors as the sensors for the LTP TMS. Platinum RTD sensors have been discarded because the requirement given in Eq. (2.1) cannot be met with the power limitations of the system —see§2.3.2.
The NTC thermistors selected (which obviously have to be space qualified) are BetaTherm, specifically, the G10K4 surface type model. They are matched glass coated3 NTC thermistor
21/f excess noise refers to that noise above the Johnson noise (or thermal noise) and that depends on, for instance,
the voltage or current applied to the device [94].
beads, mounted on aluminium housing and pot with Sty cast 2850 ft epoxy —see Figure 2.2. The nominal resistance is 10 kΩ and β is 3694 K [16].
Figure 2.2: NTC thermistor: G10K4 surface type model of BetaTherm.