Although the main focus of this guide is the application of Sensor Tech thermometers, a large part of the information applies to the measurement of food temperatures in general.
These explanations are designed to promote a shared understanding and language and are not necessarily intended to be definitive or to give a scientific definitions. With over 30 years experience manufacturing and designing thermometers to work in all the food sectors, we are continuously learning from our customers how best to suit their needs and applications. The aim of this guide is to provide some answers to those regularly asked questions.
Answers themselves will often lead to yet more question. If you wish to ask about any of our products, their application and the topics discussed, please email firstname.lastname@example.org
There are important differences between testing food products out in the field, on a production line, process tanks, pipe lines, cold stores, chillers, retail outlets, refrigerated containers, silos, grain stores and loading bays etc. They may require a specialized skill and different types of thermometers. The methods and probes used to test food temperatures will often vary with the type and size of the food product being tested.
In order to make an accurate measurement of food product, it is important to match the sensitivity and thermal mass of the probe to the conductivity and thermal mass of the product or medium being measured.
We offer probes from our standard range to the bespoke, from the very low mass non invasive 'Between Pack' probe to a range of penetration probes starting with the hypodermic or needle probe that has a 1.5mm diameter and continue with diameters 3mm, 4mm, 5mm and 6mm. In general probes with similar wall thicknesses have a thermal mass that increases in proportion to their diameters.
Diameters between 3 and 4mm are the most popular as they combine a safe practical size, having good strength and response with the ability to house a range of sensors.
The next variable is the size of the product. With small products there is always a risk that the thermal mass of the probe will cause a heating or cooling effect on the product itself. With cocktail sausage, chicken nuggets, fruits etc., it may be useful to pick up a number of these pieces on the steel probe 'Kebab Style' to simulate larger product.
Quality control carries out these temperature checks throughout the food production process. In general the best way to measure a product's temperature is to use a penetration probe and get a core temperature, however this depends very much on what the situation will allow.
The circumstances will often change as the product progresses through production from bulk product down to the smaller retail package. You only have to look at a supermarket shelf to see the huge variety of food products and the different packages used to protect and present them.
Our thermometer range TGST3, TGBP, TGS4, TGTL, TGSS, TGAL and TGPM has been designed for specific applications throughout all the food production sectors, Farming, Production, Storage, Catering and Retail and Distribution.
Penetration probe TGST3 is our most popular seller
Testing Product temperatures using a steel penetration probe is the most accurate way of getting the core temperature of the product. To achieve this it is necessary to penetrate the product to a least 10 times the diameter of the probe to a depth where both probe and product reaches a temperature equilibrium.
This means that a 3.5mm diameter probe requires a penetration depth of 35mm. This is the depth where the readings will stabilize with minimal heat flow or temperature fluctuations, this may take a little time depending on the thermal conductivity of the product. Meat is a good conductor, bread is a poor conductor which needs a larger penetration. Air is slower still and needs whole probe including the handle to get an accurate result.
In practice there is often no need to wait for the last degree to settle, as an experienced operator will know from the rate of change of the readings, whether a particular target temperature will be reached or surpassed.
The TGS4 used for Cold Store Temperatures.
Our most popular probe for cold store temperatures is this heavy duty TGS4 with double wall thickness tube. It is our toughest probe. If it gets bent the tube will not kink so it can be straightened on a vice.
Whenever the core temperature is needed it may be necessary to drill a hole in the product. In order to get an accurate and fast reading it is necessary to use a drill bit that has just slightly larger diameter than the probe.
We supply a 3.5mm drill bit to match our 3.4mm probes and a 4.5mm drill to match our 4mm TGS4 probes. A 4mm probe sitting in a 6mm hole (1⁄4 inch, often the only size available) is sitting in an air pocket and will often give an error in the direction of ambient
One way around this is to push two probes in the same 6mm hole thus forcing thermal contact with the product and improving both the accuracy and response. The strength of our drill bits were demonstrated to us by drilling a hole in a 1⁄4 inch steel plate and then forcing the drill sideways in both direction making an elongated slot. Very impressive; so breakage should never be an issue
Between Pack Temperatures.
The simplest way to test frozen product temperatures is to use a 'Between Pack' probe. Although this not a core temperature it can be accurate as long as the product has stabilized in the cold store.
When temperature needs to be taken after the product has been packed the choices are very limited. These are some of the most difficult tests in the food industry. They are also the subject of most of
the calls we receive.
'Between Pack' temperature measurement using a steel probe
On a loading bay it may be helpful to pre-cool the probe in an ice bucket prior to making the test. This will speed up the test as there is less heat transfer needed to reach equilibrium with the product. As this is usually not an option, it may help to loose some heat by pushing the probe between the boxes a number of times before settling on the final result.
Placing a round steel probe between boxes on a pallet will cause the displaced boxes to make contact with the probe, top and bottom only. This contact is in the form of a tangent only and creates air pockets drawing in air on both sides of the probe and creating an error that is always in the direction of the ambient temperature.
We often get a call from customers having issues with product being tested during a delivery. These are some of the most difficult tests in the food industry and may result in the product being unfairly rejected because the methods being used are incorrect and no not represent the true product temperature.
Our answer to this problem was to design a low mass 'Between Pack' thermometer. Because of it's accuracy this probe has the ability to win agreement on all sides, so much so that large supermarkets are now requesting that haulers making deliveries to their stores, carry these thermometers.
The TGBP is a low mass 'Between Pack' probe.
The low mass 'Between Pack' probe is a 2” x 1” plastic foil sensor about the thickness of a credit card. It has a tough, flexible mylar spine which makes it easy to slip between boxes without drawing in any air. It creates a large contact area top and bottom of the sensor and can be maneuvered deep between the boxes on the pallet giving fast and accurate temperature readings that closely represent the product within.
We demonstrate this probe by taking the temperature between vacuum packs of product, then opening one of them and taking a core temperature with a penetration probe. The readings are very similar and would indicate that the 'Between Pack' probes provide a simple and accurate means of taking temperatures without the need to penetrate the product.
In order for this test to be representative of the core temperatures, the product must have been allowed to stabilize in the fridge or freezer for some time. The accuracy that these tests provide, shows that there is less need for destructive testing and the product spoilage associated with it.
Loading Bay Temperatures.
InfraRed versus a 'Between Pack' Thermometer.
On opening the back of a refrigerated container the consignment immediately comes into contact with the outside air temperatures. When the air temperature is higher than that of the product the first wafts of air will cause the surface temperature of the product to rise. This is why it is important to take a reading as soon as the door is opened. If packed in cardboard boxes which can act as an insulator, the surface temperature of the box will rise faster towards the ambient air temperature than the product inside the box which will continue to maintain a stable temperature for some time. Using an InfraRed to scan the consignment will only read those surface temperatures. At best this test can only give a rough guide of the actual product temperatures in the box.
From the literature of most of these InfraRed thermometers the accuracy is no better than +/- 2.0°C and more. These specifications are usually the results of tests carried out in the dry stable conditions of a laboratory which often bear no resemblance to the harsh operational conditions on the shop floor or loading bay. An InfraRed thermometer taken from a pocket at body temperature will need to stabilized before any readings can be taken.
Driver Take Note !
If issues arise when testing product temperatures during delivery, it may be necessary to have a backup strategy. We recommend using the only viable, most accurate alternative available for non invasive testing.
Using a 'Between Pack' probe is the best defence of the true product temperature as they use a physical contact measurement. It comes with a calibration certificate whose accuracy can be quickly demonstrated using a slushy-ice mix in a vacuum flask.
The sensor itself is located close to the tip of the probe and only needs to be dipped less than an inch into the ice mix. This test is so convincing that it should only need a single demonstration.
The probe should not be fully immersed as it can cause water contamination at the triangulation joint where the cable and silver foil meet.
When used carefully these probes can give years of service, but this will depend very much on how they are being used. In designing these probes we had to strike a balance between robustness (slow response) and low mass (fast response). These probes need to be used with reasonable care.
It is important to identify the correct openings so as to avoid any crumpling of the probe before inserting the foil smoothly between the boxes. Once the technique for doing this has been mastered it becomes routine. We also advise that the probe should not be left between boxes and should be removed after the reading. Special care needs to be taken when the probe has been left between frozen meat boxes as the combination of the weight and the freezing action may cause damage to the probe when it is being withdrawn. Best lift the top box and take the pressure off.
TGTL is a T-Loop probe giving it a push, pull, twist action
This probe is used for deep muscle penetration in carcass meat. The handle allows for a push, pull, twist action which makes it more of an industrial probe than the TGST3. It should not be used to measure frozen product unless it is being used with a drill. This probe is very useful in certain situations but we do not recommend it for regular production lines as the probe hook-handle can get caught in moving machinery and is more awkward to carry around. The TGST3 is always the preferred option for most operatives outselling the TGTL 20/1.
Please note: When this probe is left to hang freely from the cable, it has a tendency to stick out at right angles from the wall and may pose a safety hazard.
Our standard wall mounting brackets have been designed to take care of this issue by allowing the probe hook-handle to wrap itself around the holder using the natural spring action of the cable. This simple mechanism allows the probe to be safely and quickly stored out of harms way at the back of the thermometer.
TGPM Pipe Mounted Probes.
These probes are used in food processes to measure the temperature of product flowing in the pipe lines. These are used in the various food processes e.g., creamery intake and milk pasteurisation, sauce lines, refrigerant cooling temperatures etc. For large tanks, they can be supplied with probe lengths up to 2meters long.
All our pipe line probes use 6mm (5.95mm) tube diameters which are designed to fit standard BSP fittings 1/2”, 1/4” and 1/8”. The 6mm tube is a slide fit into the BSP fittings so it can be mounted by inserting it into the pipe until it reaches bottom and then raised slightly insuring that it gets maximum immersion in the flow.. Tightening the nut a half turn compresses the two ferrules on the tube giving it a high pressure seal. When ordering these probes for use on number of pipe lines it is best to order a length that enables a depth that fits a range of your pipe diameters and help to standardise the thermometer. The 120mm length is popular and suits most applications.
TGSS (Short Stem) and TGAL (with a 1 meter lead)
These are very basic, quick and easy to install wall mounted water resistant (protected against power washers) thermometers that come with a calibration certificate. The calibration sticker is protected behind the front water proof label. They also have a 10 year battery life which gives them long term maintenance free use in Fridges, Freezers and process areas, they can also be used to accurately read environmental temperatures. The probes can be tied to existing thermostats and sensors in order to calibrate them. They are supplied with wall mounting brackets and a stainless steel screw for mounting inside or outside these areas. A washer is also supplied which can be fitted as an easy option to prevent its removal without the use of a screwdriver. The TGAL has a 1.5 meter lead which allows the meter to be mounted on the outside of a freezer/serve-over while the probe is mounted inside.
Hypodermic Probes are used mainly in test kitchens
Hypodermic probes are not popular on a production floor because of safety issues and also because they are considered too fragile for most areas.
With thin product, cutlets and burgers it is necessary to insert the probe horizontally (or sideways) as this allows the depth of penetration necessary to get an accurate result.
When testing meat burgers it is best to use a thermocouple type hypodermic or needle probe. Using a standard 3mm probe inserted in the burger from a vertical position will cause most of the heat to conduct up the probe tube. From the vertical a 3mm probe inserted in a 4oz burger would only penetrate it to about 3mm This will leave these readings very inaccurate.
'In fact, you could probably cremate the burger and still not get a 70°C reading on the meter'
The 3mm diameter probe is probably too large for a sideways insertion in a 4oz burger, best use a hypodermic (usually 1.5 mm in Diameter)
We supply these hypodermic probe thermometers to food manufacturers allowing them to accurately measure their cooking times which you will often see printed as the cooking instructions
on their their packaging.
If you are cooking these burgers on a frying pan at home or on the barbecue it is necessary to remove them on to a flat surface where they can be tested by sideways insertion. In this situation a 3mm probe, will work ok providing extra care is taken to make sure the probe is properly centered when being inserted.
Dry Block Calibrators
In the food industry there are two temperature instruments that can destroy user confidence. The first is the InfraRed thermometer which we have discussed earlier and the second is the dry block calibrator. These are very convenient to use, which makes them increasingly popular, however, in practice being able to get a consistent, accurate and repeatable reading needs a proper understanding of the parameters involved.
These calibrators use a metal equalising block with a number of holes drilled into it. These holes may be the same size and may also have a number of holes sizes designed to accept a number of different probe diameters from 3, to 6mm. In theory, when these probes are inserted in the block into a matching hole size, they are brought to the same temperature. If only it were that simple!
The first issue to consider is that the any probe inserted in the block can effect the temperature of the block itself, requiring the controller to compensate for this temperature change by incremental changes to the heating or controlling systems in the block. This may take some time as these temperature fluctuations need to stabilize. When a number of probes are being tested at pasteurization temperature this effect is more pronounced and will take much longer to stabilize.
The next problem is the hole size in the block itself. To be really accurate the steel probe needs to be a snug fit with good thermal contact between the tubes and the block wall. This slide fit should leave no air pockets between the steel probe and the block. The probe is placed in the minimum hole size that best matches its diameter. As the probe gets used, the operational ware and tear on the tube may causes it to deform or bend, so that it may no longer be able to fit the original hole.
'Slide fit, becomes a jammed fit with the slightest bend.'
It is then necessary to use the next size up. This is often too big for the probe so we end up with a probe sitting in an air pocket which can cause large errors. The error is always in the direction of ambient so that a setting of say 70°C can give readings as low as of 69°C, 68°C or lower.
Customers will often call describing these errors, so we explain the limitations of these units and advise them to do the 'Interim Calibration Test' and come back if there is still a problem, this usually returns confidence in the accuracy of the thermometers.
To help with this problem, we can supply a 6mm (5.95mm) sleeve which fits into a 6mm hole in the block and acts as a good conductor between probe and block. These are supplied with internal diameters to match our probe diameters.
For further information on these calibrators, the European co-operation for Accreditation EA-10/13 gives a very good insight into the possible errors in the design and calibration using these Dry Block Calibrators.
Jonathan Coyle - Sales
Gavin Molloy - Repairs
David Martin - Calibrations
Dunleer Enterprise Park
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